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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>3. Data model — Python 3.6.7 documentation</title> <link rel="stylesheet" href="../_static/pydoctheme.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 3.6.7 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="4. Execution model" href="executionmodel.html" /> <link rel="prev" title="2. Lexical analysis" href="lexical_analysis.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/3/reference/datamodel.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="executionmodel.html" title="4. Execution model" accesskey="N">next</a> |</li> <li class="right" > <a href="lexical_analysis.html" title="2. Lexical analysis" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">3.6.7 Documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" accesskey="U">The Python Language Reference</a> »</li> <li class="right"> <div class="inline-search" style="display: none" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> <script type="text/javascript">$('.inline-search').show(0);</script> | </li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="data-model"> <span id="datamodel"></span><h1>3. Data model<a class="headerlink" href="#data-model" title="Permalink to this headline">¶</a></h1> <div class="section" id="objects-values-and-types"> <span id="objects"></span><h2>3.1. Objects, values and types<a class="headerlink" href="#objects-values-and-types" title="Permalink to this headline">¶</a></h2> <p id="index-0"><em class="dfn">Objects</em> are Python’s abstraction for data. All data in a Python program is represented by objects or by relations between objects. (In a sense, and in conformance to Von Neumann’s model of a “stored program computer,” code is also represented by objects.)</p> <span class="target" id="index-1"></span><p>Every object has an identity, a type and a value. An object’s <em>identity</em> never changes once it has been created; you may think of it as the object’s address in memory. The ‘<a class="reference internal" href="expressions.html#is"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">is</span></code></a>’ operator compares the identity of two objects; the <a class="reference internal" href="../library/functions.html#id" title="id"><code class="xref py py-func docutils literal notranslate"><span class="pre">id()</span></code></a> function returns an integer representing its identity.</p> <div class="impl-detail compound"> <p><strong>CPython implementation detail:</strong> For CPython, <code class="docutils literal notranslate"><span class="pre">id(x)</span></code> is the memory address where <code class="docutils literal notranslate"><span class="pre">x</span></code> is stored.</p> </div> <p>An object’s type determines the operations that the object supports (e.g., “does it have a length?”) and also defines the possible values for objects of that type. The <a class="reference internal" href="../library/functions.html#type" title="type"><code class="xref py py-func docutils literal notranslate"><span class="pre">type()</span></code></a> function returns an object’s type (which is an object itself). Like its identity, an object’s <em class="dfn">type</em> is also unchangeable. <a class="footnote-reference" href="#id8" id="id1">[1]</a></p> <p>The <em>value</em> of some objects can change. Objects whose value can change are said to be <em>mutable</em>; objects whose value is unchangeable once they are created are called <em>immutable</em>. (The value of an immutable container object that contains a reference to a mutable object can change when the latter’s value is changed; however the container is still considered immutable, because the collection of objects it contains cannot be changed. So, immutability is not strictly the same as having an unchangeable value, it is more subtle.) An object’s mutability is determined by its type; for instance, numbers, strings and tuples are immutable, while dictionaries and lists are mutable.</p> <p id="index-2">Objects are never explicitly destroyed; however, when they become unreachable they may be garbage-collected. An implementation is allowed to postpone garbage collection or omit it altogether — it is a matter of implementation quality how garbage collection is implemented, as long as no objects are collected that are still reachable.</p> <div class="impl-detail compound"> <p><strong>CPython implementation detail:</strong> CPython currently uses a reference-counting scheme with (optional) delayed detection of cyclically linked garbage, which collects most objects as soon as they become unreachable, but is not guaranteed to collect garbage containing circular references. See the documentation of the <a class="reference internal" href="../library/gc.html#module-gc" title="gc: Interface to the cycle-detecting garbage collector."><code class="xref py py-mod docutils literal notranslate"><span class="pre">gc</span></code></a> module for information on controlling the collection of cyclic garbage. Other implementations act differently and CPython may change. Do not depend on immediate finalization of objects when they become unreachable (so you should always close files explicitly).</p> </div> <p>Note that the use of the implementation’s tracing or debugging facilities may keep objects alive that would normally be collectable. Also note that catching an exception with a ‘<a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a>…<a class="reference internal" href="compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a>’ statement may keep objects alive.</p> <p>Some objects contain references to “external” resources such as open files or windows. It is understood that these resources are freed when the object is garbage-collected, but since garbage collection is not guaranteed to happen, such objects also provide an explicit way to release the external resource, usually a <code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code> method. Programs are strongly recommended to explicitly close such objects. The ‘<a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a>…<a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a>’ statement and the ‘<a class="reference internal" href="compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>’ statement provide convenient ways to do this.</p> <p id="index-3">Some objects contain references to other objects; these are called <em>containers</em>. Examples of containers are tuples, lists and dictionaries. The references are part of a container’s value. In most cases, when we talk about the value of a container, we imply the values, not the identities of the contained objects; however, when we talk about the mutability of a container, only the identities of the immediately contained objects are implied. So, if an immutable container (like a tuple) contains a reference to a mutable object, its value changes if that mutable object is changed.</p> <p>Types affect almost all aspects of object behavior. Even the importance of object identity is affected in some sense: for immutable types, operations that compute new values may actually return a reference to any existing object with the same type and value, while for mutable objects this is not allowed. E.g., after <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">=</span> <span class="pre">1;</span> <span class="pre">b</span> <span class="pre">=</span> <span class="pre">1</span></code>, <code class="docutils literal notranslate"><span class="pre">a</span></code> and <code class="docutils literal notranslate"><span class="pre">b</span></code> may or may not refer to the same object with the value one, depending on the implementation, but after <code class="docutils literal notranslate"><span class="pre">c</span> <span class="pre">=</span> <span class="pre">[];</span> <span class="pre">d</span> <span class="pre">=</span> <span class="pre">[]</span></code>, <code class="docutils literal notranslate"><span class="pre">c</span></code> and <code class="docutils literal notranslate"><span class="pre">d</span></code> are guaranteed to refer to two different, unique, newly created empty lists. (Note that <code class="docutils literal notranslate"><span class="pre">c</span> <span class="pre">=</span> <span class="pre">d</span> <span class="pre">=</span> <span class="pre">[]</span></code> assigns the same object to both <code class="docutils literal notranslate"><span class="pre">c</span></code> and <code class="docutils literal notranslate"><span class="pre">d</span></code>.)</p> </div> <div class="section" id="the-standard-type-hierarchy"> <span id="types"></span><h2>3.2. The standard type hierarchy<a class="headerlink" href="#the-standard-type-hierarchy" title="Permalink to this headline">¶</a></h2> <p id="index-4">Below is a list of the types that are built into Python. Extension modules (written in C, Java, or other languages, depending on the implementation) can define additional types. Future versions of Python may add types to the type hierarchy (e.g., rational numbers, efficiently stored arrays of integers, etc.), although such additions will often be provided via the standard library instead.</p> <p id="index-5">Some of the type descriptions below contain a paragraph listing ‘special attributes.’ These are attributes that provide access to the implementation and are not intended for general use. Their definition may change in the future.</p> <dl class="docutils"> <dt>None</dt> <dd><p class="first last" id="index-6">This type has a single value. There is a single object with this value. This object is accessed through the built-in name <code class="docutils literal notranslate"><span class="pre">None</span></code>. It is used to signify the absence of a value in many situations, e.g., it is returned from functions that don’t explicitly return anything. Its truth value is false.</p> </dd> <dt>NotImplemented</dt> <dd><p class="first" id="index-7">This type has a single value. There is a single object with this value. This object is accessed through the built-in name <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>. Numeric methods and rich comparison methods should return this value if they do not implement the operation for the operands provided. (The interpreter will then try the reflected operation, or some other fallback, depending on the operator.) Its truth value is true.</p> <p class="last">See <a class="reference internal" href="../library/numbers.html#implementing-the-arithmetic-operations"><span class="std std-ref">Implementing the arithmetic operations</span></a> for more details.</p> </dd> <dt>Ellipsis</dt> <dd><p class="first last" id="index-8">This type has a single value. There is a single object with this value. This object is accessed through the literal <code class="docutils literal notranslate"><span class="pre">...</span></code> or the built-in name <code class="docutils literal notranslate"><span class="pre">Ellipsis</span></code>. Its truth value is true.</p> </dd> <dt><a class="reference internal" href="../library/numbers.html#numbers.Number" title="numbers.Number"><code class="xref py py-class docutils literal notranslate"><span class="pre">numbers.Number</span></code></a></dt> <dd><p class="first" id="index-9">These are created by numeric literals and returned as results by arithmetic operators and arithmetic built-in functions. Numeric objects are immutable; once created their value never changes. Python numbers are of course strongly related to mathematical numbers, but subject to the limitations of numerical representation in computers.</p> <p>Python distinguishes between integers, floating point numbers, and complex numbers:</p> <dl class="last docutils"> <dt><a class="reference internal" href="../library/numbers.html#numbers.Integral" title="numbers.Integral"><code class="xref py py-class docutils literal notranslate"><span class="pre">numbers.Integral</span></code></a></dt> <dd><p class="first" id="index-10">These represent elements from the mathematical set of integers (positive and negative).</p> <p>There are two types of integers:</p> <p>Integers (<a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a>)</p> <blockquote> <div>These represent numbers in an unlimited range, subject to available (virtual) memory only. For the purpose of shift and mask operations, a binary representation is assumed, and negative numbers are represented in a variant of 2’s complement which gives the illusion of an infinite string of sign bits extending to the left.</div></blockquote> <dl class="docutils"> <dt>Booleans (<a class="reference internal" href="../library/functions.html#bool" title="bool"><code class="xref py py-class docutils literal notranslate"><span class="pre">bool</span></code></a>)</dt> <dd><p class="first last" id="index-11">These represent the truth values False and True. The two objects representing the values <code class="docutils literal notranslate"><span class="pre">False</span></code> and <code class="docutils literal notranslate"><span class="pre">True</span></code> are the only Boolean objects. The Boolean type is a subtype of the integer type, and Boolean values behave like the values 0 and 1, respectively, in almost all contexts, the exception being that when converted to a string, the strings <code class="docutils literal notranslate"><span class="pre">"False"</span></code> or <code class="docutils literal notranslate"><span class="pre">"True"</span></code> are returned, respectively.</p> </dd> </dl> <p class="last" id="index-12">The rules for integer representation are intended to give the most meaningful interpretation of shift and mask operations involving negative integers.</p> </dd> <dt><a class="reference internal" href="../library/numbers.html#numbers.Real" title="numbers.Real"><code class="xref py py-class docutils literal notranslate"><span class="pre">numbers.Real</span></code></a> (<a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate"><span class="pre">float</span></code></a>)</dt> <dd><p class="first last" id="index-13">These represent machine-level double precision floating point numbers. You are at the mercy of the underlying machine architecture (and C or Java implementation) for the accepted range and handling of overflow. Python does not support single-precision floating point numbers; the savings in processor and memory usage that are usually the reason for using these are dwarfed by the overhead of using objects in Python, so there is no reason to complicate the language with two kinds of floating point numbers.</p> </dd> <dt><a class="reference internal" href="../library/numbers.html#numbers.Complex" title="numbers.Complex"><code class="xref py py-class docutils literal notranslate"><span class="pre">numbers.Complex</span></code></a> (<a class="reference internal" href="../library/functions.html#complex" title="complex"><code class="xref py py-class docutils literal notranslate"><span class="pre">complex</span></code></a>)</dt> <dd><p class="first last" id="index-14">These represent complex numbers as a pair of machine-level double precision floating point numbers. The same caveats apply as for floating point numbers. The real and imaginary parts of a complex number <code class="docutils literal notranslate"><span class="pre">z</span></code> can be retrieved through the read-only attributes <code class="docutils literal notranslate"><span class="pre">z.real</span></code> and <code class="docutils literal notranslate"><span class="pre">z.imag</span></code>.</p> </dd> </dl> </dd> <dt>Sequences</dt> <dd><p class="first" id="index-15">These represent finite ordered sets indexed by non-negative numbers. The built-in function <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a> returns the number of items of a sequence. When the length of a sequence is <em>n</em>, the index set contains the numbers 0, 1, …, <em>n</em>-1. Item <em>i</em> of sequence <em>a</em> is selected by <code class="docutils literal notranslate"><span class="pre">a[i]</span></code>.</p> <p id="index-16">Sequences also support slicing: <code class="docutils literal notranslate"><span class="pre">a[i:j]</span></code> selects all items with index <em>k</em> such that <em>i</em> <code class="docutils literal notranslate"><span class="pre"><=</span></code> <em>k</em> <code class="docutils literal notranslate"><span class="pre"><</span></code> <em>j</em>. When used as an expression, a slice is a sequence of the same type. This implies that the index set is renumbered so that it starts at 0.</p> <p>Some sequences also support “extended slicing” with a third “step” parameter: <code class="docutils literal notranslate"><span class="pre">a[i:j:k]</span></code> selects all items of <em>a</em> with index <em>x</em> where <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">=</span> <span class="pre">i</span> <span class="pre">+</span> <span class="pre">n*k</span></code>, <em>n</em> <code class="docutils literal notranslate"><span class="pre">>=</span></code> <code class="docutils literal notranslate"><span class="pre">0</span></code> and <em>i</em> <code class="docutils literal notranslate"><span class="pre"><=</span></code> <em>x</em> <code class="docutils literal notranslate"><span class="pre"><</span></code> <em>j</em>.</p> <p>Sequences are distinguished according to their mutability:</p> <dl class="last docutils"> <dt>Immutable sequences</dt> <dd><p class="first" id="index-17">An object of an immutable sequence type cannot change once it is created. (If the object contains references to other objects, these other objects may be mutable and may be changed; however, the collection of objects directly referenced by an immutable object cannot change.)</p> <p>The following types are immutable sequences:</p> <dl class="last docutils" id="index-18"> <dt>Strings</dt> <dd><p class="first last" id="index-19">A string is a sequence of values that represent Unicode code points. All the code points in the range <code class="docutils literal notranslate"><span class="pre">U+0000</span> <span class="pre">-</span> <span class="pre">U+10FFFF</span></code> can be represented in a string. Python doesn’t have a <code class="xref c c-type docutils literal notranslate"><span class="pre">char</span></code> type; instead, every code point in the string is represented as a string object with length <code class="docutils literal notranslate"><span class="pre">1</span></code>. The built-in function <a class="reference internal" href="../library/functions.html#ord" title="ord"><code class="xref py py-func docutils literal notranslate"><span class="pre">ord()</span></code></a> converts a code point from its string form to an integer in the range <code class="docutils literal notranslate"><span class="pre">0</span> <span class="pre">-</span> <span class="pre">10FFFF</span></code>; <a class="reference internal" href="../library/functions.html#chr" title="chr"><code class="xref py py-func docutils literal notranslate"><span class="pre">chr()</span></code></a> converts an integer in the range <code class="docutils literal notranslate"><span class="pre">0</span> <span class="pre">-</span> <span class="pre">10FFFF</span></code> to the corresponding length <code class="docutils literal notranslate"><span class="pre">1</span></code> string object. <a class="reference internal" href="../library/stdtypes.html#str.encode" title="str.encode"><code class="xref py py-meth docutils literal notranslate"><span class="pre">str.encode()</span></code></a> can be used to convert a <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate"><span class="pre">str</span></code></a> to <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> using the given text encoding, and <a class="reference internal" href="../library/stdtypes.html#bytes.decode" title="bytes.decode"><code class="xref py py-meth docutils literal notranslate"><span class="pre">bytes.decode()</span></code></a> can be used to achieve the opposite.</p> </dd> <dt>Tuples</dt> <dd><p class="first last" id="index-20">The items of a tuple are arbitrary Python objects. Tuples of two or more items are formed by comma-separated lists of expressions. A tuple of one item (a ‘singleton’) can be formed by affixing a comma to an expression (an expression by itself does not create a tuple, since parentheses must be usable for grouping of expressions). An empty tuple can be formed by an empty pair of parentheses.</p> </dd> <dt>Bytes</dt> <dd><p class="first last" id="index-21">A bytes object is an immutable array. The items are 8-bit bytes, represented by integers in the range 0 <= x < 256. Bytes literals (like <code class="docutils literal notranslate"><span class="pre">b'abc'</span></code>) and the built-in <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-func docutils literal notranslate"><span class="pre">bytes()</span></code></a> constructor can be used to create bytes objects. Also, bytes objects can be decoded to strings via the <a class="reference internal" href="../library/stdtypes.html#bytes.decode" title="bytes.decode"><code class="xref py py-meth docutils literal notranslate"><span class="pre">decode()</span></code></a> method.</p> </dd> </dl> </dd> <dt>Mutable sequences</dt> <dd><p class="first" id="index-22">Mutable sequences can be changed after they are created. The subscription and slicing notations can be used as the target of assignment and <a class="reference internal" href="simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> (delete) statements.</p> <p>There are currently two intrinsic mutable sequence types:</p> <dl class="docutils"> <dt>Lists</dt> <dd><p class="first last" id="index-23">The items of a list are arbitrary Python objects. Lists are formed by placing a comma-separated list of expressions in square brackets. (Note that there are no special cases needed to form lists of length 0 or 1.)</p> </dd> <dt>Byte Arrays</dt> <dd><p class="first last" id="index-24">A bytearray object is a mutable array. They are created by the built-in <a class="reference internal" href="../library/stdtypes.html#bytearray" title="bytearray"><code class="xref py py-func docutils literal notranslate"><span class="pre">bytearray()</span></code></a> constructor. Aside from being mutable (and hence unhashable), byte arrays otherwise provide the same interface and functionality as immutable <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> objects.</p> </dd> </dl> <p class="last" id="index-25">The extension module <a class="reference internal" href="../library/array.html#module-array" title="array: Space efficient arrays of uniformly typed numeric values."><code class="xref py py-mod docutils literal notranslate"><span class="pre">array</span></code></a> provides an additional example of a mutable sequence type, as does the <a class="reference internal" href="../library/collections.html#module-collections" title="collections: Container datatypes"><code class="xref py py-mod docutils literal notranslate"><span class="pre">collections</span></code></a> module.</p> </dd> </dl> </dd> <dt>Set types</dt> <dd><p class="first" id="index-26">These represent unordered, finite sets of unique, immutable objects. As such, they cannot be indexed by any subscript. However, they can be iterated over, and the built-in function <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a> returns the number of items in a set. Common uses for sets are fast membership testing, removing duplicates from a sequence, and computing mathematical operations such as intersection, union, difference, and symmetric difference.</p> <p>For set elements, the same immutability rules apply as for dictionary keys. Note that numeric types obey the normal rules for numeric comparison: if two numbers compare equal (e.g., <code class="docutils literal notranslate"><span class="pre">1</span></code> and <code class="docutils literal notranslate"><span class="pre">1.0</span></code>), only one of them can be contained in a set.</p> <p>There are currently two intrinsic set types:</p> <dl class="last docutils"> <dt>Sets</dt> <dd><p class="first last" id="index-27">These represent a mutable set. They are created by the built-in <a class="reference internal" href="../library/stdtypes.html#set" title="set"><code class="xref py py-func docutils literal notranslate"><span class="pre">set()</span></code></a> constructor and can be modified afterwards by several methods, such as <code class="xref py py-meth docutils literal notranslate"><span class="pre">add()</span></code>.</p> </dd> <dt>Frozen sets</dt> <dd><p class="first last" id="index-28">These represent an immutable set. They are created by the built-in <a class="reference internal" href="../library/stdtypes.html#frozenset" title="frozenset"><code class="xref py py-func docutils literal notranslate"><span class="pre">frozenset()</span></code></a> constructor. As a frozenset is immutable and <a class="reference internal" href="../glossary.html#term-hashable"><span class="xref std std-term">hashable</span></a>, it can be used again as an element of another set, or as a dictionary key.</p> </dd> </dl> </dd> <dt>Mappings</dt> <dd><p class="first" id="index-29">These represent finite sets of objects indexed by arbitrary index sets. The subscript notation <code class="docutils literal notranslate"><span class="pre">a[k]</span></code> selects the item indexed by <code class="docutils literal notranslate"><span class="pre">k</span></code> from the mapping <code class="docutils literal notranslate"><span class="pre">a</span></code>; this can be used in expressions and as the target of assignments or <a class="reference internal" href="simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> statements. The built-in function <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a> returns the number of items in a mapping.</p> <p>There is currently a single intrinsic mapping type:</p> <dl class="last docutils"> <dt>Dictionaries</dt> <dd><p class="first" id="index-30">These represent finite sets of objects indexed by nearly arbitrary values. The only types of values not acceptable as keys are values containing lists or dictionaries or other mutable types that are compared by value rather than by object identity, the reason being that the efficient implementation of dictionaries requires a key’s hash value to remain constant. Numeric types used for keys obey the normal rules for numeric comparison: if two numbers compare equal (e.g., <code class="docutils literal notranslate"><span class="pre">1</span></code> and <code class="docutils literal notranslate"><span class="pre">1.0</span></code>) then they can be used interchangeably to index the same dictionary entry.</p> <p>Dictionaries are mutable; they can be created by the <code class="docutils literal notranslate"><span class="pre">{...}</span></code> notation (see section <a class="reference internal" href="expressions.html#dict"><span class="std std-ref">Dictionary displays</span></a>).</p> <p class="last" id="index-31">The extension modules <a class="reference internal" href="../library/dbm.html#module-dbm.ndbm" title="dbm.ndbm: The standard "database" interface, based on ndbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.ndbm</span></code></a> and <a class="reference internal" href="../library/dbm.html#module-dbm.gnu" title="dbm.gnu: GNU's reinterpretation of dbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.gnu</span></code></a> provide additional examples of mapping types, as does the <a class="reference internal" href="../library/collections.html#module-collections" title="collections: Container datatypes"><code class="xref py py-mod docutils literal notranslate"><span class="pre">collections</span></code></a> module.</p> </dd> </dl> </dd> <dt>Callable types</dt> <dd><p class="first" id="index-32">These are the types to which the function call operation (see section <a class="reference internal" href="expressions.html#calls"><span class="std std-ref">Calls</span></a>) can be applied:</p> <dl class="last docutils"> <dt>User-defined functions</dt> <dd><p class="first" id="index-33">A user-defined function object is created by a function definition (see section <a class="reference internal" href="compound_stmts.html#function"><span class="std std-ref">Function definitions</span></a>). It should be called with an argument list containing the same number of items as the function’s formal parameter list.