D7net
Home
Console
Upload
information
Create File
Create Folder
About
Tools
:
/
usr
/
share
/
doc
/
python3-docs
/
html
/
reference
/
Filename :
expressions.html
back
Copy
<!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>6. Expressions — 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="7. Simple statements" href="simple_stmts.html" /> <link rel="prev" title="5. The import system" href="import.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/3/reference/expressions.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="simple_stmts.html" title="7. Simple statements" accesskey="N">next</a> |</li> <li class="right" > <a href="import.html" title="5. The import system" 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="expressions"> <span id="id1"></span><h1>6. Expressions<a class="headerlink" href="#expressions" title="Permalink to this headline">¶</a></h1> <p id="index-0">This chapter explains the meaning of the elements of expressions in Python.</p> <p><strong>Syntax Notes:</strong> In this and the following chapters, extended BNF notation will be used to describe syntax, not lexical analysis. When (one alternative of) a syntax rule has the form</p> <pre> <strong id="grammar-token-name">name</strong> ::= <code class="xref docutils literal notranslate"><span class="pre">othername</span></code> </pre> <p>and no semantics are given, the semantics of this form of <code class="docutils literal notranslate"><span class="pre">name</span></code> are the same as for <code class="docutils literal notranslate"><span class="pre">othername</span></code>.</p> <div class="section" id="arithmetic-conversions"> <span id="conversions"></span><h2>6.1. Arithmetic conversions<a class="headerlink" href="#arithmetic-conversions" title="Permalink to this headline">¶</a></h2> <p id="index-1">When a description of an arithmetic operator below uses the phrase “the numeric arguments are converted to a common type,” this means that the operator implementation for built-in types works as follows:</p> <ul class="simple"> <li>If either argument is a complex number, the other is converted to complex;</li> <li>otherwise, if either argument is a floating point number, the other is converted to floating point;</li> <li>otherwise, both must be integers and no conversion is necessary.</li> </ul> <p>Some additional rules apply for certain operators (e.g., a string as a left argument to the ‘%’ operator). Extensions must define their own conversion behavior.</p> </div> <div class="section" id="atoms"> <span id="id2"></span><h2>6.2. Atoms<a class="headerlink" href="#atoms" title="Permalink to this headline">¶</a></h2> <p id="index-2">Atoms are the most basic elements of expressions. The simplest atoms are identifiers or literals. Forms enclosed in parentheses, brackets or braces are also categorized syntactically as atoms. The syntax for atoms is:</p> <pre> <strong id="grammar-token-atom">atom </strong> ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> | <a class="reference internal" href="#grammar-token-literal"><code class="xref docutils literal notranslate"><span class="pre">literal</span></code></a> | <a class="reference internal" href="#grammar-token-enclosure"><code class="xref docutils literal notranslate"><span class="pre">enclosure</span></code></a> <strong id="grammar-token-enclosure">enclosure</strong> ::= <a class="reference internal" href="#grammar-token-parenth_form"><code class="xref docutils literal notranslate"><span class="pre">parenth_form</span></code></a> | <a class="reference internal" href="#grammar-token-list_display"><code class="xref docutils literal notranslate"><span class="pre">list_display</span></code></a> | <a class="reference internal" href="#grammar-token-dict_display"><code class="xref docutils literal notranslate"><span class="pre">dict_display</span></code></a> | <a class="reference internal" href="#grammar-token-set_display"><code class="xref docutils literal notranslate"><span class="pre">set_display</span></code></a> | <a class="reference internal" href="#grammar-token-generator_expression"><code class="xref docutils literal notranslate"><span class="pre">generator_expression</span></code></a> | <a class="reference internal" href="#grammar-token-yield_atom"><code class="xref docutils literal notranslate"><span class="pre">yield_atom</span></code></a> </pre> <div class="section" id="atom-identifiers"> <span id="identifiers-names"></span><h3>6.2.1. Identifiers (Names)<a class="headerlink" href="#atom-identifiers" title="Permalink to this headline">¶</a></h3> <p id="index-3">An identifier occurring as an atom is a name. See section <a class="reference internal" href="lexical_analysis.html#identifiers"><span class="std std-ref">Identifiers and keywords</span></a> for lexical definition and section <a class="reference internal" href="executionmodel.html#naming"><span class="std std-ref">Naming and binding</span></a> for documentation of naming and binding.