</p> <p>Special attributes:</p> <table border="1" class="docutils" id="index-34"> <colgroup> <col width="37%" /> <col width="46%" /> <col width="16%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Attribute</th> <th class="head">Meaning</th> <th class="head"> </th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><code class="xref py py-attr docutils literal notranslate"><span class="pre">__doc__</span></code></td> <td>The function’s documentation string, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if unavailable; not inherited by subclasses</td> <td>Writable</td> </tr> <tr class="row-odd"><td><a class="reference internal" href="../library/stdtypes.html#definition.__name__" title="definition.__name__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__name__</span></code></a></td> <td>The function’s name</td> <td>Writable</td> </tr> <tr class="row-even"><td><a class="reference internal" href="../library/stdtypes.html#definition.__qualname__" title="definition.__qualname__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__qualname__</span></code></a></td> <td><p class="first">The function’s <a class="reference internal" href="../glossary.html#term-qualified-name"><span class="xref std std-term">qualified name</span></a></p> <div class="last versionadded"> <p><span class="versionmodified">New in version 3.3.</span></p> </div> </td> <td>Writable</td> </tr> <tr class="row-odd"><td><code class="xref py py-attr docutils literal notranslate"><span class="pre">__module__</span></code></td> <td>The name of the module the function was defined in, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if unavailable.</td> <td>Writable</td> </tr> <tr class="row-even"><td><code class="xref py py-attr docutils literal notranslate"><span class="pre">__defaults__</span></code></td> <td>A tuple containing default argument values for those arguments that have defaults, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if no arguments have a default value</td> <td>Writable</td> </tr> <tr class="row-odd"><td><code class="xref py py-attr docutils literal notranslate"><span class="pre">__code__</span></code></td> <td>The code object representing the compiled function body.</td> <td>Writable</td> </tr> <tr class="row-even"><td><code class="xref py py-attr docutils literal notranslate"><span class="pre">__globals__</span></code></td> <td>A reference to the dictionary that holds the function’s global variables — the global namespace of the module in which the function was defined.</td> <td>Read-only</td> </tr> <tr class="row-odd"><td><a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a></td> <td>The namespace supporting arbitrary function attributes.</td> <td>Writable</td> </tr> <tr class="row-even"><td><code class="xref py py-attr docutils literal notranslate"><span class="pre">__closure__</span></code></td> <td><code class="docutils literal notranslate"><span class="pre">None</span></code> or a tuple of cells that contain bindings for the function’s free variables.</td> <td>Read-only</td> </tr> <tr class="row-odd"><td><code class="xref py py-attr docutils literal notranslate"><span class="pre">__annotations__</span></code></td> <td>A dict containing annotations of parameters. The keys of the dict are the parameter names, and <code class="docutils literal notranslate"><span class="pre">'return'</span></code> for the return annotation, if provided.</td> <td>Writable</td> </tr> <tr class="row-even"><td><code class="xref py py-attr docutils literal notranslate"><span class="pre">__kwdefaults__</span></code></td> <td>A dict containing defaults for keyword-only parameters.</td> <td>Writable</td> </tr> </tbody> </table> <p>Most of the attributes labelled “Writable” check the type of the assigned value.</p> <p>Function objects also support getting and setting arbitrary attributes, which can be used, for example, to attach metadata to functions. Regular attribute dot-notation is used to get and set such attributes. <em>Note that the current implementation only supports function attributes on user-defined functions. Function attributes on built-in functions may be supported in the future.</em></p> <p class="last">Additional information about a function’s definition can be retrieved from its code object; see the description of internal types below.</p> </dd> <dt>Instance methods</dt> <dd><p class="first" id="index-35">An instance method object combines a class, a class instance and any callable object (normally a user-defined function).</p> <p id="index-36">Special read-only attributes: <code class="xref py py-attr docutils literal notranslate"><span class="pre">__self__</span></code> is the class instance object, <code class="xref py py-attr docutils literal notranslate"><span class="pre">__func__</span></code> is the function object; <code class="xref py py-attr docutils literal notranslate"><span class="pre">__doc__</span></code> is the method’s documentation (same as <code class="docutils literal notranslate"><span class="pre">__func__.__doc__</span></code>); <a class="reference internal" href="../library/stdtypes.html#definition.__name__" title="definition.__name__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__name__</span></code></a> is the method name (same as <code class="docutils literal notranslate"><span class="pre">__func__.__name__</span></code>); <code class="xref py py-attr docutils literal notranslate"><span class="pre">__module__</span></code> is the name of the module the method was defined in, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if unavailable.</p> <p>Methods also support accessing (but not setting) the arbitrary function attributes on the underlying function object.</p> <p>User-defined method objects may be created when getting an attribute of a class (perhaps via an instance of that class), if that attribute is a user-defined function object or a class method object.</p> <p>When an instance method object is created by retrieving a user-defined function object from a class via one of its instances, its <code class="xref py py-attr docutils literal notranslate"><span class="pre">__self__</span></code> attribute is the instance, and the method object is said to be bound. The new method’s <code class="xref py py-attr docutils literal notranslate"><span class="pre">__func__</span></code> attribute is the original function object.</p> <p>When a user-defined method object is created by retrieving another method object from a class or instance, the behaviour is the same as for a function object, except that the <code class="xref py py-attr docutils literal notranslate"><span class="pre">__func__</span></code> attribute of the new instance is not the original method object but its <code class="xref py py-attr docutils literal notranslate"><span class="pre">__func__</span></code> attribute.</p> <p>When an instance method object is created by retrieving a class method object from a class or instance, its <code class="xref py py-attr docutils literal notranslate"><span class="pre">__self__</span></code> attribute is the class itself, and its <code class="xref py py-attr docutils literal notranslate"><span class="pre">__func__</span></code> attribute is the function object underlying the class method.</p> <p>When an instance method object is called, the underlying function (<code class="xref py py-attr docutils literal notranslate"><span class="pre">__func__</span></code>) is called, inserting the class instance (<code class="xref py py-attr docutils literal notranslate"><span class="pre">__self__</span></code>) in front of the argument list. For instance, when <code class="xref py py-class docutils literal notranslate"><span class="pre">C</span></code> is a class which contains a definition for a function <code class="xref py py-meth docutils literal notranslate"><span class="pre">f()</span></code>, and <code class="docutils literal notranslate"><span class="pre">x</span></code> is an instance of <code class="xref py py-class docutils literal notranslate"><span class="pre">C</span></code>, calling <code class="docutils literal notranslate"><span class="pre">x.f(1)</span></code> is equivalent to calling <code class="docutils literal notranslate"><span class="pre">C.f(x,</span> <span class="pre">1)</span></code>.</p> <p>When an instance method object is derived from a class method object, the “class instance” stored in <code class="xref py py-attr docutils literal notranslate"><span class="pre">__self__</span></code> will actually be the class itself, so that calling either <code class="docutils literal notranslate"><span class="pre">x.f(1)</span></code> or <code class="docutils literal notranslate"><span class="pre">C.f(1)</span></code> is equivalent to calling <code class="docutils literal notranslate"><span class="pre">f(C,1)</span></code> where <code class="docutils literal notranslate"><span class="pre">f</span></code> is the underlying function.</p> <p class="last">Note that the transformation from function object to instance method object happens each time the attribute is retrieved from the instance. In some cases, a fruitful optimization is to assign the attribute to a local variable and call that local variable. Also notice that this transformation only happens for user-defined functions; other callable objects (and all non-callable objects) are retrieved without transformation. It is also important to note that user-defined functions which are attributes of a class instance are not converted to bound methods; this <em>only</em> happens when the function is an attribute of the class.</p> </dd> <dt>Generator functions</dt> <dd><p class="first last" id="index-37">A function or method which uses the <a class="reference internal" href="simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement (see section <a class="reference internal" href="simple_stmts.html#yield"><span class="std std-ref">The yield statement</span></a>) is called a <em class="dfn">generator function</em>. Such a function, when called, always returns an iterator object which can be used to execute the body of the function: calling the iterator’s <a class="reference internal" href="../library/stdtypes.html#iterator.__next__" title="iterator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">iterator.__next__()</span></code></a> method will cause the function to execute until it provides a value using the <a class="reference internal" href="simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement. When the function executes a <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement or falls off the end, a <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a> exception is raised and the iterator will have reached the end of the set of values to be returned.</p> </dd> <dt>Coroutine functions</dt> <dd><p class="first last" id="index-38">A function or method which is defined using <a class="reference internal" href="compound_stmts.html#async-def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">def</span></code></a> is called a <em class="dfn">coroutine function</em>. Such a function, when called, returns a <a class="reference internal" href="../glossary.html#term-coroutine"><span class="xref std std-term">coroutine</span></a> object. It may contain <a class="reference internal" href="expressions.html#await"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">await</span></code></a> expressions, as well as <a class="reference internal" href="compound_stmts.html#async-with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">with</span></code></a> and <a class="reference internal" href="compound_stmts.html#async-for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code></a> statements. See also the <a class="reference internal" href="#coroutine-objects"><span class="std std-ref">Coroutine Objects</span></a> section.</p> </dd> <dt>Asynchronous generator functions</dt> <dd><p class="first" id="index-39">A function or method which is defined using <a class="reference internal" href="compound_stmts.html#async-def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">def</span></code></a> and which uses the <a class="reference internal" href="simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement is called a <em class="dfn">asynchronous generator function</em>. Such a function, when called, returns an asynchronous iterator object which can be used in an <a class="reference internal" href="compound_stmts.html#async-for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code></a> statement to execute the body of the function.</p> <p class="last">Calling the asynchronous iterator’s <code class="xref py py-meth docutils literal notranslate"><span class="pre">aiterator.__anext__()</span></code> method will return an <a class="reference internal" href="../glossary.html#term-awaitable"><span class="xref std std-term">awaitable</span></a> which when awaited will execute until it provides a value using the <a class="reference internal" href="simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> expression. When the function executes an empty <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement or falls off the end, a <a class="reference internal" href="../library/exceptions.html#StopAsyncIteration" title="StopAsyncIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopAsyncIteration</span></code></a> exception is raised and the asynchronous iterator will have reached the end of the set of values to be yielded.</p> </dd> <dt>Built-in functions</dt> <dd><p class="first last" id="index-40">A built-in function object is a wrapper around a C function. Examples of built-in functions are <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a> and <a class="reference internal" href="../library/math.html#math.sin" title="math.sin"><code class="xref py py-func docutils literal notranslate"><span class="pre">math.sin()</span></code></a> (<a class="reference internal" href="../library/math.html#module-math" title="math: Mathematical functions (sin() etc.)."><code class="xref py py-mod docutils literal notranslate"><span class="pre">math</span></code></a> is a standard built-in module). The number and type of the arguments are determined by the C function. Special read-only attributes: <code class="xref py py-attr docutils literal notranslate"><span class="pre">__doc__</span></code> is the function’s documentation string, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if unavailable; <a class="reference internal" href="../library/stdtypes.html#definition.__name__" title="definition.__name__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__name__</span></code></a> is the function’s name; <code class="xref py py-attr docutils literal notranslate"><span class="pre">__self__</span></code> is set to <code class="docutils literal notranslate"><span class="pre">None</span></code> (but see the next item); <code class="xref py py-attr docutils literal notranslate"><span class="pre">__module__</span></code> is the name of the module the function was defined in or <code class="docutils literal notranslate"><span class="pre">None</span></code> if unavailable.</p> </dd> <dt>Built-in methods</dt> <dd><p class="first last" id="index-41">This is really a different disguise of a built-in function, this time containing an object passed to the C function as an implicit extra argument. An example of a built-in method is <code class="docutils literal notranslate"><span class="pre">alist.append()</span></code>, assuming <em>alist</em> is a list object. In this case, the special read-only attribute <code class="xref py py-attr docutils literal notranslate"><span class="pre">__self__</span></code> is set to the object denoted by <em>alist</em>.</p> </dd> <dt>Classes</dt> <dd>Classes are callable. These objects normally act as factories for new instances of themselves, but variations are possible for class types that override <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a>. The arguments of the call are passed to <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> and, in the typical case, to <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> to initialize the new instance.</dd> <dt>Class Instances</dt> <dd>Instances of arbitrary classes can be made callable by defining a <a class="reference internal" href="#object.__call__" title="object.__call__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__call__()</span></code></a> method in their class.</dd> </dl> </dd> <dt>Modules</dt> <dd><p class="first" id="index-42">Modules are a basic organizational unit of Python code, and are created by the <a class="reference internal" href="import.html#importsystem"><span class="std std-ref">import system</span></a> as invoked either by the <a class="reference internal" href="simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> statement (see <a class="reference internal" href="simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a>), or by calling functions such as <a class="reference internal" href="../library/importlib.html#importlib.import_module" title="importlib.import_module"><code class="xref py py-func docutils literal notranslate"><span class="pre">importlib.import_module()</span></code></a> and built-in <a class="reference internal" href="../library/functions.html#__import__" title="__import__"><code class="xref py py-func docutils literal notranslate"><span class="pre">__import__()</span></code></a>. A module object has a namespace implemented by a dictionary object (this is the dictionary referenced by the <code class="docutils literal notranslate"><span class="pre">__globals__</span></code> attribute of functions defined in the module). Attribute references are translated to lookups in this dictionary, e.g., <code class="docutils literal notranslate"><span class="pre">m.x</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">m.__dict__["x"]</span></code>. A module object does not contain the code object used to initialize the module (since it isn’t needed once the initialization is done).</p> <p>Attribute assignment updates the module’s namespace dictionary, e.g., <code class="docutils literal notranslate"><span class="pre">m.x</span> <span class="pre">=</span> <span class="pre">1</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">m.__dict__["x"]</span> <span class="pre">=</span> <span class="pre">1</span></code>.</p> <p id="index-43">Predefined (writable) attributes: <a class="reference internal" href="import.html#__name__" title="__name__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__name__</span></code></a> is the module’s name; <code class="xref py py-attr docutils literal notranslate"><span class="pre">__doc__</span></code> is the module’s documentation string, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if unavailable; <code class="xref py py-attr docutils literal notranslate"><span class="pre">__annotations__</span></code> (optional) is a dictionary containing <a class="reference internal" href="../glossary.html#term-variable-annotation"><span class="xref std std-term">variable annotations</span></a> collected during module body execution; <a class="reference internal" href="import.html#__file__" title="__file__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__file__</span></code></a> is the pathname of the file from which the module was loaded, if it was loaded from a file. The <a class="reference internal" href="import.html#__file__" title="__file__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__file__</span></code></a> attribute may be missing for certain types of modules, such as C modules that are statically linked into the interpreter; for extension modules loaded dynamically from a shared library, it is the pathname of the shared library file.</p> <p id="index-44">Special read-only attribute: <a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a> is the module’s namespace as a dictionary object.</p> <div class="impl-detail last compound"> <p><strong>CPython implementation detail:</strong> Because of the way CPython clears module dictionaries, the module dictionary will be cleared when the module falls out of scope even if the dictionary still has live references. To avoid this, copy the dictionary or keep the module around while using its dictionary directly.</p> </div> </dd> <dt>Custom classes</dt> <dd><p class="first">Custom class types are typically created by class definitions (see section <a class="reference internal" href="compound_stmts.html#class"><span class="std std-ref">Class definitions</span></a>). A class has a namespace implemented by a dictionary object. Class attribute references are translated to lookups in this dictionary, e.g., <code class="docutils literal notranslate"><span class="pre">C.x</span></code> is translated to <code class="docutils literal notranslate"><span class="pre">C.__dict__["x"]</span></code> (although there are a number of hooks which allow for other means of locating attributes). When the attribute name is not found there, the attribute search continues in the base classes. This search of the base classes uses the C3 method resolution order which behaves correctly even in the presence of ‘diamond’ inheritance structures where there are multiple inheritance paths leading back to a common ancestor. Additional details on the C3 MRO used by Python can be found in the documentation accompanying the 2.3 release at <a class="reference external" href="https://www.python.org/download/releases/2.3/mro/">https://www.python.org/download/releases/2.3/mro/</a>.</p> <p id="index-45">When a class attribute reference (for class <code class="xref py py-class docutils literal notranslate"><span class="pre">C</span></code>, say) would yield a class method object, it is transformed into an instance method object whose <code class="xref py py-attr docutils literal notranslate"><span class="pre">__self__</span></code> attribute is <code class="xref py py-class docutils literal notranslate"><span class="pre">C</span></code>. When it would yield a static method object, it is transformed into the object wrapped by the static method object. See section <a class="reference internal" href="#descriptors"><span class="std std-ref">Implementing Descriptors</span></a> for another way in which attributes retrieved from a class may differ from those actually contained in its <a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a>.</p> <p id="index-46">Class attribute assignments update the class’s dictionary, never the dictionary of a base class.</p> <p id="index-47">A class object can be called (see above) to yield a class instance (see below).</p> <p class="last" id="index-48">Special attributes: <a class="reference internal" href="../library/stdtypes.html#definition.__name__" title="definition.__name__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__name__</span></code></a> is the class name; <code class="xref py py-attr docutils literal notranslate"><span class="pre">__module__</span></code> is the module name in which the class was defined; <a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a> is the dictionary containing the class’s namespace; <a class="reference internal" href="../library/stdtypes.html#class.__bases__" title="class.__bases__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__bases__</span></code></a> is a tuple containing the base classes, in the order of their occurrence in the base class list; <code class="xref py py-attr docutils literal notranslate"><span class="pre">__doc__</span></code> is the class’s documentation string, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if undefined; <code class="xref py py-attr docutils literal notranslate"><span class="pre">__annotations__</span></code> (optional) is a dictionary containing <a class="reference internal" href="../glossary.html#term-variable-annotation"><span class="xref std std-term">variable annotations</span></a> collected during class body execution.</p> </dd> <dt>Class instances</dt> <dd><p class="first" id="index-49">A class instance is created by calling a class object (see above). A class instance has a namespace implemented as a dictionary which is the first place in which attribute references are searched. When an attribute is not found there, and the instance’s class has an attribute by that name, the search continues with the class attributes. If a class attribute is found that is a user-defined function object, it is transformed into an instance method object whose <code class="xref py py-attr docutils literal notranslate"><span class="pre">__self__</span></code> attribute is the instance. Static method and class method objects are also transformed; see above under “Classes”. See section <a class="reference internal" href="#descriptors"><span class="std std-ref">Implementing Descriptors</span></a> for another way in which attributes of a class retrieved via its instances may differ from the objects actually stored in the class’s <a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a>. If no class attribute is found, and the object’s class has a <a class="reference internal" href="#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattr__()</span></code></a> method, that is called to satisfy the lookup.</p> <p id="index-50">Attribute assignments and deletions update the instance’s dictionary, never a class’s dictionary. If the class has a <a class="reference internal" href="#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a> or <a class="reference internal" href="#object.__delattr__" title="object.__delattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delattr__()</span></code></a> method, this is called instead of updating the instance dictionary directly.</p> <p id="index-51">Class instances can pretend to be numbers, sequences, or mappings if they have methods with certain special names. See section <a class="reference internal" href="#specialnames"><span class="std std-ref">Special method names</span></a>.</p> <p class="last" id="index-52">Special attributes: <a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a> is the attribute dictionary; <a class="reference internal" href="../library/stdtypes.html#instance.__class__" title="instance.__class__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__class__</span></code></a> is the instance’s class.</p> </dd> <dt>I/O objects (also known as file objects)</dt> <dd><p class="first" id="index-53">A <a class="reference internal" href="../glossary.html#term-file-object"><span class="xref std std-term">file object</span></a> represents an open file. Various shortcuts are available to create file objects: the <a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a> built-in function, and also <a class="reference internal" href="../library/os.html#os.popen" title="os.popen"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.popen()</span></code></a>, <a class="reference internal" href="../library/os.html#os.fdopen" title="os.fdopen"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.fdopen()</span></code></a>, and the <a class="reference internal" href="../library/socket.html#socket.socket.makefile" title="socket.socket.makefile"><code class="xref py py-meth docutils literal notranslate"><span class="pre">makefile()</span></code></a> method of socket objects (and perhaps by other functions or methods provided by extension modules).</p> <p class="last">The objects <code class="docutils literal notranslate"><span class="pre">sys.stdin</span></code>, <code class="docutils literal notranslate"><span class="pre">sys.stdout</span></code> and <code class="docutils literal notranslate"><span class="pre">sys.stderr</span></code> are initialized to file objects corresponding to the interpreter’s standard input, output and error streams; they are all open in text mode and therefore follow the interface defined by the <a class="reference internal" href="../library/io.html#io.TextIOBase" title="io.TextIOBase"><code class="xref py py-class docutils literal notranslate"><span class="pre">io.TextIOBase</span></code></a> abstract class.</p> </dd> <dt>Internal types</dt> <dd><p class="first" id="index-54">A few types used internally by the interpreter are exposed to the user. Their definitions may change with future versions of the interpreter, but they are mentioned here for completeness.</p> <dl class="docutils" id="index-55"> <dt>Code objects</dt> <dd><p class="first">Code objects represent <em>byte-compiled</em> executable Python code, or <a class="reference internal" href="../glossary.html#term-bytecode"><span class="xref std std-term">bytecode</span></a>. The difference between a code object and a function object is that the function object contains an explicit reference to the function’s globals (the module in which it was defined), while a code object contains no context; also the default argument values are stored in the function object, not in the code object (because they represent values calculated at run-time). Unlike function objects, code objects are immutable and contain no references (directly or indirectly) to mutable objects.</p> <p id="index-56">Special read-only attributes: <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_name</span></code> gives the function name; <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_argcount</span></code> is the number of positional arguments (including arguments with default values); <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_nlocals</span></code> is the number of local variables used by the function (including arguments); <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_varnames</span></code> is a tuple containing the names of the local variables (starting with the argument names); <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_cellvars</span></code> is a tuple containing the names of local variables that are referenced by nested functions; <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_freevars</span></code> is a tuple containing the names of free variables; <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_code</span></code> is a string representing the sequence of bytecode instructions; <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_consts</span></code> is a tuple containing the literals used by the bytecode; <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_names</span></code> is a tuple containing the names used by the bytecode; <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_filename</span></code> is the filename from which the code was compiled; <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_firstlineno</span></code> is the first line number of the function; <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_lnotab</span></code> is a string encoding the mapping from bytecode offsets to line numbers (for details see the source code of the interpreter); <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_stacksize</span></code> is the required stack size (including local variables); <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_flags</span></code> is an integer encoding a number of flags for the interpreter.</p> <p id="index-57">The following flag bits are defined for <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_flags</span></code>: bit <code class="docutils literal notranslate"><span class="pre">0x04</span></code> is set if the function uses the <code class="docutils literal notranslate"><span class="pre">*arguments</span></code> syntax to accept an arbitrary number of positional arguments; bit <code class="docutils literal notranslate"><span class="pre">0x08</span></code> is set if the function uses the <code class="docutils literal notranslate"><span class="pre">**keywords</span></code> syntax to accept arbitrary keyword arguments; bit <code class="docutils literal notranslate"><span class="pre">0x20</span></code> is set if the function is a generator.</p> <p>Future feature declarations (<code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">__future__</span> <span class="pre">import</span> <span class="pre">division</span></code>) also use bits in <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_flags</span></code> to indicate whether a code object was compiled with a particular feature enabled: bit <code class="docutils literal notranslate"><span class="pre">0x2000</span></code> is set if the function was compiled with future division enabled; bits <code class="docutils literal notranslate"><span class="pre">0x10</span></code> and <code class="docutils literal notranslate"><span class="pre">0x1000</span></code> were used in earlier versions of Python.</p> <p>Other bits in <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_flags</span></code> are reserved for internal use.</p> <p class="last" id="index-58">If a code object represents a function, the first item in <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_consts</span></code> is the documentation string of the function, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if undefined.</p> </dd> </dl> <dl class="last docutils" id="frame-objects"> <dt>Frame objects</dt> <dd><p class="first" id="index-59">Frame objects represent execution frames. They may occur in traceback objects (see below).</p> <p id="index-60">Special read-only attributes: <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_back</span></code> is to the previous stack frame (towards the caller), or <code class="docutils literal notranslate"><span class="pre">None</span></code> if this is the bottom stack frame; <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_code</span></code> is the code object being executed in this frame; <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_locals</span></code> is the dictionary used to look up local variables; <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_globals</span></code> is used for global variables; <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_builtins</span></code> is used for built-in (intrinsic) names; <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_lasti</span></code> gives the precise instruction (this is an index into the bytecode string of the code object).</p> <p id="index-61">Special writable attributes: <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_trace</span></code>, if not <code class="docutils literal notranslate"><span class="pre">None</span></code>, is a function called at the start of each source code line (this is used by the debugger); <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_lineno</span></code> is the current line number of the frame — writing to this from within a trace function jumps to the given line (only for the bottom-most frame). A debugger can implement a Jump command (aka Set Next Statement) by writing to f_lineno.</p> <p>Frame objects support one method:</p> <dl class="last method"> <dt id="frame.clear"> <code class="descclassname">frame.</code><code class="descname">clear</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#frame.clear" title="Permalink to this definition">¶</a></dt> <dd><p>This method clears all references to local variables held by the frame. Also, if the frame belonged to a generator, the generator is finalized. This helps break reference cycles involving frame objects (for example when catching an exception and storing its traceback for later use).</p> <p><a class="reference internal" href="../library/exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeError</span></code></a> is raised if the frame is currently executing.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 3.4.</span></p> </div> </dd></dl> </dd> <dt>Traceback objects</dt> <dd><p class="first" id="index-62">Traceback objects represent a stack trace of an exception. A traceback object is created when an exception occurs. When the search for an exception handler unwinds the execution stack, at each unwound level a traceback object is inserted in front of the current traceback. When an exception handler is entered, the stack trace is made available to the program. (See section <a class="reference internal" href="compound_stmts.html#try"><span class="std std-ref">The try statement</span></a>.) It is accessible as the third item of the tuple returned by <code class="docutils literal notranslate"><span class="pre">sys.exc_info()</span></code>. When the program contains no suitable handler, the stack trace is written (nicely formatted) to the standard error stream; if the interpreter is interactive, it is also made available to the user as <code class="docutils literal notranslate"><span class="pre">sys.last_traceback</span></code>.</p> <p class="last" id="index-63">Special read-only attributes: <code class="xref py py-attr docutils literal notranslate"><span class="pre">tb_next</span></code> is the next level in the stack trace (towards the frame where the exception occurred), or <code class="docutils literal notranslate"><span class="pre">None</span></code> if there is no next level; <code class="xref py py-attr docutils literal notranslate"><span class="pre">tb_frame</span></code> points to the execution frame of the current level; <code class="xref py py-attr docutils literal notranslate"><span class="pre">tb_lineno</span></code> gives the line number where the exception occurred; <code class="xref py py-attr docutils literal notranslate"><span class="pre">tb_lasti</span></code> indicates the precise instruction. The line number and last instruction in the traceback may differ from the line number of its frame object if the exception occurred in a <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement with no matching except clause or with a finally clause.</p> </dd> <dt>Slice objects</dt> <dd><p class="first" id="index-64">Slice objects are used to represent slices for <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> methods. They are also created by the built-in <a class="reference internal" href="../library/functions.html#slice" title="slice"><code class="xref py py-func docutils literal notranslate"><span class="pre">slice()</span></code></a> function.</p> <p id="index-65">Special read-only attributes: <code class="xref py py-attr docutils literal notranslate"><span class="pre">start</span></code> is the lower bound; <code class="xref py py-attr docutils literal notranslate"><span class="pre">stop</span></code> is the upper bound; <code class="xref py py-attr docutils literal notranslate"><span class="pre">step</span></code> is the step value; each is <code class="docutils literal notranslate"><span class="pre">None</span></code> if omitted. These attributes can have any type.</p> <p>Slice objects support one method:</p> <dl class="last method"> <dt id="slice.indices"> <code class="descclassname">slice.</code><code class="descname">indices</code><span class="sig-paren">(</span><em>self</em>, <em>length</em><span class="sig-paren">)</span><a class="headerlink" href="#slice.indices" title="Permalink to this definition">¶</a></dt> <dd><p>This method takes a single integer argument <em>length</em> and computes information about the slice that the slice object would describe if applied to a sequence of <em>length</em> items. It returns a tuple of three integers; respectively these are the <em>start</em> and <em>stop</em> indices and the <em>step</em> or stride length of the slice. Missing or out-of-bounds indices are handled in a manner consistent with regular slices.</p> </dd></dl> </dd> <dt>Static method objects</dt> <dd>Static method objects provide a way of defeating the transformation of function objects to method objects described above. A static method object is a wrapper around any other object, usually a user-defined method object. When a static method object is retrieved from a class or a class instance, the object actually returned is the wrapped object, which is not subject to any further transformation. Static method objects are not themselves callable, although the objects they wrap usually are. Static method objects are created by the built-in <a class="reference internal" href="../library/functions.html#staticmethod" title="staticmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">staticmethod()</span></code></a> constructor.</dd> <dt>Class method objects</dt> <dd>A class method object, like a static method object, is a wrapper around another object that alters the way in which that object is retrieved from classes and class instances. The behaviour of class method objects upon such retrieval is described above, under “User-defined methods”. Class method objects are created by the built-in <a class="reference internal" href="../library/functions.html#classmethod" title="classmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">classmethod()</span></code></a> constructor.</dd> </dl> </dd> </dl> </div> <div class="section" id="special-method-names"> <span id="specialnames"></span><h2>3.3. Special method names<a class="headerlink" href="#special-method-names" title="Permalink to this headline">¶</a></h2> <p id="index-66">A class can implement certain operations that are invoked by special syntax (such as arithmetic operations or subscripting and slicing) by defining methods with special names. This is Python’s approach to <em class="dfn">operator overloading</em>, allowing classes to define their own behavior with respect to language operators. For instance, if a class defines a method named <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>, and <code class="docutils literal notranslate"><span class="pre">x</span></code> is an instance of this class, then <code class="docutils literal notranslate"><span class="pre">x[i]</span></code> is roughly equivalent to <code class="docutils literal notranslate"><span class="pre">type(x).__getitem__(x,</span> <span class="pre">i)</span></code>. Except where mentioned, attempts to execute an operation raise an exception when no appropriate method is defined (typically <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> or <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>).</p> <p>Setting a special method to <code class="docutils literal notranslate"><span class="pre">None</span></code> indicates that the corresponding operation is not available. For example, if a class sets <a class="reference internal" href="#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a> to <code class="docutils literal notranslate"><span class="pre">None</span></code>, the class is not iterable, so calling <a class="reference internal" href="../library/functions.html#iter" title="iter"><code class="xref py py-func docutils literal notranslate"><span class="pre">iter()</span></code></a> on its instances will raise a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> (without falling back to <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>). <a class="footnote-reference" href="#id9" id="id2">[2]</a></p> <p>When implementing a class that emulates any built-in type, it is important that the emulation only be implemented to the degree that it makes sense for the object being modelled. For example, some sequences may work well with retrieval of individual elements, but extracting a slice may not make sense. (One example of this is the <code class="xref py py-class docutils literal notranslate"><span class="pre">NodeList</span></code> interface in the W3C’s Document Object Model.)</p> <div class="section" id="basic-customization"> <span id="customization"></span><h3>3.3.1. Basic customization<a class="headerlink" href="#basic-customization" title="Permalink to this headline">¶</a></h3> <dl class="method"> <dt id="object.__new__"> <code class="descclassname">object.</code><code class="descname">__new__</code><span class="sig-paren">(</span><em>cls</em><span class="optional">[</span>, <em>...</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#object.__new__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-67">Called to create a new instance of class <em>cls</em>. <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> is a static method (special-cased so you need not declare it as such) that takes the class of which an instance was requested as its first argument. The remaining arguments are those passed to the object constructor expression (the call to the class). The return value of <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> should be the new object instance (usually an instance of <em>cls</em>).</p> <p>Typical implementations create a new instance of the class by invoking the superclass’s <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> method using <code class="docutils literal notranslate"><span class="pre">super().__new__(cls[,</span> <span class="pre">...])</span></code> with appropriate arguments and then modifying the newly-created instance as necessary before returning it.</p> <p>If <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> returns an instance of <em>cls</em>, then the new instance’s <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method will be invoked like <code class="docutils literal notranslate"><span class="pre">__init__(self[,</span> <span class="pre">...])</span></code>, where <em>self</em> is the new instance and the remaining arguments are the same as were passed to <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a>.</p> <p>If <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> does not return an instance of <em>cls</em>, then the new instance’s <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method will not be invoked.</p> <p><a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> is intended mainly to allow subclasses of immutable types (like int, str, or tuple) to customize instance creation. It is also commonly overridden in custom metaclasses in order to customize class creation.</p> </dd></dl> <dl class="method"> <dt id="object.__init__"> <code class="descclassname">object.</code><code class="descname">__init__</code><span class="sig-paren">(</span><em>self</em><span class="optional">[</span>, <em>...</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#object.__init__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-68">Called after the instance has been created (by <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a>), but before it is returned to the caller. The arguments are those passed to the class constructor expression. If a base class has an <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method, the derived class’s <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method, if any, must explicitly call it to ensure proper initialization of the base class part of the instance; for example: <code class="docutils literal notranslate"><span class="pre">super().__init__([args...])</span></code>.</p> <p>Because <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> and <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> work together in constructing objects (<a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> to create it, and <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> to customize it), no non-<code class="docutils literal notranslate"><span class="pre">None</span></code> value may be returned by <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a>; doing so will cause a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> to be raised at runtime.</p> </dd></dl> <dl class="method"> <dt id="object.__del__"> <code class="descclassname">object.</code><code class="descname">__del__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__del__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-69">Called when the instance is about to be destroyed. This is also called a finalizer or (improperly) a destructor. If a base class has a <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method, the derived class’s <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method, if any, must explicitly call it to ensure proper deletion of the base class part of the instance.</p> <p>It is possible (though not recommended!) for the <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method to postpone destruction of the instance by creating a new reference to it. This is called object <em>resurrection</em>. It is implementation-dependent whether <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> is called a second time when a resurrected object is about to be destroyed; the current <a class="reference internal" href="../glossary.html#term-cpython"><span class="xref std std-term">CPython</span></a> implementation only calls it once.</p> <p>It is not guaranteed that <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> methods are called for objects that still exist when the interpreter exits.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last"><code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">x</span></code> doesn’t directly call <code class="docutils literal notranslate"><span class="pre">x.__del__()</span></code> — the former decrements the reference count for <code class="docutils literal notranslate"><span class="pre">x</span></code> by one, and the latter is only called when <code class="docutils literal notranslate"><span class="pre">x</span></code>’s reference count reaches zero.</p> </div> <div class="impl-detail compound"> <p class="compound-first"><strong>CPython implementation detail:</strong> It is possible for a reference cycle to prevent the reference count of an object from going to zero. In this case, the cycle will be later detected and deleted by the <a class="reference internal" href="../glossary.html#term-garbage-collection"><span class="xref std std-term">cyclic garbage collector</span></a>. A common cause of reference cycles is when an exception has been caught in a local variable. The frame’s locals then reference the exception, which references its own traceback, which references the locals of all frames caught in the traceback.</p> <div class="compound-last admonition seealso"> <p class="first admonition-title">See also</p> <p class="last">Documentation for the <a class="reference internal" href="../library/gc.html#module-gc" title="gc: Interface to the cycle-detecting garbage collector."><code class="xref py py-mod docutils literal notranslate"><span class="pre">gc</span></code></a> module.</p> </div> </div> <div class="admonition warning"> <p class="first admonition-title">Warning</p> <p>Due to the precarious circumstances under which <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> methods are invoked, exceptions that occur during their execution are ignored, and a warning is printed to <code class="docutils literal notranslate"><span class="pre">sys.stderr</span></code> instead. In particular:</p> <ul class="last simple"> <li><a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> can be invoked when arbitrary code is being executed, including from any arbitrary thread. If <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> needs to take a lock or invoke any other blocking resource, it may deadlock as the resource may already be taken by the code that gets interrupted to execute <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a>.</li> <li><a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> can be executed during interpreter shutdown. As a consequence, the global variables it needs to access (including other modules) may already have been deleted or set to <code class="docutils literal notranslate"><span class="pre">None</span></code>. Python guarantees that globals whose name begins with a single underscore are deleted from their module before other globals are deleted; if no other references to such globals exist, this may help in assuring that imported modules are still available at the time when the <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method is called.</li> </ul> </div> <span class="target" id="index-70"></span></dd></dl> <dl class="method"> <dt id="object.__repr__"> <code class="descclassname">object.</code><code class="descname">__repr__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__repr__" title="Permalink to this definition">¶</a></dt> <dd><p>Called by the <a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal notranslate"><span class="pre">repr()</span></code></a> built-in function to compute the “official” string representation of an object. If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment). If this is not possible, a string of the form <code class="docutils literal notranslate"><span class="pre"><...some</span> <span class="pre">useful</span> <span class="pre">description...></span></code> should be returned. The return value must be a string object. If a class defines <a class="reference internal" href="#object.__repr__" title="object.__repr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__repr__()</span></code></a> but not <a class="reference internal" href="#object.__str__" title="object.__str__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__str__()</span></code></a>, then <a class="reference internal" href="#object.__repr__" title="object.__repr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__repr__()</span></code></a> is also used when an “informal” string representation of instances of that class is required.</p> <p>This is typically used for debugging, so it is important that the representation is information-rich and unambiguous.</p> <span class="target" id="index-71"></span></dd></dl> <dl class="method"> <dt id="object.__str__"> <code class="descclassname">object.</code><code class="descname">__str__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__str__" title="Permalink to this definition">¶</a></dt> <dd><p>Called by <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-func docutils literal notranslate"><span class="pre">str(object)</span></code></a> and the built-in functions <a class="reference internal" href="../library/functions.html#format" title="format"><code class="xref py py-func docutils literal notranslate"><span class="pre">format()</span></code></a> and <a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate"><span class="pre">print()</span></code></a> to compute the “informal” or nicely printable string representation of an object. The return value must be a <a class="reference internal" href="../library/stdtypes.html#textseq"><span class="std std-ref">string</span></a> object.</p> <p>This method differs from <a class="reference internal" href="#object.__repr__" title="object.__repr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__repr__()</span></code></a> in that there is no expectation that <a class="reference internal" href="#object.__str__" title="object.__str__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__str__()</span></code></a> return a valid Python expression: a more convenient or concise representation can be used.</p> <p>The default implementation defined by the built-in type <a class="reference internal" href="../library/functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate"><span class="pre">object</span></code></a> calls <a class="reference internal" href="#object.__repr__" title="object.__repr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__repr__()</span></code></a>.</p> </dd></dl> <dl class="method"> <dt id="object.__bytes__"> <code class="descclassname">object.</code><code class="descname">__bytes__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__bytes__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-72">Called by <a class="reference internal" href="../library/functions.html#func-bytes"><span class="std std-ref">bytes</span></a> to compute a byte-string representation of an object. This should return a <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> object.</p> <span class="target" id="index-73"></span></dd></dl> <dl class="method"> <dt id="object.__format__"> <code class="descclassname">object.