</p> <p id="index-4">When the name is bound to an object, evaluation of the atom yields that object. When a name is not bound, an attempt to evaluate it raises a <a class="reference internal" href="../library/exceptions.html#NameError" title="NameError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">NameError</span></code></a> exception.</p> <p id="index-5"><strong>Private name mangling:</strong> When an identifier that textually occurs in a class definition begins with two or more underscore characters and does not end in two or more underscores, it is considered a <em class="dfn">private name</em> of that class. Private names are transformed to a longer form before code is generated for them. The transformation inserts the class name, with leading underscores removed and a single underscore inserted, in front of the name. For example, the identifier <code class="docutils literal notranslate"><span class="pre">__spam</span></code> occurring in a class named <code class="docutils literal notranslate"><span class="pre">Ham</span></code> will be transformed to <code class="docutils literal notranslate"><span class="pre">_Ham__spam</span></code>. This transformation is independent of the syntactical context in which the identifier is used. If the transformed name is extremely long (longer than 255 characters), implementation defined truncation may happen. If the class name consists only of underscores, no transformation is done.</p> </div> <div class="section" id="literals"> <span id="atom-literals"></span><h3>6.2.2. Literals<a class="headerlink" href="#literals" title="Permalink to this headline">¶</a></h3> <p id="index-6">Python supports string and bytes literals and various numeric literals:</p> <pre> <strong id="grammar-token-literal">literal</strong> ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-stringliteral"><code class="xref docutils literal notranslate"><span class="pre">stringliteral</span></code></a> | <a class="reference internal" href="lexical_analysis.html#grammar-token-bytesliteral"><code class="xref docutils literal notranslate"><span class="pre">bytesliteral</span></code></a> | <a class="reference internal" href="lexical_analysis.html#grammar-token-integer"><code class="xref docutils literal notranslate"><span class="pre">integer</span></code></a> | <a class="reference internal" href="lexical_analysis.html#grammar-token-floatnumber"><code class="xref docutils literal notranslate"><span class="pre">floatnumber</span></code></a> | <a class="reference internal" href="lexical_analysis.html#grammar-token-imagnumber"><code class="xref docutils literal notranslate"><span class="pre">imagnumber</span></code></a> </pre> <p>Evaluation of a literal yields an object of the given type (string, bytes, integer, floating point number, complex number) with the given value. The value may be approximated in the case of floating point and imaginary (complex) literals. See section <a class="reference internal" href="lexical_analysis.html#literals"><span class="std std-ref">Literals</span></a> for details.</p> <p id="index-7">All literals correspond to immutable data types, and hence the object’s identity is less important than its value. Multiple evaluations of literals with the same value (either the same occurrence in the program text or a different occurrence) may obtain the same object or a different object with the same value.</p> </div> <div class="section" id="parenthesized-forms"> <span id="parenthesized"></span><h3>6.2.3. Parenthesized forms<a class="headerlink" href="#parenthesized-forms" title="Permalink to this headline">¶</a></h3> <p id="index-8">A parenthesized form is an optional expression list enclosed in parentheses:</p> <pre> <strong id="grammar-token-parenth_form">parenth_form</strong> ::= "(" [<a class="reference internal" href="#grammar-token-starred_expression"><code class="xref docutils literal notranslate"><span class="pre">starred_expression</span></code></a>] ")" </pre> <p>A parenthesized expression list yields whatever that expression list yields: if the list contains at least one comma, it yields a tuple; otherwise, it yields the single expression that makes up the expression list.</p> <p id="index-9">An empty pair of parentheses yields an empty tuple object. Since tuples are immutable, the rules for literals apply (i.e., two occurrences of the empty tuple may or may not yield the same object).</p> <p id="index-10">Note that tuples are not formed by the parentheses, but rather by use of the comma operator. The exception is the empty tuple, for which parentheses <em>are</em> required — allowing unparenthesized “nothing” in expressions would cause ambiguities and allow common typos to pass uncaught.</p> </div> <div class="section" id="displays-for-lists-sets-and-dictionaries"> <span id="comprehensions"></span><h3>6.2.4. Displays for lists, sets and dictionaries<a class="headerlink" href="#displays-for-lists-sets-and-dictionaries" title="Permalink to this headline">¶</a></h3> <p>For constructing a list, a set or a dictionary Python provides special syntax called “displays”, each of them in two flavors:</p> <ul class="simple"> <li>either the container contents are listed explicitly, or</li> <li>they are computed via a set of looping and filtering instructions, called a <em class="dfn">comprehension</em>.</li> </ul> <p>Common syntax elements for comprehensions are:</p> <pre> <strong id="grammar-token-comprehension">comprehension</strong> ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> <a class="reference internal" href="#grammar-token-comp_for"><code class="xref docutils literal notranslate"><span class="pre">comp_for</span></code></a> <strong id="grammar-token-comp_for">comp_for </strong> ::= [ASYNC] "for" <a class="reference internal" href="simple_stmts.html#grammar-token-target_list"><code class="xref docutils literal notranslate"><span class="pre">target_list</span></code></a> "in" <a class="reference internal" href="#grammar-token-or_test"><code class="xref docutils literal notranslate"><span class="pre">or_test</span></code></a> [<a class="reference internal" href="#grammar-token-comp_iter"><code class="xref docutils literal notranslate"><span class="pre">comp_iter</span></code></a>] <strong id="grammar-token-comp_iter">comp_iter </strong> ::= <a class="reference internal" href="#grammar-token-comp_for"><code class="xref docutils literal notranslate"><span class="pre">comp_for</span></code></a> | <a class="reference internal" href="#grammar-token-comp_if"><code class="xref docutils literal notranslate"><span class="pre">comp_if</span></code></a> <strong id="grammar-token-comp_if">comp_if </strong> ::= "if" <a class="reference internal" href="#grammar-token-expression_nocond"><code class="xref docutils literal notranslate"><span class="pre">expression_nocond</span></code></a> [<a class="reference internal" href="#grammar-token-comp_iter"><code class="xref docutils literal notranslate"><span class="pre">comp_iter</span></code></a>] </pre> <p>The comprehension consists of a single expression followed by at least one <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> clause and zero or more <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> or <a class="reference internal" href="compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> clauses. In this case, the elements of the new container are those that would be produced by considering each of the <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> or <a class="reference internal" href="compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> clauses a block, nesting from left to right, and evaluating the expression to produce an element each time the innermost block is reached.</p> <p>Note that the comprehension is executed in a separate scope, so names assigned to in the target list don’t “leak” into the enclosing scope.</p> <p>Since Python 3.6, in an <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> function, 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> clause may be used to iterate over a <a class="reference internal" href="../glossary.html#term-asynchronous-iterator"><span class="xref std std-term">asynchronous iterator</span></a>. A comprehension in an <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> function may consist of either a <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> or <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> clause following the leading expression, may contain additional <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> or <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> clauses, and may also use <a class="reference internal" href="#await"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">await</span></code></a> expressions. If a comprehension contains either <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> clauses or <a class="reference internal" href="#await"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">await</span></code></a> expressions it is called an <em class="dfn">asynchronous comprehension</em>. An asynchronous comprehension may suspend the execution of the coroutine function in which it appears. See also <span class="target" id="index-11"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0530"><strong>PEP 530</strong></a>.</p> </div> <div class="section" id="list-displays"> <span id="lists"></span><h3>6.2.5. List displays<a class="headerlink" href="#list-displays" title="Permalink to this headline">¶</a></h3> <p id="index-12">A list display is a possibly empty series of expressions enclosed in square brackets:</p> <pre> <strong id="grammar-token-list_display">list_display</strong> ::= "[" [<a class="reference internal" href="#grammar-token-starred_list"><code class="xref docutils literal notranslate"><span class="pre">starred_list</span></code></a> | <a class="reference internal" href="#grammar-token-comprehension"><code class="xref docutils literal notranslate"><span class="pre">comprehension</span></code></a>] "]" </pre> <p>A list display yields a new list object, the contents being specified by either a list of expressions or a comprehension. When a comma-separated list of expressions is supplied, its elements are evaluated from left to right and placed into the list object in that order. When a comprehension is supplied, the list is constructed from the elements resulting from the comprehension.