</code><code class="descname">__format__</code><span class="sig-paren">(</span><em>self</em>, <em>format_spec</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__format__" title="Permalink to this definition">¶</a></dt> <dd><p>Called by the <a class="reference internal" href="../library/functions.html#format" title="format"><code class="xref py py-func docutils literal notranslate"><span class="pre">format()</span></code></a> built-in function, and by extension, evaluation of <a class="reference internal" href="lexical_analysis.html#f-strings"><span class="std std-ref">formatted string literals</span></a> and the <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal notranslate"><span class="pre">str.format()</span></code></a> method, to produce a “formatted” string representation of an object. The <code class="docutils literal notranslate"><span class="pre">format_spec</span></code> argument is a string that contains a description of the formatting options desired. The interpretation of the <code class="docutils literal notranslate"><span class="pre">format_spec</span></code> argument is up to the type implementing <a class="reference internal" href="#object.__format__" title="object.__format__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__format__()</span></code></a>, however most classes will either delegate formatting to one of the built-in types, or use a similar formatting option syntax.</p> <p>See <a class="reference internal" href="../library/string.html#formatspec"><span class="std std-ref">Format Specification Mini-Language</span></a> for a description of the standard formatting syntax.</p> <p>The return value must be a string object.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.4: </span>The __format__ method of <code class="docutils literal notranslate"><span class="pre">object</span></code> itself raises a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> if passed any non-empty string.</p> </div> </dd></dl> <span class="target" id="richcmpfuncs"></span><dl class="method"> <dt id="object.__lt__"> <code class="descclassname">object.</code><code class="descname">__lt__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__lt__" title="Permalink to this definition">¶</a></dt> <dt id="object.__le__"> <code class="descclassname">object.</code><code class="descname">__le__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__le__" title="Permalink to this definition">¶</a></dt> <dt id="object.__eq__"> <code class="descclassname">object.</code><code class="descname">__eq__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__eq__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ne__"> <code class="descclassname">object.</code><code class="descname">__ne__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__ne__" title="Permalink to this definition">¶</a></dt> <dt id="object.__gt__"> <code class="descclassname">object.</code><code class="descname">__gt__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__gt__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ge__"> <code class="descclassname">object.</code><code class="descname">__ge__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__ge__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-74">These are the so-called “rich comparison” methods. The correspondence between operator symbols and method names is as follows: <code class="docutils literal notranslate"><span class="pre">x<y</span></code> calls <code class="docutils literal notranslate"><span class="pre">x.__lt__(y)</span></code>, <code class="docutils literal notranslate"><span class="pre">x<=y</span></code> calls <code class="docutils literal notranslate"><span class="pre">x.__le__(y)</span></code>, <code class="docutils literal notranslate"><span class="pre">x==y</span></code> calls <code class="docutils literal notranslate"><span class="pre">x.__eq__(y)</span></code>, <code class="docutils literal notranslate"><span class="pre">x!=y</span></code> calls <code class="docutils literal notranslate"><span class="pre">x.__ne__(y)</span></code>, <code class="docutils literal notranslate"><span class="pre">x>y</span></code> calls <code class="docutils literal notranslate"><span class="pre">x.__gt__(y)</span></code>, and <code class="docutils literal notranslate"><span class="pre">x>=y</span></code> calls <code class="docutils literal notranslate"><span class="pre">x.__ge__(y)</span></code>.</p> <p>A rich comparison method may return the singleton <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code> if it does not implement the operation for a given pair of arguments. By convention, <code class="docutils literal notranslate"><span class="pre">False</span></code> and <code class="docutils literal notranslate"><span class="pre">True</span></code> are returned for a successful comparison. However, these methods can return any value, so if the comparison operator is used in a Boolean context (e.g., in the condition of an <code class="docutils literal notranslate"><span class="pre">if</span></code> statement), Python will call <a class="reference internal" href="../library/functions.html#bool" title="bool"><code class="xref py py-func docutils literal notranslate"><span class="pre">bool()</span></code></a> on the value to determine if the result is true or false.</p> <p>By default, <a class="reference internal" href="#object.__ne__" title="object.__ne__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__ne__()</span></code></a> delegates to <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> and inverts the result unless it is <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>. There are no other implied relationships among the comparison operators, for example, the truth of <code class="docutils literal notranslate"><span class="pre">(x<y</span> <span class="pre">or</span> <span class="pre">x==y)</span></code> does not imply <code class="docutils literal notranslate"><span class="pre">x<=y</span></code>. To automatically generate ordering operations from a single root operation, see <a class="reference internal" href="../library/functools.html#functools.total_ordering" title="functools.total_ordering"><code class="xref py py-func docutils literal notranslate"><span class="pre">functools.total_ordering()</span></code></a>.</p> <p>See the paragraph on <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> for some important notes on creating <a class="reference internal" href="../glossary.html#term-hashable"><span class="xref std std-term">hashable</span></a> objects which support custom comparison operations and are usable as dictionary keys.</p> <p>There are no swapped-argument versions of these methods (to be used when the left argument does not support the operation but the right argument does); rather, <a class="reference internal" href="#object.__lt__" title="object.__lt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__lt__()</span></code></a> and <a class="reference internal" href="#object.__gt__" title="object.__gt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__gt__()</span></code></a> are each other’s reflection, <a class="reference internal" href="#object.__le__" title="object.__le__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__le__()</span></code></a> and <a class="reference internal" href="#object.__ge__" title="object.__ge__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__ge__()</span></code></a> are each other’s reflection, and <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> and <a class="reference internal" href="#object.__ne__" title="object.__ne__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__ne__()</span></code></a> are their own reflection. If the operands are of different types, and right operand’s type is a direct or indirect subclass of the left operand’s type, the reflected method of the right operand has priority, otherwise the left operand’s method has priority. Virtual subclassing is not considered.</p> </dd></dl> <dl class="method"> <dt id="object.__hash__"> <code class="descclassname">object.</code><code class="descname">__hash__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__hash__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-75">Called by built-in function <a class="reference internal" href="../library/functions.html#hash" title="hash"><code class="xref py py-func docutils literal notranslate"><span class="pre">hash()</span></code></a> and for operations on members of hashed collections including <a class="reference internal" href="../library/stdtypes.html#set" title="set"><code class="xref py py-class docutils literal notranslate"><span class="pre">set</span></code></a>, <a class="reference internal" href="../library/stdtypes.html#frozenset" title="frozenset"><code class="xref py py-class docutils literal notranslate"><span class="pre">frozenset</span></code></a>, and <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate"><span class="pre">dict</span></code></a>. <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> should return an integer. The only required property is that objects which compare equal have the same hash value; it is advised to mix together the hash values of the components of the object that also play a part in comparison of objects by packing them into a tuple and hashing the tuple. Example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">__hash__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">return</span> <span class="nb">hash</span><span class="p">((</span><span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">nick</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">color</span><span class="p">))</span> </pre></div> </div> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last"><a class="reference internal" href="../library/functions.html#hash" title="hash"><code class="xref py py-func docutils literal notranslate"><span class="pre">hash()</span></code></a> truncates the value returned from an object’s custom <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> method to the size of a <code class="xref c c-type docutils literal notranslate"><span class="pre">Py_ssize_t</span></code>. This is typically 8 bytes on 64-bit builds and 4 bytes on 32-bit builds. If an object’s <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> must interoperate on builds of different bit sizes, be sure to check the width on all supported builds. An easy way to do this is with <code class="docutils literal notranslate"><span class="pre">python</span> <span class="pre">-c</span> <span class="pre">"import</span> <span class="pre">sys;</span> <span class="pre">print(sys.hash_info.width)"</span></code>.</p> </div> <p>If a class does not define an <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> method it should not define a <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> operation either; if it defines <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> but not <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a>, its instances will not be usable as items in hashable collections. If a class defines mutable objects and implements an <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> method, it should not implement <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a>, since the implementation of hashable collections requires that a key’s hash value is immutable (if the object’s hash value changes, it will be in the wrong hash bucket).</p> <p>User-defined classes have <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> and <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> methods by default; with them, all objects compare unequal (except with themselves) and <code class="docutils literal notranslate"><span class="pre">x.__hash__()</span></code> returns an appropriate value such that <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">y</span></code> implies both that <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">is</span> <span class="pre">y</span></code> and <code class="docutils literal notranslate"><span class="pre">hash(x)</span> <span class="pre">==</span> <span class="pre">hash(y)</span></code>.</p> <p>A class that overrides <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> and does not define <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> will have its <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> implicitly set to <code class="docutils literal notranslate"><span class="pre">None</span></code>. When the <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> method of a class is <code class="docutils literal notranslate"><span class="pre">None</span></code>, instances of the class will raise an appropriate <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> when a program attempts to retrieve their hash value, and will also be correctly identified as unhashable when checking <code class="docutils literal notranslate"><span class="pre">isinstance(obj,</span> <span class="pre">collections.Hashable)</span></code>.</p> <p>If a class that overrides <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> needs to retain the implementation of <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> from a parent class, the interpreter must be told this explicitly by setting <code class="docutils literal notranslate"><span class="pre">__hash__</span> <span class="pre">=</span> <span class="pre"><ParentClass>.__hash__</span></code>.</p> <p>If a class that does not override <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> wishes to suppress hash support, it should include <code class="docutils literal notranslate"><span class="pre">__hash__</span> <span class="pre">=</span> <span class="pre">None</span></code> in the class definition. A class which defines its own <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> that explicitly raises a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> would be incorrectly identified as hashable by an <code class="docutils literal notranslate"><span class="pre">isinstance(obj,</span> <span class="pre">collections.Hashable)</span></code> call.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p>By default, the <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> values of str, bytes and datetime objects are “salted” with an unpredictable random value. Although they remain constant within an individual Python process, they are not predictable between repeated invocations of Python.</p> <p>This is intended to provide protection against a denial-of-service caused by carefully-chosen inputs that exploit the worst case performance of a dict insertion, O(n^2) complexity. See <a class="reference external" href="http://www.ocert.org/advisories/ocert-2011-003.html">http://www.ocert.org/advisories/ocert-2011-003.html</a> for details.</p> <p>Changing hash values affects the iteration order of dicts, sets and other mappings. Python has never made guarantees about this ordering (and it typically varies between 32-bit and 64-bit builds).</p> <p class="last">See also <span class="target" id="index-76"></span><a class="reference internal" href="../using/cmdline.html#envvar-PYTHONHASHSEED"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONHASHSEED</span></code></a>.</p> </div> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.3: </span>Hash randomization is enabled by default.</p> </div> </dd></dl> <dl class="method"> <dt id="object.__bool__"> <code class="descclassname">object.</code><code class="descname">__bool__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__bool__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-77">Called to implement truth value testing and the built-in operation <code class="docutils literal notranslate"><span class="pre">bool()</span></code>; should return <code class="docutils literal notranslate"><span class="pre">False</span></code> or <code class="docutils literal notranslate"><span class="pre">True</span></code>. When this method is not defined, <a class="reference internal" href="#object.__len__" title="object.__len__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__len__()</span></code></a> is called, if it is defined, and the object is considered true if its result is nonzero. If a class defines neither <a class="reference internal" href="#object.__len__" title="object.__len__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__len__()</span></code></a> nor <a class="reference internal" href="#object.__bool__" title="object.__bool__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__bool__()</span></code></a>, all its instances are considered true.</p> </dd></dl> </div> <div class="section" id="customizing-attribute-access"> <span id="attribute-access"></span><h3>3.3.2. Customizing attribute access<a class="headerlink" href="#customizing-attribute-access" title="Permalink to this headline">¶</a></h3> <p>The following methods can be defined to customize the meaning of attribute access (use of, assignment to, or deletion of <code class="docutils literal notranslate"><span class="pre">x.name</span></code>) for class instances.</p> <dl class="method"> <dt id="object.__getattr__"> <code class="descclassname">object.</code><code class="descname">__getattr__</code><span class="sig-paren">(</span><em>self</em>, <em>name</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__getattr__" title="Permalink to this definition">¶</a></dt> <dd><p>Called when the default attribute access fails with an <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> (either <a class="reference internal" href="#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a> raises an <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> because <em>name</em> is not an instance attribute or an attribute in the class tree for <code class="docutils literal notranslate"><span class="pre">self</span></code>; or <a class="reference internal" href="#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a> of a <em>name</em> property raises <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a>). This method should either return the (computed) attribute value or raise an <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> exception.</p> <p>Note that if the attribute is found through the normal mechanism, <a class="reference internal" href="#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattr__()</span></code></a> is not called. (This is an intentional asymmetry between <a class="reference internal" href="#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattr__()</span></code></a> and <a class="reference internal" href="#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a>.) This is done both for efficiency reasons and because otherwise <a class="reference internal" href="#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattr__()</span></code></a> would have no way to access other attributes of the instance. Note that at least for instance variables, you can fake total control by not inserting any values in the instance attribute dictionary (but instead inserting them in another object). See the <a class="reference internal" href="#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a> method below for a way to actually get total control over attribute access.</p> </dd></dl> <dl class="method"> <dt id="object.__getattribute__"> <code class="descclassname">object.</code><code class="descname">__getattribute__</code><span class="sig-paren">(</span><em>self</em>, <em>name</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__getattribute__" title="Permalink to this definition">¶</a></dt> <dd><p>Called unconditionally to implement attribute accesses for instances of the class. If the class also defines <a class="reference internal" href="#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattr__()</span></code></a>, the latter will not be called unless <a class="reference internal" href="#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a> either calls it explicitly or raises an <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a>. This method should return the (computed) attribute value or raise an <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> exception. In order to avoid infinite recursion in this method, its implementation should always call the base class method with the same name to access any attributes it needs, for example, <code class="docutils literal notranslate"><span class="pre">object.__getattribute__(self,</span> <span class="pre">name)</span></code>.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">This method may still be bypassed when looking up special methods as the result of implicit invocation via language syntax or built-in functions. See <a class="reference internal" href="#special-lookup"><span class="std std-ref">Special method lookup</span></a>.</p> </div> </dd></dl> <dl class="method"> <dt id="object.__setattr__"> <code class="descclassname">object.</code><code class="descname">__setattr__</code><span class="sig-paren">(</span><em>self</em>, <em>name</em>, <em>value</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__setattr__" title="Permalink to this definition">¶</a></dt> <dd><p>Called when an attribute assignment is attempted. This is called instead of the normal mechanism (i.e. store the value in the instance dictionary). <em>name</em> is the attribute name, <em>value</em> is the value to be assigned to it.</p> <p>If <a class="reference internal" href="#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a> wants to assign to an instance attribute, it should call the base class method with the same name, for example, <code class="docutils literal notranslate"><span class="pre">object.__setattr__(self,</span> <span class="pre">name,</span> <span class="pre">value)</span></code>.</p> </dd></dl> <dl class="method"> <dt id="object.__delattr__"> <code class="descclassname">object.</code><code class="descname">__delattr__</code><span class="sig-paren">(</span><em>self</em>, <em>name</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__delattr__" title="Permalink to this definition">¶</a></dt> <dd><p>Like <a class="reference internal" href="#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a> but for attribute deletion instead of assignment. This should only be implemented if <code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">obj.name</span></code> is meaningful for the object.</p> </dd></dl> <dl class="method"> <dt id="object.__dir__"> <code class="descclassname">object.</code><code class="descname">__dir__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__dir__" title="Permalink to this definition">¶</a></dt> <dd><p>Called when <a class="reference internal" href="../library/functions.html#dir" title="dir"><code class="xref py py-func docutils literal notranslate"><span class="pre">dir()</span></code></a> is called on the object. A sequence must be returned. <a class="reference internal" href="../library/functions.html#dir" title="dir"><code class="xref py py-func docutils literal notranslate"><span class="pre">dir()</span></code></a> converts the returned sequence to a list and sorts it.</p> </dd></dl> <div class="section" id="customizing-module-attribute-access"> <h4>3.3.2.1. Customizing module attribute access<a class="headerlink" href="#customizing-module-attribute-access" title="Permalink to this headline">¶</a></h4> <p id="index-78">For a more fine grained customization of the module behavior (setting attributes, properties, etc.), one can set the <code class="docutils literal notranslate"><span class="pre">__class__</span></code> attribute of a module object to a subclass of <a class="reference internal" href="../library/types.html#types.ModuleType" title="types.ModuleType"><code class="xref py py-class docutils literal notranslate"><span class="pre">types.ModuleType</span></code></a>. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">sys</span> <span class="kn">from</span> <span class="nn">types</span> <span class="k">import</span> <span class="n">ModuleType</span> <span class="k">class</span> <span class="nc">VerboseModule</span><span class="p">(</span><span class="n">ModuleType</span><span class="p">):</span> <span class="k">def</span> <span class="nf">__repr__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">return</span> <span class="n">f</span><span class="s1">'Verbose </span><span class="si">{self.__name__}</span><span class="s1">'</span> <span class="k">def</span> <span class="nf">__setattr__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">attr</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span> <span class="nb">print</span><span class="p">(</span><span class="n">f</span><span class="s1">'Setting </span><span class="si">{attr}</span><span class="s1">...'</span><span class="p">)</span> <span class="nb">setattr</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">attr</span><span class="p">,</span> <span class="n">value</span><span class="p">)</span> <span class="n">sys</span><span class="o">.</span><span class="n">modules</span><span class="p">[</span><span class="vm">__name__</span><span class="p">]</span><span class="o">.</span><span class="vm">__class__</span> <span class="o">=</span> <span class="n">VerboseModule</span> </pre></div> </div> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">Setting module <code class="docutils literal notranslate"><span class="pre">__class__</span></code> only affects lookups made using the attribute access syntax – directly accessing the module globals (whether by code within the module, or via a reference to the module’s globals dictionary) is unaffected.</p> </div> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.5: </span><code class="docutils literal notranslate"><span class="pre">__class__</span></code> module attribute is now writable.</p> </div> </div> <div class="section" id="implementing-descriptors"> <span id="descriptors"></span><h4>3.3.2.2. Implementing Descriptors<a class="headerlink" href="#implementing-descriptors" title="Permalink to this headline">¶</a></h4> <p>The following methods only apply when an instance of the class containing the method (a so-called <em>descriptor</em> class) appears in an <em>owner</em> class (the descriptor must be in either the owner’s class dictionary or in the class dictionary for one of its parents). In the examples below, “the attribute” refers to the attribute whose name is the key of the property in the owner class’ <a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a>.</p> <dl class="method"> <dt id="object.__get__"> <code class="descclassname">object.</code><code class="descname">__get__</code><span class="sig-paren">(</span><em>self</em>, <em>instance</em>, <em>owner</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__get__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to get the attribute of the owner class (class attribute access) or of an instance of that class (instance attribute access). <em>owner</em> is always the owner class, while <em>instance</em> is the instance that the attribute was accessed through, or <code class="docutils literal notranslate"><span class="pre">None</span></code> when the attribute is accessed through the <em>owner</em>. This method should return the (computed) attribute value or raise an <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> exception.</p> </dd></dl> <dl class="method"> <dt id="object.__set__"> <code class="descclassname">object.</code><code class="descname">__set__</code><span class="sig-paren">(</span><em>self</em>, <em>instance</em>, <em>value</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__set__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to set the attribute on an instance <em>instance</em> of the owner class to a new value, <em>value</em>.</p> </dd></dl> <dl class="method"> <dt id="object.__delete__"> <code class="descclassname">object.</code><code class="descname">__delete__</code><span class="sig-paren">(</span><em>self</em>, <em>instance</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__delete__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to delete the attribute on an instance <em>instance</em> of the owner class.</p> </dd></dl> <dl class="method"> <dt id="object.__set_name__"> <code class="descclassname">object.</code><code class="descname">__set_name__</code><span class="sig-paren">(</span><em>self</em>, <em>owner</em>, <em>name</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__set_name__" title="Permalink to this definition">¶</a></dt> <dd><p>Called at the time the owning class <em>owner</em> is created. The descriptor has been assigned to <em>name</em>.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 3.6.</span></p> </div> </dd></dl> <p>The attribute <code class="xref py py-attr docutils literal notranslate"><span class="pre">__objclass__</span></code> is interpreted by the <a class="reference internal" href="../library/inspect.html#module-inspect" title="inspect: Extract information and source code from live objects."><code class="xref py py-mod docutils literal notranslate"><span class="pre">inspect</span></code></a> module as specifying the class where this object was defined (setting this appropriately can assist in runtime introspection of dynamic class attributes). For callables, it may indicate that an instance of the given type (or a subclass) is expected or required as the first positional argument (for example, CPython sets this attribute for unbound methods that are implemented in C).</p> </div> <div class="section" id="invoking-descriptors"> <span id="descriptor-invocation"></span><h4>3.3.2.3. Invoking Descriptors<a class="headerlink" href="#invoking-descriptors" title="Permalink to this headline">¶</a></h4> <p>In general, a descriptor is an object attribute with “binding behavior”, one whose attribute access has been overridden by methods in the descriptor protocol: <a class="reference internal" href="#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a>, <a class="reference internal" href="#object.__set__" title="object.__set__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__set__()</span></code></a>, and <a class="reference internal" href="#object.__delete__" title="object.__delete__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delete__()</span></code></a>. If any of those methods are defined for an object, it is said to be a descriptor.</p> <p>The default behavior for attribute access is to get, set, or delete the attribute from an object’s dictionary. For instance, <code class="docutils literal notranslate"><span class="pre">a.x</span></code> has a lookup chain starting with <code class="docutils literal notranslate"><span class="pre">a.__dict__['x']</span></code>, then <code class="docutils literal notranslate"><span class="pre">type(a).__dict__['x']</span></code>, and continuing through the base classes of <code class="docutils literal notranslate"><span class="pre">type(a)</span></code> excluding metaclasses.</p> <p>However, if the looked-up value is an object defining one of the descriptor methods, then Python may override the default behavior and invoke the descriptor method instead. Where this occurs in the precedence chain depends on which descriptor methods were defined and how they were called.</p> <p>The starting point for descriptor invocation is a binding, <code class="docutils literal notranslate"><span class="pre">a.x</span></code>. How the arguments are assembled depends on <code class="docutils literal notranslate"><span class="pre">a</span></code>:</p> <dl class="docutils"> <dt>Direct Call</dt> <dd>The simplest and least common call is when user code directly invokes a descriptor method: <code class="docutils literal notranslate"><span class="pre">x.__get__(a)</span></code>.</dd> <dt>Instance Binding</dt> <dd>If binding to an object instance, <code class="docutils literal notranslate"><span class="pre">a.x</span></code> is transformed into the call: <code class="docutils literal notranslate"><span class="pre">type(a).__dict__['x'].__get__(a,</span> <span class="pre">type(a))</span></code>.</dd> <dt>Class Binding</dt> <dd>If binding to a class, <code class="docutils literal notranslate"><span class="pre">A.x</span></code> is transformed into the call: <code class="docutils literal notranslate"><span class="pre">A.__dict__['x'].__get__(None,</span> <span class="pre">A)</span></code>.</dd> <dt>Super Binding</dt> <dd>If <code class="docutils literal notranslate"><span class="pre">a</span></code> is an instance of <a class="reference internal" href="../library/functions.html#super" title="super"><code class="xref py py-class docutils literal notranslate"><span class="pre">super</span></code></a>, then the binding <code class="docutils literal notranslate"><span class="pre">super(B,</span> <span class="pre">obj).m()</span></code> searches <code class="docutils literal notranslate"><span class="pre">obj.__class__.__mro__</span></code> for the base class <code class="docutils literal notranslate"><span class="pre">A</span></code> immediately preceding <code class="docutils literal notranslate"><span class="pre">B</span></code> and then invokes the descriptor with the call: <code class="docutils literal notranslate"><span class="pre">A.__dict__['m'].__get__(obj,</span> <span class="pre">obj.__class__)</span></code>.</dd> </dl> <p>For instance bindings, the precedence of descriptor invocation depends on the which descriptor methods are defined. A descriptor can define any combination of <a class="reference internal" href="#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a>, <a class="reference internal" href="#object.__set__" title="object.__set__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__set__()</span></code></a> and <a class="reference internal" href="#object.__delete__" title="object.__delete__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delete__()</span></code></a>. If it does not define <a class="reference internal" href="#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a>, then accessing the attribute will return the descriptor object itself unless there is a value in the object’s instance dictionary. If the descriptor defines <a class="reference internal" href="#object.__set__" title="object.__set__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__set__()</span></code></a> and/or <a class="reference internal" href="#object.__delete__" title="object.__delete__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delete__()</span></code></a>, it is a data descriptor; if it defines neither, it is a non-data descriptor. Normally, data descriptors define both <a class="reference internal" href="#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a> and <a class="reference internal" href="#object.__set__" title="object.__set__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__set__()</span></code></a>, while non-data descriptors have just the <a class="reference internal" href="#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a> method. Data descriptors with <a class="reference internal" href="#object.__set__" title="object.__set__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__set__()</span></code></a> and <a class="reference internal" href="#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a> defined always override a redefinition in an instance dictionary. In contrast, non-data descriptors can be overridden by instances.</p> <p>Python methods (including <a class="reference internal" href="../library/functions.html#staticmethod" title="staticmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">staticmethod()</span></code></a> and <a class="reference internal" href="../library/functions.html#classmethod" title="classmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">classmethod()</span></code></a>) are implemented as non-data descriptors. Accordingly, instances can redefine and override methods. This allows individual instances to acquire behaviors that differ from other instances of the same class.</p> <p>The <a class="reference internal" href="../library/functions.html#property" title="property"><code class="xref py py-func docutils literal notranslate"><span class="pre">property()</span></code></a> function is implemented as a data descriptor. Accordingly, instances cannot override the behavior of a property.</p> </div> <div class="section" id="slots"> <span id="id3"></span><h4>3.3.2.4. __slots__<a class="headerlink" href="#slots" title="Permalink to this headline">¶</a></h4> <p><em>__slots__</em> allow us to explicitly declare data members (like properties) and deny the creation of <em>__dict__</em> and <em>__weakref__</em> (unless explicitly declared in <em>__slots__</em> or available in a parent.)</p> <p>The space saved over using <em>__dict__</em> can be significant.</p> <dl class="data"> <dt id="object.__slots__"> <code class="descclassname">object.</code><code class="descname">__slots__</code><a class="headerlink" href="#object.__slots__" title="Permalink to this definition">¶</a></dt> <dd><p>This class variable can be assigned a string, iterable, or sequence of strings with variable names used by instances. <em>__slots__</em> reserves space for the declared variables and prevents the automatic creation of <em>__dict__</em> and <em>__weakref__</em> for each instance.</p> </dd></dl> <div class="section" id="notes-on-using-slots"> <h5>3.3.2.4.1. Notes on using <em>__slots__</em><a class="headerlink" href="#notes-on-using-slots" title="Permalink to this headline">¶</a></h5> <ul class="simple"> <li>When inheriting from a class without <em>__slots__</em>, the <em>__dict__</em> and <em>__weakref__</em> attribute of the instances will always be accessible.</li> <li>Without a <em>__dict__</em> variable, instances cannot be assigned new variables not listed in the <em>__slots__</em> definition. Attempts to assign to an unlisted variable name raises <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a>. If dynamic assignment of new variables is desired, then add <code class="docutils literal notranslate"><span class="pre">'__dict__'</span></code> to the sequence of strings in the <em>__slots__</em> declaration.</li> <li>Without a <em>__weakref__</em> variable for each instance, classes defining <em>__slots__</em> do not support weak references to its instances. If weak reference support is needed, then add <code class="docutils literal notranslate"><span class="pre">'__weakref__'</span></code> to the sequence of strings in the <em>__slots__</em> declaration.</li> <li><em>__slots__</em> are implemented at the class level by creating descriptors (<a class="reference internal" href="#descriptors"><span class="std std-ref">Implementing Descriptors</span></a>) for each variable name. As a result, class attributes cannot be used to set default values for instance variables defined by <em>__slots__</em>; otherwise, the class attribute would overwrite the descriptor assignment.</li> <li>The action of a <em>__slots__</em> declaration is not limited to the class where it is defined. <em>__slots__</em> declared in parents are available in child classes. However, child subclasses will get a <em>__dict__</em> and <em>__weakref__</em> unless they also define <em>__slots__</em> (which should only contain names of any <em>additional</em> slots).</li> <li>If a class defines a slot also defined in a base class, the instance variable defined by the base class slot is inaccessible (except by retrieving its descriptor directly from the base class). This renders the meaning of the program undefined. In the future, a check may be added to prevent this.</li> <li>Nonempty <em>__slots__</em> does not work for classes derived from “variable-length” built-in types such as <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a>, <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> and <a class="reference internal" href="../library/stdtypes.html#tuple" title="tuple"><code class="xref py py-class docutils literal notranslate"><span class="pre">tuple</span></code></a>.</li> <li>Any non-string iterable may be assigned to <em>__slots__</em>. Mappings may also be used; however, in the future, special meaning may be assigned to the values corresponding to each key.</li> <li><em>__class__</em> assignment works only if both classes have the same <em>__slots__</em>.</li> <li>Multiple inheritance with multiple slotted parent classes can be used, but only one parent is allowed to have attributes created by slots (the other bases must have empty slot layouts) - violations raise <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>.</li> </ul> </div> </div> </div> <div class="section" id="customizing-class-creation"> <span id="class-customization"></span><h3>3.3.3. Customizing class creation<a class="headerlink" href="#customizing-class-creation" title="Permalink to this headline">¶</a></h3> <p>Whenever a class inherits from another class, <em>__init_subclass__</em> is called on that class. This way, it is possible to write classes which change the behavior of subclasses. This is closely related to class decorators, but where class decorators only affect the specific class they’re applied to, <code class="docutils literal notranslate"><span class="pre">__init_subclass__</span></code> solely applies to future subclasses of the class defining the method.</p> <dl class="classmethod"> <dt id="object.__init_subclass__"> <em class="property">classmethod </em><code class="descclassname">object.</code><code class="descname">__init_subclass__</code><span class="sig-paren">(</span><em>cls</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__init_subclass__" title="Permalink to this definition">¶</a></dt> <dd><p>This method is called whenever the containing class is subclassed. <em>cls</em> is then the new subclass. If defined as a normal instance method, this method is implicitly converted to a class method.</p> <p>Keyword arguments which are given to a new class are passed to the parent’s class <code class="docutils literal notranslate"><span class="pre">__init_subclass__</span></code>. For compatibility with other classes using <code class="docutils literal notranslate"><span class="pre">__init_subclass__</span></code>, one should take out the needed keyword arguments and pass the others over to the base class, as in:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Philosopher</span><span class="p">:</span> <span class="k">def</span> <span class="nf">__init_subclass__</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">default_name</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span> <span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="n">__init_subclass__</span><span class="p">(</span><span class="o">**</span><span class="n">kwargs</span><span class="p">)</span> <span class="bp">cls</span><span class="o">.</span><span class="n">default_name</span> <span class="o">=</span> <span class="n">default_name</span> <span class="k">class</span> <span class="nc">AustralianPhilosopher</span><span class="p">(</span><span class="n">Philosopher</span><span class="p">,</span> <span class="n">default_name</span><span class="o">=</span><span class="s2">"Bruce"</span><span class="p">):</span> <span class="k">pass</span> </pre></div> </div> <p>The default implementation <code class="docutils literal notranslate"><span class="pre">object.__init_subclass__</span></code> does nothing, but raises an error if it is called with any arguments.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">The metaclass hint <code class="docutils literal notranslate"><span class="pre">metaclass</span></code> is consumed by the rest of the type machinery, and is never passed to <code class="docutils literal notranslate"><span class="pre">__init_subclass__</span></code> implementations. The actual metaclass (rather than the explicit hint) can be accessed as <code class="docutils literal notranslate"><span class="pre">type(cls)</span></code>.</p> </div> <div class="versionadded"> <p><span class="versionmodified">New in version 3.6.</span></p> </div> </dd></dl> <div class="section" id="metaclasses"> <span id="id4"></span><h4>3.3.3.1. Metaclasses<a class="headerlink" href="#metaclasses" title="Permalink to this headline">¶</a></h4> <p id="index-79">By default, classes are constructed using <a class="reference internal" href="../library/functions.html#type" title="type"><code class="xref py py-func docutils literal notranslate"><span class="pre">type()</span></code></a>. The class body is executed in a new namespace and the class name is bound locally to the result of <code class="docutils literal notranslate"><span class="pre">type(name,</span> <span class="pre">bases,</span> <span class="pre">namespace)</span></code>.</p> <p>The class creation process can be customized by passing the <code class="docutils literal notranslate"><span class="pre">metaclass</span></code> keyword argument in the class definition line, or by inheriting from an existing class that included such an argument. In the following example, both <code class="docutils literal notranslate"><span class="pre">MyClass</span></code> and <code class="docutils literal notranslate"><span class="pre">MySubclass</span></code> are instances of <code class="docutils literal notranslate"><span class="pre">Meta</span></code>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Meta</span><span class="p">(</span><span class="nb">type</span><span class="p">):</span> <span class="k">pass</span> <span class="k">class</span> <span class="nc">MyClass</span><span class="p">(</span><span class="n">metaclass</span><span class="o">=</span><span class="n">Meta</span><span class="p">):</span> <span class="k">pass</span> <span class="k">class</span> <span class="nc">MySubclass</span><span class="p">(</span><span class="n">MyClass</span><span class="p">):</span> <span class="k">pass</span> </pre></div> </div> <p>Any other keyword arguments that are specified in the class definition are passed through to all metaclass operations described below.</p> <p>When a class definition is executed, the following steps occur:</p> <ul class="simple"> <li>the appropriate metaclass is determined</li> <li>the class namespace is prepared</li> <li>the class body is executed</li> <li>the class object is created</li> </ul> </div> <div class="section" id="determining-the-appropriate-metaclass"> <h4>3.3.3.2. Determining the appropriate metaclass<a class="headerlink" href="#determining-the-appropriate-metaclass" title="Permalink to this headline">¶</a></h4> <p id="index-80">The appropriate metaclass for a class definition is determined as follows:</p> <ul class="simple"> <li>if no bases and no explicit metaclass are given, then <a class="reference internal" href="../library/functions.html#type" title="type"><code class="xref py py-func docutils literal notranslate"><span class="pre">type()</span></code></a> is used</li> <li>if an explicit metaclass is given and it is <em>not</em> an instance of <a class="reference internal" href="../library/functions.html#type" title="type"><code class="xref py py-func docutils literal notranslate"><span class="pre">type()</span></code></a>, then it is used directly as the metaclass</li> <li>if an instance of <a class="reference internal" href="../library/functions.html#type" title="type"><code class="xref py py-func docutils literal notranslate"><span class="pre">type()</span></code></a> is given as the explicit metaclass, or bases are defined, then the most derived metaclass is used</li> </ul> <p>The most derived metaclass is selected from the explicitly specified metaclass (if any) and the metaclasses (i.e. <code class="docutils literal notranslate"><span class="pre">type(cls)</span></code>) of all specified base classes. The most derived metaclass is one which is a subtype of <em>all</em> of these candidate metaclasses. If none of the candidate metaclasses meets that criterion, then the class definition will fail with <code class="docutils literal notranslate"><span class="pre">TypeError</span></code>.</p> </div> <div class="section" id="preparing-the-class-namespace"> <span id="prepare"></span><h4>3.3.3.3. Preparing the class namespace<a class="headerlink" href="#preparing-the-class-namespace" title="Permalink to this headline">¶</a></h4> <p id="index-81">Once the appropriate metaclass has been identified, then the class namespace is prepared. If the metaclass has a <code class="docutils literal notranslate"><span class="pre">__prepare__</span></code> attribute, it is called as <code class="docutils literal notranslate"><span class="pre">namespace</span> <span class="pre">=</span> <span class="pre">metaclass.__prepare__(name,</span> <span class="pre">bases,</span> <span class="pre">**kwds)</span></code> (where the additional keyword arguments, if any, come from the class definition).</p> <p>If the metaclass has no <code class="docutils literal notranslate"><span class="pre">__prepare__</span></code> attribute, then the class namespace is initialised as an empty ordered mapping.</p> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><span class="target" id="index-82"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3115"><strong>PEP 3115</strong></a> - Metaclasses in Python 3000</dt> <dd>Introduced the <code class="docutils literal notranslate"><span class="pre">__prepare__</span></code> namespace hook</dd> </dl> </div> </div> <div class="section" id="executing-the-class-body"> <h4>3.3.3.4. Executing the class body<a class="headerlink" href="#executing-the-class-body" title="Permalink to this headline">¶</a></h4> <p id="index-83">The class body is executed (approximately) as <code class="docutils literal notranslate"><span class="pre">exec(body,</span> <span class="pre">globals(),</span> <span class="pre">namespace)</span></code>. The key difference from a normal call to <a class="reference internal" href="../library/functions.html#exec" title="exec"><code class="xref py py-func docutils literal notranslate"><span class="pre">exec()</span></code></a> is that lexical scoping allows the class body (including any methods) to reference names from the current and outer scopes when the class definition occurs inside a function.</p> <p>However, even when the class definition occurs inside the function, methods defined inside the class still cannot see names defined at the class scope. Class variables must be accessed through the first parameter of instance or class methods, or through the implicit lexically scoped <code class="docutils literal notranslate"><span class="pre">__class__</span></code> reference described in the next section.</p> </div> <div class="section" id="creating-the-class-object"> <span id="class-object-creation"></span><h4>3.3.3.5. Creating the class object<a class="headerlink" href="#creating-the-class-object" title="Permalink to this headline">¶</a></h4> <p id="index-84">Once the class namespace has been populated by executing the class body, the class object is created by calling <code class="docutils literal notranslate"><span class="pre">metaclass(name,</span> <span class="pre">bases,</span> <span class="pre">namespace,</span> <span class="pre">**kwds)</span></code> (the additional keywords passed here are the same as those passed to <code class="docutils literal notranslate"><span class="pre">__prepare__</span></code>).</p> <p>This class object is the one that will be referenced by the zero-argument form of <a class="reference internal" href="../library/functions.html#super" title="super"><code class="xref py py-func docutils literal notranslate"><span class="pre">super()</span></code></a>. <code class="docutils literal notranslate"><span class="pre">__class__</span></code> is an implicit closure reference created by the compiler if any methods in a class body refer to either <code class="docutils literal notranslate"><span class="pre">__class__</span></code> or <code class="docutils literal notranslate"><span class="pre">super</span></code>. This allows the zero argument form of <a class="reference internal" href="../library/functions.html#super" title="super"><code class="xref py py-func docutils literal notranslate"><span class="pre">super()</span></code></a> to correctly identify the class being defined based on lexical scoping, while the class or instance that was used to make the current call is identified based on the first argument passed to the method.</p> <div class="impl-detail compound"> <p><strong>CPython implementation detail:</strong> In CPython 3.6 and later, the <code class="docutils literal notranslate"><span class="pre">__class__</span></code> cell is passed to the metaclass as a <code class="docutils literal notranslate"><span class="pre">__classcell__</span></code> entry in the class namespace. If present, this must be propagated up to the <code class="docutils literal notranslate"><span class="pre">type.__new__</span></code> call in order for the class to be initialised correctly. Failing to do so will result in a <a class="reference internal" href="../library/exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate"><span class="pre">DeprecationWarning</span></code></a> in Python 3.6, and a <a class="reference internal" href="../library/exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeError</span></code></a> in Python 3.8.</p> </div> <p>When using the default metaclass <a class="reference internal" href="../library/functions.html#type" title="type"><code class="xref py py-class docutils literal notranslate"><span class="pre">type</span></code></a>, or any metaclass that ultimately calls <code class="docutils literal notranslate"><span class="pre">type.__new__</span></code>, the following additional customisation steps are invoked after creating the class object:</p> <ul class="simple"> <li>first, <code class="docutils literal notranslate"><span class="pre">type.__new__</span></code> collects all of the descriptors in the class namespace that define a <a class="reference internal" href="#object.__set_name__" title="object.__set_name__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__set_name__()</span></code></a> method;</li> <li>second, all of these <code class="docutils literal notranslate"><span class="pre">__set_name__</span></code> methods are called with the class being defined and the assigned name of that particular descriptor; and</li> <li>finally, the <a class="reference internal" href="#object.__init_subclass__" title="object.__init_subclass__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init_subclass__()</span></code></a> hook is called on the immediate parent of the new class in its method resolution order.</li> </ul> <p>After the class object is created, it is passed to the class decorators included in the class definition (if any) and the resulting object is bound in the local namespace as the defined class.</p> <p>When a new class is created by <code class="docutils literal notranslate"><span class="pre">type.__new__</span></code>, the object provided as the namespace parameter is copied to a new ordered mapping and the original object is discarded. The new copy is wrapped in a read-only proxy, which becomes the <a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a> attribute of the class object.</p> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><span class="target" id="index-85"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3135"><strong>PEP 3135</strong></a> - New super</dt> <dd>Describes the implicit <code class="docutils literal notranslate"><span class="pre">__class__</span></code> closure reference</dd> </dl> </div> </div> <div class="section" id="metaclass-example"> <h4>3.3.3.6. Metaclass example<a class="headerlink" href="#metaclass-example" title="Permalink to this headline">¶</a></h4> <p>The potential uses for metaclasses are boundless. Some ideas that have been explored include enum, logging, interface checking, automatic delegation, automatic property creation, proxies, frameworks, and automatic resource locking/synchronization.</p> <p>Here is an example of a metaclass that uses an <a class="reference internal" href="../library/collections.html#collections.OrderedDict" title="collections.OrderedDict"><code class="xref py py-class docutils literal notranslate"><span class="pre">collections.OrderedDict</span></code></a> to remember the order that class variables are defined:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">OrderedClass</span><span class="p">(</span><span class="nb">type</span><span class="p">):</span> <span class="nd">@classmethod</span> <span class="k">def</span> <span class="nf">__prepare__</span><span class="p">(</span><span class="n">metacls</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">bases</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">):</span> <span class="k">return</span> <span class="n">collections</span><span class="o">.</span><span class="n">OrderedDict</span><span class="p">()</span> <span class="k">def</span> <span class="nf">__new__</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">bases</span><span class="p">,</span> <span class="n">namespace</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">):</span> <span class="n">result</span> <span class="o">=</span> <span class="nb">type</span><span class="o">.</span><span class="fm">__new__</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">bases</span><span class="p">,</span> <span class="nb">dict</span><span class="p">(</span><span class="n">namespace</span><span class="p">))</span> <span class="n">result</span><span class="o">.</span><span class="n">members</span> <span class="o">=</span> <span class="nb">tuple</span><span class="p">(</span><span class="n">namespace</span><span class="p">)</span> <span class="k">return</span> <span class="n">result</span> <span class="k">class</span> <span class="nc">A</span><span class="p">(</span><span class="n">metaclass</span><span class="o">=</span><span class="n">OrderedClass</span><span class="p">):</span> <span class="k">def</span> <span class="nf">one</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">pass</span> <span class="k">def</span> <span class="nf">two</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">pass</span> <span class="k">def</span> <span class="nf">three</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">pass</span> <span class="k">def</span> <span class="nf">four</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">pass</span> <span class="o">>>></span> <span class="n">A</span><span class="o">.</span><span class="n">members</span> <span class="p">(</span><span class="s1">'__module__'</span><span class="p">,</span> <span class="s1">'one'</span><span class="p">,</span> <span class="s1">'two'</span><span class="p">,</span> <span class="s1">'three'</span><span class="p">,</span> <span class="s1">'four'</span><span class="p">)</span> </pre></div> </div> <p>When the class definition for <em>A</em> gets executed, the process begins with calling the metaclass’s <code class="xref py py-meth docutils literal notranslate"><span class="pre">__prepare__()</span></code> method which returns an empty <a class="reference internal" href="../library/collections.html#collections.OrderedDict" title="collections.OrderedDict"><code class="xref py py-class docutils literal notranslate"><span class="pre">collections.OrderedDict</span></code></a>. That mapping records the methods and attributes of <em>A</em> as they are defined within the body of the class statement. Once those definitions are executed, the ordered dictionary is fully populated and the metaclass’s <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> method gets invoked. That method builds the new type and it saves the ordered dictionary keys in an attribute called <code class="docutils literal notranslate"><span class="pre">members</span></code>.</p> </div> </div> <div class="section" id="customizing-instance-and-subclass-checks"> <h3>3.3.4. Customizing instance and subclass checks<a class="headerlink" href="#customizing-instance-and-subclass-checks" title="Permalink to this headline">¶</a></h3> <p>The following methods are used to override the default behavior of the <a class="reference internal" href="../library/functions.html#isinstance" title="isinstance"><code class="xref py py-func docutils literal notranslate"><span class="pre">isinstance()</span></code></a> and <a class="reference internal" href="../library/functions.html#issubclass" title="issubclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">issubclass()</span></code></a> built-in functions.</p> <p>In particular, the metaclass <a class="reference internal" href="../library/abc.html#abc.ABCMeta" title="abc.ABCMeta"><code class="xref py py-class docutils literal notranslate"><span class="pre">abc.ABCMeta</span></code></a> implements these methods in order to allow the addition of Abstract Base Classes (ABCs) as “virtual base classes” to any class or type (including built-in types), including other ABCs.</p> <dl class="method"> <dt id="class.__instancecheck__"> <code class="descclassname">class.</code><code class="descname">__instancecheck__</code><span class="sig-paren">(</span><em>self</em>, <em>instance</em><span class="sig-paren">)</span><a class="headerlink" href="#class.__instancecheck__" title="Permalink to this definition">¶</a></dt> <dd><p>Return true if <em>instance</em> should be considered a (direct or indirect) instance of <em>class</em>. If defined, called to implement <code class="docutils literal notranslate"><span class="pre">isinstance(instance,</span> <span class="pre">class)</span></code>.</p> </dd></dl> <dl class="method"> <dt id="class.__subclasscheck__"> <code class="descclassname">class.</code><code class="descname">__subclasscheck__</code><span class="sig-paren">(</span><em>self</em>, <em>subclass</em><span class="sig-paren">)</span><a class="headerlink" href="#class.__subclasscheck__" title="Permalink to this definition">¶</a></dt> <dd><p>Return true if <em>subclass</em> should be considered a (direct or indirect) subclass of <em>class</em>. If defined, called to implement <code class="docutils literal notranslate"><span class="pre">issubclass(subclass,</span> <span class="pre">class)</span></code>.</p> </dd></dl> <p>Note that these methods are looked up on the type (metaclass) of a class. They cannot be defined as class methods in the actual class. This is consistent with the lookup of special methods that are called on instances, only in this case the instance is itself a class.</p> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><span class="target" id="index-86"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3119"><strong>PEP 3119</strong></a> - Introducing Abstract Base Classes</dt> <dd>Includes the specification for customizing <a class="reference internal" href="../library/functions.html#isinstance" title="isinstance"><code class="xref py py-func docutils literal notranslate"><span class="pre">isinstance()</span></code></a> and <a class="reference internal" href="../library/functions.html#issubclass" title="issubclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">issubclass()</span></code></a> behavior through <a class="reference internal" href="#class.__instancecheck__" title="class.__instancecheck__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__instancecheck__()</span></code></a> and <a class="reference internal" href="#class.__subclasscheck__" title="class.__subclasscheck__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__subclasscheck__()</span></code></a>, with motivation for this functionality in the context of adding Abstract Base Classes (see the <a class="reference internal" href="../library/abc.html#module-abc" title="abc: Abstract base classes according to PEP 3119."><code class="xref py py-mod docutils literal notranslate"><span class="pre">abc</span></code></a> module) to the language.</dd> </dl> </div> </div> <div class="section" id="emulating-callable-objects"> <span id="callable-types"></span><h3>3.3.5. Emulating callable objects<a class="headerlink" href="#emulating-callable-objects" title="Permalink to this headline">¶</a></h3> <dl class="method"> <dt id="object.__call__"> <code class="descclassname">object.</code><code class="descname">__call__</code><span class="sig-paren">(</span><em>self</em><span class="optional">[</span>, <em>args...</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#object.__call__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-87">Called when the instance is “called” as a function; if this method is defined, <code class="docutils literal notranslate"><span class="pre">x(arg1,</span> <span class="pre">arg2,</span> <span class="pre">...)</span></code> is a shorthand for <code class="docutils literal notranslate"><span class="pre">x.__call__(arg1,</span> <span class="pre">arg2,</span> <span class="pre">...)</span></code>.</p> </dd></dl> </div> <div class="section" id="emulating-container-types"> <span id="sequence-types"></span><h3>3.3.6. Emulating container types<a class="headerlink" href="#emulating-container-types" title="Permalink to this headline">¶</a></h3> <p>The following methods can be defined to implement container objects. Containers usually are sequences (such as lists or tuples) or mappings (like dictionaries), but can represent other containers as well. The first set of methods is used either to emulate a sequence or to emulate a mapping; the difference is that for a sequence, the allowable keys should be the integers <em>k</em> for which <code class="docutils literal notranslate"><span class="pre">0</span> <span class="pre"><=</span> <span class="pre">k</span> <span class="pre"><</span> <span class="pre">N</span></code> where <em>N</em> is the length of the sequence, or slice objects, which define a range of items. It is also recommended that mappings provide the methods <code class="xref py py-meth docutils literal notranslate"><span class="pre">keys()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">values()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">items()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">get()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">clear()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">setdefault()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">pop()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">popitem()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">copy()</span></code>, and <code class="xref py py-meth docutils literal notranslate"><span class="pre">update()</span></code> behaving similar to those for Python’s standard dictionary objects. The <a class="reference internal" href="../library/collections.html#module-collections" title="collections: Container datatypes"><code class="xref py py-mod docutils literal notranslate"><span class="pre">collections</span></code></a> module provides a <a class="reference internal" href="../library/collections.abc.html#collections.abc.MutableMapping" title="collections.abc.MutableMapping"><code class="xref py py-class docutils literal notranslate"><span class="pre">MutableMapping</span></code></a> abstract base class to help create those methods from a base set of <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>, <a class="reference internal" href="#object.__setitem__" title="object.__setitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setitem__()</span></code></a>, <a class="reference internal" href="#object.__delitem__" title="object.__delitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delitem__()</span></code></a>, and <code class="xref py py-meth docutils literal notranslate"><span class="pre">keys()</span></code>. Mutable sequences should provide methods <code class="xref py py-meth docutils literal notranslate"><span class="pre">append()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">count()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">index()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">extend()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">insert()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">pop()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">remove()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">reverse()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">sort()</span></code>, like Python standard list objects. Finally, sequence types should implement addition (meaning concatenation) and multiplication (meaning repetition) by defining the methods <a class="reference internal" href="#object.__add__" title="object.__add__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__add__()</span></code></a>, <a class="reference internal" href="#object.__radd__" title="object.__radd__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__radd__()</span></code></a>, <a class="reference internal" href="#object.__iadd__" title="object.__iadd__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iadd__()</span></code></a>, <a class="reference internal" href="#object.__mul__" title="object.__mul__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__mul__()</span></code></a>, <a class="reference internal" href="#object.__rmul__" title="object.__rmul__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__rmul__()</span></code></a> and <a class="reference internal" href="#object.__imul__" title="object.__imul__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__imul__()</span></code></a> described below; they should not define other numerical operators. It is recommended that both mappings and sequences implement the <a class="reference internal" href="#object.__contains__" title="object.__contains__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__contains__()</span></code></a> method to allow efficient use of the <code class="docutils literal notranslate"><span class="pre">in</span></code> operator; for mappings, <code class="docutils literal notranslate"><span class="pre">in</span></code> should search the mapping’s keys; for sequences, it should search through the values. It is further recommended that both mappings and sequences implement the <a class="reference internal" href="#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a> method to allow efficient iteration through the container; for mappings, <a class="reference internal" href="#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a> should be the same as <code class="xref py py-meth docutils literal notranslate"><span class="pre">keys()</span></code>; for sequences, it should iterate through the values.</p> <dl class="method"> <dt id="object.__len__"> <code class="descclassname">object.</code><code class="descname">__len__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__len__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-88">Called to implement the built-in function <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a>. Should return the length of the object, an integer <code class="docutils literal notranslate"><span class="pre">>=</span></code> 0. Also, an object that doesn’t define a <a class="reference internal" href="#object.__bool__" title="object.__bool__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__bool__()</span></code></a> method and whose <a class="reference internal" href="#object.__len__" title="object.__len__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__len__()</span></code></a> method returns zero is considered to be false in a Boolean context.</p> <div class="impl-detail compound"> <p><strong>CPython implementation detail:</strong> In CPython, the length is required to be at most <a class="reference internal" href="../library/sys.html#sys.maxsize" title="sys.maxsize"><code class="xref py py-attr docutils literal notranslate"><span class="pre">sys.maxsize</span></code></a>. If the length is larger than <code class="xref py py-attr docutils literal notranslate"><span class="pre">sys.maxsize</span></code> some features (such as <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a>) may raise <a class="reference internal" href="../library/exceptions.html#OverflowError" title="OverflowError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OverflowError</span></code></a>. To prevent raising <code class="xref py py-exc docutils literal notranslate"><span class="pre">OverflowError</span></code> by truth value testing, an object must define a <a class="reference internal" href="#object.__bool__" title="object.__bool__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__bool__()</span></code></a> method.</p> </div> </dd></dl> <dl class="method"> <dt id="object.__length_hint__"> <code class="descclassname">object.</code><code class="descname">__length_hint__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__length_hint__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to implement <a class="reference internal" href="../library/operator.html#operator.length_hint" title="operator.length_hint"><code class="xref py py-func docutils literal notranslate"><span class="pre">operator.length_hint()</span></code></a>. Should return an estimated length for the object (which may be greater or less than the actual length). The length must be an integer <code class="docutils literal notranslate"><span class="pre">>=</span></code> 0. This method is purely an optimization and is never required for correctness.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 3.4.</span></p> </div> </dd></dl> <div class="admonition note"> <p class="first admonition-title">Note</p> <p>Slicing is done exclusively with the following three methods. A call like</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">a</span><span class="p">[</span><span class="mi">1</span><span class="p">:</span><span class="mi">2</span><span class="p">]</span> <span class="o">=</span> <span class="n">b</span> </pre></div> </div> <p>is translated to</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">a</span><span class="p">[</span><span class="nb">slice</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="kc">None</span><span class="p">)]</span> <span class="o">=</span> <span class="n">b</span> </pre></div> </div> <p class="last">and so forth. Missing slice items are always filled in with <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p> </div> <dl class="method"> <dt id="object.__getitem__"> <code class="descclassname">object.</code><code class="descname">__getitem__</code><span class="sig-paren">(</span><em>self</em>, <em>key</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__getitem__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-89">Called to implement evaluation of <code class="docutils literal notranslate"><span class="pre">self[key]</span></code>. For sequence types, the accepted keys should be integers and slice objects. Note that the special interpretation of negative indexes (if the class wishes to emulate a sequence type) is up to the <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> method. If <em>key</em> is of an inappropriate type, <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> may be raised; if of a value outside the set of indexes for the sequence (after any special interpretation of negative values), <a class="reference internal" href="../library/exceptions.html#IndexError" title="IndexError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">IndexError</span></code></a> should be raised. For mapping types, if <em>key</em> is missing (not in the container), <a class="reference internal" href="../library/exceptions.html#KeyError" title="KeyError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyError</span></code></a> should be raised.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last"><a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> loops expect that an <a class="reference internal" href="../library/exceptions.html#IndexError" title="IndexError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">IndexError</span></code></a> will be raised for illegal indexes to allow proper detection of the end of the sequence.</p> </div> </dd></dl> <dl class="method"> <dt id="object.__missing__"> <code class="descclassname">object.</code><code class="descname">__missing__</code><span class="sig-paren">(</span><em>self</em>, <em>key</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__missing__" title="Permalink to this definition">¶</a></dt> <dd><p>Called by <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate"><span class="pre">dict</span></code></a>.<a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> to implement <code class="docutils literal notranslate"><span class="pre">self[key]</span></code> for dict subclasses when key is not in the dictionary.</p> </dd></dl> <dl class="method"> <dt id="object.__setitem__"> <code class="descclassname">object.</code><code class="descname">__setitem__</code><span class="sig-paren">(</span><em>self</em>, <em>key</em>, <em>value</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__setitem__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to implement assignment to <code class="docutils literal notranslate"><span class="pre">self[key]</span></code>. Same note as for <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>. This should only be implemented for mappings if the objects support changes to the values for keys, or if new keys can be added, or for sequences if elements can be replaced. The same exceptions should be raised for improper <em>key</em> values as for the <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> method.</p> </dd></dl> <dl class="method"> <dt id="object.__delitem__"> <code class="descclassname">object.</code><code class="descname">__delitem__</code><span class="sig-paren">(</span><em>self</em>, <em>key</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__delitem__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to implement deletion of <code class="docutils literal notranslate"><span class="pre">self[key]</span></code>. Same note as for <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>. This should only be implemented for mappings if the objects support removal of keys, or for sequences if elements can be removed from the sequence. The same exceptions should be raised for improper <em>key</em> values as for the <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> method.</p> </dd></dl> <dl class="method"> <dt id="object.__iter__"> <code class="descclassname">object.</code><code class="descname">__iter__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__iter__" title="Permalink to this definition">¶</a></dt> <dd><p>This method is called when an iterator is required for a container. This method should return a new iterator object that can iterate over all the objects in the container. For mappings, it should iterate over the keys of the container.</p> <p>Iterator objects also need to implement this method; they are required to return themselves. For more information on iterator objects, see <a class="reference internal" href="../library/stdtypes.html#typeiter"><span class="std std-ref">Iterator Types</span></a>.</p> </dd></dl> <dl class="method"> <dt id="object.__reversed__"> <code class="descclassname">object.</code><code class="descname">__reversed__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__reversed__" title="Permalink to this definition">¶</a></dt> <dd><p>Called (if present) by the <a class="reference internal" href="../library/functions.html#reversed" title="reversed"><code class="xref py py-func docutils literal notranslate"><span class="pre">reversed()</span></code></a> built-in to implement reverse iteration. It should return a new iterator object that iterates over all the objects in the container in reverse order.</p> <p>If the <a class="reference internal" href="#object.__reversed__" title="object.__reversed__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__reversed__()</span></code></a> method is not provided, the <a class="reference internal" href="../library/functions.html#reversed" title="reversed"><code class="xref py py-func docutils literal notranslate"><span class="pre">reversed()</span></code></a> built-in will fall back to using the sequence protocol (<a class="reference internal" href="#object.__len__" title="object.__len__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__len__()</span></code></a> and <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>). Objects that support the sequence protocol should only provide <a class="reference internal" href="#object.__reversed__" title="object.__reversed__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__reversed__()</span></code></a> if they can provide an implementation that is more efficient than the one provided by <a class="reference internal" href="../library/functions.html#reversed" title="reversed"><code class="xref py py-func docutils literal notranslate"><span class="pre">reversed()</span></code></a>.</p> </dd></dl> <p>The membership test operators (<a class="reference internal" href="expressions.html#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a> and <a class="reference internal" href="expressions.html#not-in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">not</span> <span class="pre">in</span></code></a>) are normally implemented as an iteration through a sequence. However, container objects can supply the following special method with a more efficient implementation, which also does not require the object be a sequence.</p> <dl class="method"> <dt id="object.__contains__"> <code class="descclassname">object.</code><code class="descname">__contains__</code><span class="sig-paren">(</span><em>self</em>, <em>item</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__contains__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to implement membership test operators. Should return true if <em>item</em> is in <em>self</em>, false otherwise. For mapping objects, this should consider the keys of the mapping rather than the values or the key-item pairs.</p> <p>For objects that don’t define <a class="reference internal" href="#object.__contains__" title="object.__contains__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__contains__()</span></code></a>, the membership test first tries iteration via <a class="reference internal" href="#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a>, then the old sequence iteration protocol via <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>, see <a class="reference internal" href="expressions.html#membership-test-details"><span class="std std-ref">this section in the language reference</span></a>.</p> </dd></dl> </div> <div class="section" id="emulating-numeric-types"> <span id="numeric-types"></span><h3>3.3.7. Emulating numeric types<a class="headerlink" href="#emulating-numeric-types" title="Permalink to this headline">¶</a></h3> <p>The following methods can be defined to emulate numeric objects. Methods corresponding to operations that are not supported by the particular kind of number implemented (e.g., bitwise operations for non-integral numbers) should be left undefined.</p> <dl class="method"> <dt id="object.__add__"> <code class="descclassname">object.</code><code class="descname">__add__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__add__" title="Permalink to this definition">¶</a></dt> <dt id="object.__sub__"> <code class="descclassname">object.</code><code class="descname">__sub__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__sub__" title="Permalink to this definition">¶</a></dt> <dt id="object.__mul__"> <code class="descclassname">object.</code><code class="descname">__mul__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__mul__" title="Permalink to this definition">¶</a></dt> <dt id="object.__matmul__"> <code class="descclassname">object.</code><code class="descname">__matmul__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__matmul__" title="Permalink to this definition">¶</a></dt> <dt id="object.__truediv__"> <code class="descclassname">object.</code><code class="descname">__truediv__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__truediv__" title="Permalink to this definition">¶</a></dt> <dt id="object.__floordiv__"> <code class="descclassname">object.</code><code class="descname">__floordiv__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__floordiv__" title="Permalink to this definition">¶</a></dt> <dt id="object.__mod__"> <code class="descclassname">object.</code><code class="descname">__mod__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__mod__" title="Permalink to this definition">¶</a></dt> <dt id="object.__divmod__"> <code class="descclassname">object.</code><code class="descname">__divmod__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__divmod__" title="Permalink to this definition">¶</a></dt> <dt id="object.__pow__"> <code class="descclassname">object.</code><code class="descname">__pow__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="optional">[</span>, <em>modulo</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#object.__pow__" title="Permalink to this definition">¶</a></dt> <dt id="object.__lshift__"> <code class="descclassname">object.</code><code class="descname">__lshift__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__lshift__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rshift__"> <code class="descclassname">object.</code><code class="descname">__rshift__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rshift__" title="Permalink to this definition">¶</a></dt> <dt id="object.__and__"> <code class="descclassname">object.</code><code class="descname">__and__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__and__" title="Permalink to this definition">¶</a></dt> <dt id="object.__xor__"> <code class="descclassname">object.</code><code class="descname">__xor__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__xor__" title="Permalink to this definition">¶</a></dt> <dt id="object.__or__"> <code class="descclassname">object.</code><code class="descname">__or__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__or__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-90">These methods are called to implement the binary arithmetic operations (<code class="docutils literal notranslate"><span class="pre">+</span></code>, <code class="docutils literal notranslate"><span class="pre">-</span></code>, <code class="docutils literal notranslate"><span class="pre">*</span></code>, <code class="docutils literal notranslate"><span class="pre">@</span></code>, <code class="docutils literal notranslate"><span class="pre">/</span></code>, <code class="docutils literal notranslate"><span class="pre">//</span></code>, <code class="docutils literal notranslate"><span class="pre">%</span></code>, <a class="reference internal" href="../library/functions.html#divmod" title="divmod"><code class="xref py py-func docutils literal notranslate"><span class="pre">divmod()</span></code></a>, <a class="reference internal" href="../library/functions.html#pow" title="pow"><code class="xref py py-func docutils literal notranslate"><span class="pre">pow()</span></code></a>, <code class="docutils literal notranslate"><span class="pre">**</span></code>, <code class="docutils literal notranslate"><span class="pre"><<</span></code>, <code class="docutils literal notranslate"><span class="pre">>></span></code>, <code class="docutils literal notranslate"><span class="pre">&</span></code>, <code class="docutils literal notranslate"><span class="pre">^</span></code>, <code class="docutils literal notranslate"><span class="pre">|</span></code>). For instance, to evaluate the expression <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">+</span> <span class="pre">y</span></code>, where <em>x</em> is an instance of a class that has an <a class="reference internal" href="#object.__add__" title="object.__add__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__add__()</span></code></a> method, <code class="docutils literal notranslate"><span class="pre">x.__add__(y)</span></code> is called. The <a class="reference internal" href="#object.__divmod__" title="object.__divmod__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__divmod__()</span></code></a> method should be the equivalent to using <a class="reference internal" href="#object.__floordiv__" title="object.__floordiv__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__floordiv__()</span></code></a> and <a class="reference internal" href="#object.__mod__" title="object.__mod__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__mod__()</span></code></a>; it should not be related to <a class="reference internal" href="#object.__truediv__" title="object.__truediv__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__truediv__()</span></code></a>. Note that <a class="reference internal" href="#object.__pow__" title="object.__pow__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__pow__()</span></code></a> should be defined to accept an optional third argument if the ternary version of the built-in <a class="reference internal" href="../library/functions.html#pow" title="pow"><code class="xref py py-func docutils literal notranslate"><span class="pre">pow()</span></code></a> function is to be supported.</p> <p>If one of those methods does not support the operation with the supplied arguments, it should return <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>.</p> </dd></dl> <dl class="method"> <dt id="object.__radd__"> <code class="descclassname">object.</code><code class="descname">__radd__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__radd__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rsub__"> <code class="descclassname">object.</code><code class="descname">__rsub__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rsub__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rmul__"> <code class="descclassname">object.</code><code class="descname">__rmul__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rmul__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rmatmul__"> <code class="descclassname">object.</code><code class="descname">__rmatmul__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rmatmul__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rtruediv__"> <code class="descclassname">object.</code><code class="descname">__rtruediv__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rtruediv__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rfloordiv__"> <code class="descclassname">object.</code><code class="descname">__rfloordiv__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rfloordiv__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rmod__"> <code class="descclassname">object.</code><code class="descname">__rmod__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rmod__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rdivmod__"> <code class="descclassname">object.</code><code class="descname">__rdivmod__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rdivmod__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rpow__"> <code class="descclassname">object.</code><code class="descname">__rpow__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rpow__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rlshift__"> <code class="descclassname">object.</code><code class="descname">__rlshift__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rlshift__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rrshift__"> <code class="descclassname">object.</code><code class="descname">__rrshift__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rrshift__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rand__"> <code class="descclassname">object.</code><code class="descname">__rand__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rand__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rxor__"> <code class="descclassname">object.</code><code class="descname">__rxor__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rxor__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ror__"> <code class="descclassname">object.</code><code class="descname">__ror__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__ror__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-91">These methods are called to implement the binary arithmetic operations (<code class="docutils literal notranslate"><span class="pre">+</span></code>, <code class="docutils literal notranslate"><span class="pre">-</span></code>, <code class="docutils literal notranslate"><span class="pre">*</span></code>, <code class="docutils literal notranslate"><span class="pre">@</span></code>, <code class="docutils literal notranslate"><span class="pre">/</span></code>, <code class="docutils literal notranslate"><span class="pre">//</span></code>, <code class="docutils literal notranslate"><span class="pre">%</span></code>, <a class="reference internal" href="../library/functions.html#divmod" title="divmod"><code class="xref py py-func docutils literal notranslate"><span class="pre">divmod()</span></code></a>, <a class="reference internal" href="../library/functions.html#pow" title="pow"><code class="xref py py-func docutils literal notranslate"><span class="pre">pow()</span></code></a>, <code class="docutils literal notranslate"><span class="pre">**</span></code>, <code class="docutils literal notranslate"><span class="pre"><<</span></code>, <code class="docutils literal notranslate"><span class="pre">>></span></code>, <code class="docutils literal notranslate"><span class="pre">&</span></code>, <code class="docutils literal notranslate"><span class="pre">^</span></code>, <code class="docutils literal notranslate"><span class="pre">|</span></code>) with reflected (swapped) operands. These functions are only called if the left operand does not support the corresponding operation <a class="footnote-reference" href="#id10" id="id5">[3]</a> and the operands are of different types. <a class="footnote-reference" href="#id11" id="id6">[4]</a> For instance, to evaluate the expression <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">-</span> <span class="pre">y</span></code>, where <em>y</em> is an instance of a class that has an <a class="reference internal" href="#object.__rsub__" title="object.__rsub__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__rsub__()</span></code></a> method, <code class="docutils literal notranslate"><span class="pre">y.__rsub__(x)</span></code> is called if <code class="docutils literal notranslate"><span class="pre">x.__sub__(y)</span></code> returns <em>NotImplemented</em>.</p> <p id="index-92">Note that ternary <a class="reference internal" href="../library/functions.html#pow" title="pow"><code class="xref py py-func docutils literal notranslate"><span class="pre">pow()</span></code></a> will not try calling <a class="reference internal" href="#object.__rpow__" title="object.__rpow__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__rpow__()</span></code></a> (the coercion rules would become too complicated).</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">If the right operand’s type is a subclass of the left operand’s type and that subclass provides the reflected method for the operation, this method will be called before the left operand’s non-reflected method. This behavior allows subclasses to override their ancestors’ operations.</p> </div> </dd></dl> <dl class="method"> <dt id="object.__iadd__"> <code class="descclassname">object.</code><code class="descname">__iadd__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__iadd__" title="Permalink to this definition">¶</a></dt> <dt id="object.__isub__"> <code class="descclassname">object.</code><code class="descname">__isub__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__isub__" title="Permalink to this definition">¶</a></dt> <dt id="object.__imul__"> <code class="descclassname">object.</code><code class="descname">__imul__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__imul__" title="Permalink to this definition">¶</a></dt> <dt id="object.__imatmul__"> <code class="descclassname">object.</code><code class="descname">__imatmul__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__imatmul__" title="Permalink to this definition">¶</a></dt> <dt id="object.__itruediv__"> <code class="descclassname">object.</code><code class="descname">__itruediv__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__itruediv__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ifloordiv__"> <code class="descclassname">object.</code><code class="descname">__ifloordiv__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__ifloordiv__" title="Permalink to this definition">¶</a></dt> <dt id="object.__imod__"> <code class="descclassname">object.</code><code class="descname">__imod__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__imod__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ipow__"> <code class="descclassname">object.</code><code class="descname">__ipow__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="optional">[</span>, <em>modulo</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#object.__ipow__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ilshift__"> <code class="descclassname">object.</code><code class="descname">__ilshift__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__ilshift__" title="Permalink to this definition">¶</a></dt> <dt id="object.__irshift__"> <code class="descclassname">object.</code><code class="descname">__irshift__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__irshift__" title="Permalink to this definition">¶</a></dt> <dt id="object.__iand__"> <code class="descclassname">object.</code><code class="descname">__iand__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__iand__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ixor__"> <code class="descclassname">object.</code><code class="descname">__ixor__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__ixor__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ior__"> <code class="descclassname">object.</code><code class="descname">__ior__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__ior__" title="Permalink to this definition">¶</a></dt> <dd><p>These methods are called to implement the augmented arithmetic assignments (<code class="docutils literal notranslate"><span class="pre">+=</span></code>, <code class="docutils literal notranslate"><span class="pre">-=</span></code>, <code class="docutils literal notranslate"><span class="pre">*=</span></code>, <code class="docutils literal notranslate"><span class="pre">@=</span></code>, <code class="docutils literal notranslate"><span class="pre">/=</span></code>, <code class="docutils literal notranslate"><span class="pre">//=</span></code>, <code class="docutils literal notranslate"><span class="pre">%=</span></code>, <code class="docutils literal notranslate"><span class="pre">**=</span></code>, <code class="docutils literal notranslate"><span class="pre"><<=</span></code>, <code class="docutils literal notranslate"><span class="pre">>>=</span></code>, <code class="docutils literal notranslate"><span class="pre">&=</span></code>, <code class="docutils literal notranslate"><span class="pre">^=</span></code>, <code class="docutils literal notranslate"><span class="pre">|=</span></code>). These methods should attempt to do the operation in-place (modifying <em>self</em>) and return the result (which could be, but does not have to be, <em>self</em>). If a specific method is not defined, the augmented assignment falls back to the normal methods. For instance, if <em>x</em> is an instance of a class with an <a class="reference internal" href="#object.__iadd__" title="object.__iadd__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iadd__()</span></code></a> method, <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">+=</span> <span class="pre">y</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">=</span> <span class="pre">x.__iadd__(y)</span></code> . Otherwise, <code class="docutils literal notranslate"><span class="pre">x.__add__(y)</span></code> and <code class="docutils literal notranslate"><span class="pre">y.__radd__(x)</span></code> are considered, as with the evaluation of <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">+</span> <span class="pre">y</span></code>. In certain situations, augmented assignment can result in unexpected errors (see <a class="reference internal" href="../faq/programming.html#faq-augmented-assignment-tuple-error"><span class="std std-ref">Why does a_tuple[i] += [‘item’] raise an exception when the addition works?</span></a>), but this behavior is in fact part of the data model.</p> </dd></dl> <dl class="method"> <dt id="object.__neg__"> <code class="descclassname">object.</code><code class="descname">__neg__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__neg__" title="Permalink to this definition">¶</a></dt> <dt id="object.__pos__"> <code class="descclassname">object.</code><code class="descname">__pos__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__pos__" title="Permalink to this definition">¶</a></dt> <dt id="object.__abs__"> <code class="descclassname">object.</code><code class="descname">__abs__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__abs__" title="Permalink to this definition">¶</a></dt> <dt id="object.__invert__"> <code class="descclassname">object.</code><code class="descname">__invert__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__invert__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-93">Called to implement the unary arithmetic operations (<code class="docutils literal notranslate"><span class="pre">-</span></code>, <code class="docutils literal notranslate"><span class="pre">+</span></code>, <a class="reference internal" href="../library/functions.html#abs" title="abs"><code class="xref py py-func docutils literal notranslate"><span class="pre">abs()</span></code></a> and <code class="docutils literal notranslate"><span class="pre">~</span></code>).</p> </dd></dl> <dl class="method"> <dt id="object.__complex__"> <code class="descclassname">object.</code><code class="descname">__complex__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__complex__" title="Permalink to this definition">¶</a></dt> <dt id="object.__int__"> <code class="descclassname">object.</code><code class="descname">__int__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__int__" title="Permalink to this definition">¶</a></dt> <dt id="object.__float__"> <code class="descclassname">object.</code><code class="descname">__float__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__float__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-94">Called to implement the built-in functions <a class="reference internal" href="../library/functions.html#complex" title="complex"><code class="xref py py-func docutils literal notranslate"><span class="pre">complex()</span></code></a>, <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-func docutils literal notranslate"><span class="pre">int()</span></code></a> and <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-func docutils literal notranslate"><span class="pre">float()</span></code></a>. Should return a value of the appropriate type.</p> </dd></dl> <dl class="method"> <dt id="object.__index__"> <code class="descclassname">object.</code><code class="descname">__index__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__index__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to implement <a class="reference internal" href="../library/operator.html#operator.index" title="operator.index"><code class="xref py py-func docutils literal notranslate"><span class="pre">operator.index()</span></code></a>, and whenever Python needs to losslessly convert the numeric object to an integer object (such as in slicing, or in the built-in <a class="reference internal" href="../library/functions.html#bin" title="bin"><code class="xref py py-func docutils literal notranslate"><span class="pre">bin()</span></code></a>, <a class="reference internal" href="../library/functions.html#hex" title="hex"><code class="xref py py-func docutils literal notranslate"><span class="pre">hex()</span></code></a> and <a class="reference internal" href="../library/functions.html#oct" title="oct"><code class="xref py py-func docutils literal notranslate"><span class="pre">oct()</span></code></a> functions). Presence of this method indicates that the numeric object is an integer type. Must return an integer.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">In order to have a coherent integer type class, when <a class="reference internal" href="#object.__index__" title="object.__index__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__index__()</span></code></a> is defined <a class="reference internal" href="#object.__int__" title="object.__int__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__int__()</span></code></a> should also be defined, and both should return the same value.</p> </div> </dd></dl> <dl class="method"> <dt id="object.__round__"> <code class="descclassname">object.</code><code class="descname">__round__</code><span class="sig-paren">(</span><em>self</em><span class="optional">[</span>, <em>ndigits</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#object.__round__" title="Permalink to this definition">¶</a></dt> <dt id="object.__trunc__"> <code class="descclassname">object.</code><code class="descname">__trunc__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__trunc__" title="Permalink to this definition">¶</a></dt> <dt id="object.__floor__"> <code class="descclassname">object.</code><code class="descname">__floor__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__floor__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ceil__"> <code class="descclassname">object.</code><code class="descname">__ceil__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__ceil__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-95">Called to implement the built-in function <a class="reference internal" href="../library/functions.html#round" title="round"><code class="xref py py-func docutils literal notranslate"><span class="pre">round()</span></code></a> and <a class="reference internal" href="../library/math.html#module-math" title="math: Mathematical functions (sin() etc.)."><code class="xref py py-mod docutils literal notranslate"><span class="pre">math</span></code></a> functions <a class="reference internal" href="../library/math.html#math.trunc" title="math.trunc"><code class="xref py py-func docutils literal notranslate"><span class="pre">trunc()</span></code></a>, <a class="reference internal" href="../library/math.html#math.floor" title="math.floor"><code class="xref py py-func docutils literal notranslate"><span class="pre">floor()</span></code></a> and <a class="reference internal" href="../library/math.html#math.ceil" title="math.ceil"><code class="xref py py-func docutils literal notranslate"><span class="pre">ceil()</span></code></a>. Unless <em>ndigits</em> is passed to <code class="xref py py-meth docutils literal notranslate"><span class="pre">__round__()</span></code> all these methods should return the value of the object truncated to an <a class="reference internal" href="../library/numbers.html#numbers.Integral" title="numbers.Integral"><code class="xref py py-class docutils literal notranslate"><span class="pre">Integral</span></code></a> (typically an <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a>).</p> <p>If <a class="reference internal" href="#object.__int__" title="object.__int__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__int__()</span></code></a> is not defined then the built-in function <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-func docutils literal notranslate"><span class="pre">int()</span></code></a> falls back to <a class="reference internal" href="#object.__trunc__" title="object.__trunc__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__trunc__()</span></code></a>.</p> </dd></dl> </div> <div class="section" id="with-statement-context-managers"> <span id="context-managers"></span><h3>3.3.8. With Statement Context Managers<a class="headerlink" href="#with-statement-context-managers" title="Permalink to this headline">¶</a></h3> <p>A <em class="dfn">context manager</em> is an object that defines the runtime context to be established when executing a <a class="reference internal" href="compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement. The context manager handles the entry into, and the exit from, the desired runtime context for the execution of the block of code. Context managers are normally invoked using the <a class="reference internal" href="compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement (described in section <a class="reference internal" href="compound_stmts.html#with"><span class="std std-ref">The with statement</span></a>), but can also be used by directly invoking their methods.</p> <p id="index-96">Typical uses of context managers include saving and restoring various kinds of global state, locking and unlocking resources, closing opened files, etc.</p> <p>For more information on context managers, see <a class="reference internal" href="../library/stdtypes.html#typecontextmanager"><span class="std std-ref">Context Manager Types</span></a>.</p> <dl class="method"> <dt id="object.__enter__"> <code class="descclassname">object.</code><code class="descname">__enter__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__enter__" title="Permalink to this definition">¶</a></dt> <dd><p>Enter the runtime context related to this object. The <a class="reference internal" href="compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement will bind this method’s return value to the target(s) specified in the <a class="reference internal" href="compound_stmts.html#as"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">as</span></code></a> clause of the statement, if any.</p> </dd></dl> <dl class="method"> <dt id="object.__exit__"> <code class="descclassname">object.</code><code class="descname">__exit__</code><span class="sig-paren">(</span><em>self</em>, <em>exc_type</em>, <em>exc_value</em>, <em>traceback</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__exit__" title="Permalink to this definition">¶</a></dt> <dd><p>Exit the runtime context related to this object. The parameters describe the exception that caused the context to be exited. If the context was exited without an exception, all three arguments will be <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a>.</p> <p>If an exception is supplied, and the method wishes to suppress the exception (i.e., prevent it from being propagated), it should return a true value. Otherwise, the exception will be processed normally upon exit from this method.</p> <p>Note that <a class="reference internal" href="#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> methods should not reraise the passed-in exception; this is the caller’s responsibility.</p> </dd></dl> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><span class="target" id="index-97"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0343"><strong>PEP 343</strong></a> - The “with” statement</dt> <dd>The specification, background, and examples for the Python <a class="reference internal" href="compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement.</dd> </dl> </div> </div> <div class="section" id="special-method-lookup"> <span id="special-lookup"></span><h3>3.3.9. Special method lookup<a class="headerlink" href="#special-method-lookup" title="Permalink to this headline">¶</a></h3> <p>For custom classes, implicit invocations of special methods are only guaranteed to work correctly if defined on an object’s type, not in the object’s instance dictionary. That behaviour is the reason why the following code raises an exception:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span> <span class="gp">... </span> <span class="k">pass</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">c</span> <span class="o">=</span> <span class="n">C</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">c</span><span class="o">.</span><span class="fm">__len__</span> <span class="o">=</span> <span class="k">lambda</span><span class="p">:</span> <span class="mi">5</span> <span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">c</span><span class="p">)</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span> <span class="gr">TypeError</span>: <span class="n">object of type 'C' has no len()</span> </pre></div> </div> <p>The rationale behind this behaviour lies with a number of special methods such as <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> and <a class="reference internal" href="#object.__repr__" title="object.__repr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__repr__()</span></code></a> that are implemented by all objects, including type objects. If the implicit lookup of these methods used the conventional lookup process, they would fail when invoked on the type object itself:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="mi">1</span> <span class="o">.</span><span class="fm">__hash__</span><span class="p">()</span> <span class="o">==</span> <span class="nb">hash</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="go">True</span> <span class="gp">>>> </span><span class="nb">int</span><span class="o">.</span><span class="fm">__hash__</span><span class="p">()</span> <span class="o">==</span> <span class="nb">hash</span><span class="p">(</span><span class="nb">int</span><span class="p">)</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span> <span class="gr">TypeError</span>: <span class="n">descriptor '__hash__' of 'int' object needs an argument</span> </pre></div> </div> <p>Incorrectly attempting to invoke an unbound method of a class in this way is sometimes referred to as ‘metaclass confusion’, and is avoided by bypassing the instance when looking up special methods:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">type</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span><span class="o">.</span><span class="fm">__hash__</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="o">==</span> <span class="nb">hash</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="go">True</span> <span class="gp">>>> </span><span class="nb">type</span><span class="p">(</span><span class="nb">int</span><span class="p">)</span><span class="o">.</span><span class="fm">__hash__</span><span class="p">(</span><span class="nb">int</span><span class="p">)</span> <span class="o">==</span> <span class="nb">hash</span><span class="p">(</span><span class="nb">int</span><span class="p">)</span> <span class="go">True</span> </pre></div> </div> <p>In addition to bypassing any instance attributes in the interest of correctness, implicit special method lookup generally also bypasses the <a class="reference internal" href="#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a> method even of the object’s metaclass:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">Meta</span><span class="p">(</span><span class="nb">type</span><span class="p">):</span> <span class="gp">... </span> <span class="k">def</span> <span class="nf">__getattribute__</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">):</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Metaclass getattribute invoked"</span><span class="p">)</span> <span class="gp">... </span> <span class="k">return</span> <span class="nb">type</span><span class="o">.</span><span class="fm">__getattribute__</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">)</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="nb">object</span><span class="p">,</span> <span class="n">metaclass</span><span class="o">=</span><span class="n">Meta</span><span class="p">):</span> <span class="gp">... </span> <span class="k">def</span> <span class="nf">__len__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="gp">... </span> <span class="k">return</span> <span class="mi">10</span> <span class="gp">... </span> <span class="k">def</span> <span class="nf">__getattribute__</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">):</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Class getattribute invoked"</span><span class="p">)</span> <span class="gp">... </span> <span class="k">return</span> <span class="nb">object</span><span class="o">.</span><span class="fm">__getattribute__</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">)</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">c</span> <span class="o">=</span> <span class="n">C</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">c</span><span class="o">.</span><span class="fm">__len__</span><span class="p">()</span> <span class="c1"># Explicit lookup via instance</span> <span class="go">Class getattribute invoked</span> <span class="go">10</span> <span class="gp">>>> </span><span class="nb">type</span><span class="p">(</span><span class="n">c</span><span class="p">)</span><span class="o">.</span><span class="fm">__len__</span><span class="p">(</span><span class="n">c</span><span class="p">)</span> <span class="c1"># Explicit lookup via type</span> <span class="go">Metaclass getattribute invoked</span> <span class="go">10</span> <span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">c</span><span class="p">)</span> <span class="c1"># Implicit lookup</span> <span class="go">10</span> </pre></div> </div> <p>Bypassing the <a class="reference internal" href="#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a> machinery in this fashion provides significant scope for speed optimisations within the interpreter, at the cost of some flexibility in the handling of special methods (the special method <em>must</em> be set on the class object itself in order to be consistently invoked by the interpreter).</p> </div> </div> <div class="section" id="coroutines"> <span id="index-98"></span><h2>3.4. Coroutines<a class="headerlink" href="#coroutines" title="Permalink to this headline">¶</a></h2> <div class="section" id="awaitable-objects"> <h3>3.4.1. Awaitable Objects<a class="headerlink" href="#awaitable-objects" title="Permalink to this headline">¶</a></h3> <p>An <a class="reference internal" href="../glossary.html#term-awaitable"><span class="xref std std-term">awaitable</span></a> object generally implements an <a class="reference internal" href="#object.__await__" title="object.__await__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__await__()</span></code></a> method. <a class="reference internal" href="../glossary.html#term-coroutine"><span class="xref std std-term">Coroutine</span></a> objects returned from <a class="reference internal" href="compound_stmts.html#async-def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">def</span></code></a> functions are awaitable.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">The <a class="reference internal" href="../glossary.html#term-generator-iterator"><span class="xref std std-term">generator iterator</span></a> objects returned from generators decorated with <a class="reference internal" href="../library/types.html#types.coroutine" title="types.coroutine"><code class="xref py py-func docutils literal notranslate"><span class="pre">types.coroutine()</span></code></a> or <a class="reference internal" href="../library/asyncio-task.html#asyncio.coroutine" title="asyncio.coroutine"><code class="xref py py-func docutils literal notranslate"><span class="pre">asyncio.coroutine()</span></code></a> are also awaitable, but they do not implement <a class="reference internal" href="#object.__await__" title="object.__await__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__await__()</span></code></a>.</p> </div> <dl class="method"> <dt id="object.__await__"> <code class="descclassname">object.</code><code class="descname">__await__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__await__" title="Permalink to this definition">¶</a></dt> <dd><p>Must return an <a class="reference internal" href="../glossary.html#term-iterator"><span class="xref std std-term">iterator</span></a>. Should be used to implement <a class="reference internal" href="../glossary.html#term-awaitable"><span class="xref std std-term">awaitable</span></a> objects. For instance, <a class="reference internal" href="../library/asyncio-task.html#asyncio.Future" title="asyncio.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">asyncio.Future</span></code></a> implements this method to be compatible with the <a class="reference internal" href="expressions.html#await"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">await</span></code></a> expression.</p> </dd></dl> <div class="versionadded"> <p><span class="versionmodified">New in version 3.5.</span></p> </div> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <p class="last"><span class="target" id="index-99"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0492"><strong>PEP 492</strong></a> for additional information about awaitable objects.</p> </div> </div> <div class="section" id="coroutine-objects"> <span id="id7"></span><h3>3.4.2. Coroutine Objects<a class="headerlink" href="#coroutine-objects" title="Permalink to this headline">¶</a></h3> <p><a class="reference internal" href="../glossary.html#term-coroutine"><span class="xref std std-term">Coroutine</span></a> objects are <a class="reference internal" href="../glossary.html#term-awaitable"><span class="xref std std-term">awaitable</span></a> objects. A coroutine’s execution can be controlled by calling <a class="reference internal" href="#object.__await__" title="object.__await__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__await__()</span></code></a> and iterating over the result. When the coroutine has finished executing and returns, the iterator raises <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a>, and the exception’s <code class="xref py py-attr docutils literal notranslate"><span class="pre">value</span></code> attribute holds the return value. If the coroutine raises an exception, it is propagated by the iterator. Coroutines should not directly raise unhandled <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a> exceptions.</p> <p>Coroutines also have the methods listed below, which are analogous to those of generators (see <a class="reference internal" href="expressions.html#generator-methods"><span class="std std-ref">Generator-iterator methods</span></a>). However, unlike generators, coroutines do not directly support iteration.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.5.2: </span>It is a <a class="reference internal" href="../library/exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeError</span></code></a> to await on a coroutine more than once.</p> </div> <dl class="method"> <dt id="coroutine.send"> <code class="descclassname">coroutine.</code><code class="descname">send</code><span class="sig-paren">(</span><em>value</em><span class="sig-paren">)</span><a class="headerlink" href="#coroutine.send" title="Permalink to this definition">¶</a></dt> <dd><p>Starts or resumes execution of the coroutine. If <em>value</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code>, this is equivalent to advancing the iterator returned by <a class="reference internal" href="#object.__await__" title="object.__await__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__await__()</span></code></a>. If <em>value</em> is not <code class="docutils literal notranslate"><span class="pre">None</span></code>, this method delegates to the <a class="reference internal" href="expressions.html#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code></a> method of the iterator that caused the coroutine to suspend. The result (return value, <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a>, or other exception) is the same as when iterating over the <a class="reference internal" href="#object.__await__" title="object.__await__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__await__()</span></code></a> return value, described above.</p> </dd></dl> <dl class="method"> <dt id="coroutine.throw"> <code class="descclassname">coroutine.</code><code class="descname">throw</code><span class="sig-paren">(</span><em>type</em><span class="optional">[</span>, <em>value</em><span class="optional">[</span>, <em>traceback</em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#coroutine.throw" title="Permalink to this definition">¶</a></dt> <dd><p>Raises the specified exception in the coroutine. This method delegates to the <a class="reference internal" href="expressions.html#generator.throw" title="generator.throw"><code class="xref py py-meth docutils literal notranslate"><span class="pre">throw()</span></code></a> method of the iterator that caused the coroutine to suspend, if it has such a method. Otherwise, the exception is raised at the suspension point. The result (return value, <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a>, or other exception) is the same as when iterating over the <a class="reference internal" href="#object.__await__" title="object.__await__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__await__()</span></code></a> return value, described above. If the exception is not caught in the coroutine, it propagates back to the caller.</p> </dd></dl> <dl class="method"> <dt id="coroutine.close"> <code class="descclassname">coroutine.</code><code class="descname">close</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#coroutine.close" title="Permalink to this definition">¶</a></dt> <dd><p>Causes the coroutine to clean itself up and exit. If the coroutine is suspended, this method first delegates to the <a class="reference internal" href="expressions.html#generator.close" title="generator.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code></a> method of the iterator that caused the coroutine to suspend, if it has such a method. Then it raises <a class="reference internal" href="../library/exceptions.html#GeneratorExit" title="GeneratorExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">GeneratorExit</span></code></a> at the suspension point, causing the coroutine to immediately clean itself up. Finally, the coroutine is marked as having finished executing, even if it was never started.</p> <p>Coroutine objects are automatically closed using the above process when they are about to be destroyed.</p> </dd></dl> </div> <div class="section" id="asynchronous-iterators"> <span id="async-iterators"></span><h3>3.4.3. Asynchronous Iterators<a class="headerlink" href="#asynchronous-iterators" title="Permalink to this headline">¶</a></h3> <p>An <em>asynchronous iterable</em> is able to call asynchronous code in its <code class="docutils literal notranslate"><span class="pre">__aiter__</span></code> implementation, and an <em>asynchronous iterator</em> can call asynchronous code in its <code class="docutils literal notranslate"><span class="pre">__anext__</span></code> method.</p> <p>Asynchronous iterators can be used in an <a class="reference internal" href="compound_stmts.html#async-for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code></a> statement.</p> <dl class="method"> <dt id="object.__aiter__"> <code class="descclassname">object.</code><code class="descname">__aiter__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__aiter__" title="Permalink to this definition">¶</a></dt> <dd><p>Must return an <em>asynchronous iterator</em> object.</p> </dd></dl> <dl class="method"> <dt id="object.__anext__"> <code class="descclassname">object.</code><code class="descname">__anext__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__anext__" title="Permalink to this definition">¶</a></dt> <dd><p>Must return an <em>awaitable</em> resulting in a next value of the iterator. Should raise a <a class="reference internal" href="../library/exceptions.html#StopAsyncIteration" title="StopAsyncIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopAsyncIteration</span></code></a> error when the iteration is over.</p> </dd></dl> <p>An example of an asynchronous iterable object:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Reader</span><span class="p">:</span> <span class="k">async</span> <span class="k">def</span> <span class="nf">readline</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="o">...</span> <span class="k">def</span> <span class="nf">__aiter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">return</span> <span class="bp">self</span> <span class="k">async</span> <span class="k">def</span> <span class="nf">__anext__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="n">val</span> <span class="o">=</span> <span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span> <span class="k">if</span> <span class="n">val</span> <span class="o">==</span> <span class="sa">b</span><span class="s1">''</span><span class="p">:</span> <span class="k">raise</span> <span class="n">StopAsyncIteration</span> <span class="k">return</span> <span class="n">val</span> </pre></div> </div> <div class="versionadded"> <p><span class="versionmodified">New in version 3.5.</span></p> </div> <div class="admonition note"> <p class="first admonition-title">Note</p> <div class="last versionchanged"> <p><span class="versionmodified">Changed in version 3.5.2: </span>Starting with CPython 3.5.2, <code class="docutils literal notranslate"><span class="pre">__aiter__</span></code> can directly return <a class="reference internal" href="../glossary.html#term-asynchronous-iterator"><span class="xref std std-term">asynchronous iterators</span></a>. Returning an <a class="reference internal" href="../glossary.html#term-awaitable"><span class="xref std std-term">awaitable</span></a> object will result in a <a class="reference internal" href="../library/exceptions.html#PendingDeprecationWarning" title="PendingDeprecationWarning"><code class="xref py py-exc docutils literal notranslate"><span class="pre">PendingDeprecationWarning</span></code></a>.</p> <p>The recommended way of writing backwards compatible code in CPython 3.5.x is to continue returning awaitables from <code class="docutils literal notranslate"><span class="pre">__aiter__</span></code>. If you want to avoid the PendingDeprecationWarning and keep the code backwards compatible, the following decorator can be used:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">functools</span> <span class="kn">import</span> <span class="nn">sys</span> <span class="k">if</span> <span class="n">sys</span><span class="o">.</span><span class="n">version_info</span> <span class="o"><</span> <span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">2</span><span class="p">):</span> <span class="k">def</span> <span class="nf">aiter_compat</span><span class="p">(</span><span class="n">func</span><span class="p">):</span> <span class="nd">@functools</span><span class="o">.</span><span class="n">wraps</span><span class="p">(</span><span class="n">func</span><span class="p">)</span> <span class="k">async</span> <span class="k">def</span> <span class="nf">wrapper</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">return</span> <span class="n">func</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span> <span class="k">return</span> <span class="n">wrapper</span> <span class="k">else</span><span class="p">:</span> <span class="k">def</span> <span class="nf">aiter_compat</span><span class="p">(</span><span class="n">func</span><span class="p">):</span> <span class="k">return</span> <span class="n">func</span> </pre></div> </div> <p>Example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">AsyncIterator</span><span class="p">:</span> <span class="nd">@aiter_compat</span> <span class="k">def</span> <span class="nf">__aiter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">return</span> <span class="bp">self</span> <span class="k">async</span> <span class="k">def</span> <span class="nf">__anext__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="o">...</span> </pre></div> </div> <p>Starting with CPython 3.6, the <a class="reference internal" href="../library/exceptions.html#PendingDeprecationWarning" title="PendingDeprecationWarning"><code class="xref py py-exc docutils literal notranslate"><span class="pre">PendingDeprecationWarning</span></code></a> will be replaced with the <a class="reference internal" href="../library/exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate"><span class="pre">DeprecationWarning</span></code></a>. In CPython 3.7, returning an awaitable from <code class="docutils literal notranslate"><span class="pre">__aiter__</span></code> will result in a <a class="reference internal" href="../library/exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeError</span></code></a>.</p> </div> </div> </div> <div class="section" id="asynchronous-context-managers"> <h3>3.4.4. Asynchronous Context Managers<a class="headerlink" href="#asynchronous-context-managers" title="Permalink to this headline">¶</a></h3> <p>An <em>asynchronous context manager</em> is a <em>context manager</em> that is able to suspend execution in its <code class="docutils literal notranslate"><span class="pre">__aenter__</span></code> and <code class="docutils literal notranslate"><span class="pre">__aexit__</span></code> methods.</p> <p>Asynchronous context managers can be used in an <a class="reference internal" href="compound_stmts.html#async-with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">with</span></code></a> statement.</p> <dl class="method"> <dt id="object.__aenter__"> <code class="descclassname">object.</code><code class="descname">__aenter__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__aenter__" title="Permalink to this definition">¶</a></dt> <dd><p>This method is semantically similar to the <a class="reference internal" href="#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a>, with only difference that it must return an <em>awaitable</em>.</p> </dd></dl> <dl class="method"> <dt id="object.__aexit__"> <code class="descclassname">object.</code><code class="descname">__aexit__</code><span class="sig-paren">(</span><em>self</em>, <em>exc_type</em>, <em>exc_value</em>, <em>traceback</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__aexit__" title="Permalink to this definition">¶</a></dt> <dd><p>This method is semantically similar to the <a class="reference internal" href="#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a>, with only difference that it must return an <em>awaitable</em>.</p> </dd></dl> <p>An example of an asynchronous context manager class:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">AsyncContextManager</span><span class="p">:</span> <span class="k">async</span> <span class="k">def</span> <span class="nf">__aenter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">await</span> <span class="n">log</span><span class="p">(</span><span class="s1">'entering context'</span><span class="p">)</span> <span class="k">async</span> <span class="k">def</span> <span class="nf">__aexit__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">exc_type</span><span class="p">,</span> <span class="n">exc</span><span class="p">,</span> <span class="n">tb</span><span class="p">):</span> <span class="k">await</span> <span class="n">log</span><span class="p">(</span><span class="s1">'exiting context'</span><span class="p">)</span> </pre></div> </div> <div class="versionadded"> <p><span class="versionmodified">New in version 3.5.</span></p> </div> <p class="rubric">Footnotes</p> <table class="docutils footnote" frame="void" id="id8" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>It <em>is</em> possible in some cases to change an object’s type, under certain controlled conditions. It generally isn’t a good idea though, since it can lead to some very strange behaviour if it is handled incorrectly.</td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id9" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id2">[2]</a></td><td>The <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a>, <a class="reference internal" href="#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a>, <a class="reference internal" href="#object.__reversed__" title="object.__reversed__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__reversed__()</span></code></a>, and <a class="reference internal" href="#object.__contains__" title="object.__contains__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__contains__()</span></code></a> methods have special handling for this; others will still raise a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>, but may do so by relying on the behavior that <code class="docutils literal notranslate"><span class="pre">None</span></code> is not callable.</td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id10" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id5">[3]</a></td><td>“Does not support” here means that the class has no such method, or the method returns <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>. Do not set the method to <code class="docutils literal notranslate"><span class="pre">None</span></code> if you want to force fallback to the right operand’s reflected method—that will instead have the opposite effect of explicitly <em>blocking</em> such fallback.</td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id11" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id6">[4]</a></td><td>For operands of the same type, it is assumed that if the non-reflected method (such as <a class="reference internal" href="#object.__add__" title="object.__add__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__add__()</span></code></a>) fails the operation is not supported, which is why the reflected method is not called.</td></tr> </tbody> </table> </div> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">3. Data model</a><ul> <li><a class="reference internal" href="#objects-values-and-types">3.1. Objects, values and types</a></li> <li><a class="reference internal" href="#the-standard-type-hierarchy">3.2. The standard type hierarchy</a></li> <li><a class="reference internal" href="#special-method-names">3.3. Special method names</a><ul> <li><a class="reference internal" href="#basic-customization">3.3.1. Basic customization</a></li> <li><a class="reference internal" href="#customizing-attribute-access">3.3.2. Customizing attribute access</a><ul> <li><a class="reference internal" href="#customizing-module-attribute-access">3.3.2.1. Customizing module attribute access</a></li> <li><a class="reference internal" href="#implementing-descriptors">3.3.2.2. Implementing Descriptors</a></li> <li><a class="reference internal" href="#invoking-descriptors">3.3.2.3. Invoking Descriptors</a></li> <li><a class="reference internal" href="#slots">3.3.2.4. __slots__</a><ul> <li><a class="reference internal" href="#notes-on-using-slots">3.3.2.4.1. Notes on using <em>__slots__</em></a></li> </ul> </li> </ul> </li> <li><a class="reference internal" href="#customizing-class-creation">3.3.3. Customizing class creation</a><ul> <li><a class="reference internal" href="#metaclasses">3.3.3.1. Metaclasses</a></li> <li><a class="reference internal" href="#determining-the-appropriate-metaclass">3.3.3.2. Determining the appropriate metaclass</a></li> <li><a class="reference internal" href="#preparing-the-class-namespace">3.3.3.3. Preparing the class namespace</a></li> <li><a class="reference internal" href="#executing-the-class-body">3.3.3.4. Executing the class body</a></li> <li><a class="reference internal" href="#creating-the-class-object">3.3.3.5. Creating the class object</a></li> <li><a class="reference internal" href="#metaclass-example">3.3.3.6. Metaclass example</a></li> </ul> </li> <li><a class="reference internal" href="#customizing-instance-and-subclass-checks">3.3.4. Customizing instance and subclass checks</a></li> <li><a class="reference internal" href="#emulating-callable-objects">3.3.5. Emulating callable objects</a></li> <li><a class="reference internal" href="#emulating-container-types">3.3.6. Emulating container types</a></li> <li><a class="reference internal" href="#emulating-numeric-types">3.3.7. Emulating numeric types</a></li> <li><a class="reference internal" href="#with-statement-context-managers">3.3.8. With Statement Context Managers</a></li> <li><a class="reference internal" href="#special-method-lookup">3.3.9. Special method lookup</a></li> </ul> </li> <li><a class="reference internal" href="#coroutines">3.4. Coroutines</a><ul> <li><a class="reference internal" href="#awaitable-objects">3.4.1. Awaitable Objects</a></li> <li><a class="reference internal" href="#coroutine-objects">3.4.2. Coroutine Objects</a></li> <li><a class="reference internal" href="#asynchronous-iterators">3.4.3. Asynchronous Iterators</a></li> <li><a class="reference internal" href="#asynchronous-context-managers">3.4.4. Asynchronous Context Managers</a></li> </ul> </li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="lexical_analysis.html" title="previous chapter">2. Lexical analysis</a></p> <h4>Next topic</h4> <p class="topless"><a href="executionmodel.html" title="next chapter">4. 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