</p> </div> <div class="section" id="set-displays"> <span id="set"></span><h3>6.2.6. Set displays<a class="headerlink" href="#set-displays" title="Permalink to this headline">¶</a></h3> <p id="index-13">A set display is denoted by curly braces and distinguishable from dictionary displays by the lack of colons separating keys and values:</p> <pre> <strong id="grammar-token-set_display">set_display</strong> ::= "{" (<a class="reference internal" href="#grammar-token-starred_list"><code class="xref docutils literal notranslate"><span class="pre">starred_list</span></code></a> | <a class="reference internal" href="#grammar-token-comprehension"><code class="xref docutils literal notranslate"><span class="pre">comprehension</span></code></a>) "}" </pre> <p>A set display yields a new mutable set object, the contents being specified by either a sequence of expressions or a comprehension. When a comma-separated list of expressions is supplied, its elements are evaluated from left to right and added to the set object. When a comprehension is supplied, the set is constructed from the elements resulting from the comprehension.</p> <p>An empty set cannot be constructed with <code class="docutils literal notranslate"><span class="pre">{}</span></code>; this literal constructs an empty dictionary.</p> </div> <div class="section" id="dictionary-displays"> <span id="dict"></span><h3>6.2.7. Dictionary displays<a class="headerlink" href="#dictionary-displays" title="Permalink to this headline">¶</a></h3> <p id="index-14">A dictionary display is a possibly empty series of key/datum pairs enclosed in curly braces:</p> <pre> <strong id="grammar-token-dict_display">dict_display </strong> ::= "{" [<a class="reference internal" href="#grammar-token-key_datum_list"><code class="xref docutils literal notranslate"><span class="pre">key_datum_list</span></code></a> | <a class="reference internal" href="#grammar-token-dict_comprehension"><code class="xref docutils literal notranslate"><span class="pre">dict_comprehension</span></code></a>] "}" <strong id="grammar-token-key_datum_list">key_datum_list </strong> ::= <a class="reference internal" href="#grammar-token-key_datum"><code class="xref docutils literal notranslate"><span class="pre">key_datum</span></code></a> ("," <a class="reference internal" href="#grammar-token-key_datum"><code class="xref docutils literal notranslate"><span class="pre">key_datum</span></code></a>)* [","] <strong id="grammar-token-key_datum">key_datum </strong> ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> ":" <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> | "**" <a class="reference internal" href="#grammar-token-or_expr"><code class="xref docutils literal notranslate"><span class="pre">or_expr</span></code></a> <strong id="grammar-token-dict_comprehension">dict_comprehension</strong> ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> ":" <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> <a class="reference internal" href="#grammar-token-comp_for"><code class="xref docutils literal notranslate"><span class="pre">comp_for</span></code></a> </pre> <p>A dictionary display yields a new dictionary object.</p> <p>If a comma-separated sequence of key/datum pairs is given, they are evaluated from left to right to define the entries of the dictionary: each key object is used as a key into the dictionary to store the corresponding datum. This means that you can specify the same key multiple times in the key/datum list, and the final dictionary’s value for that key will be the last one given.</p> <p id="index-15">A double asterisk <code class="docutils literal notranslate"><span class="pre">**</span></code> denotes <em class="dfn">dictionary unpacking</em>. Its operand must be a <a class="reference internal" href="../glossary.html#term-mapping"><span class="xref std std-term">mapping</span></a>. Each mapping item is added to the new dictionary. Later values replace values already set by earlier key/datum pairs and earlier dictionary unpackings.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 3.5: </span>Unpacking into dictionary displays, originally proposed by <span class="target" id="index-16"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0448"><strong>PEP 448</strong></a>.</p> </div> <p>A dict comprehension, in contrast to list and set comprehensions, needs two expressions separated with a colon followed by the usual “for” and “if” clauses. When the comprehension is run, the resulting key and value elements are inserted in the new dictionary in the order they are produced.</p> <p id="index-17">Restrictions on the types of the key values are listed earlier in section <a class="reference internal" href="datamodel.html#types"><span class="std std-ref">The standard type hierarchy</span></a>. (To summarize, the key type should be <a class="reference internal" href="../glossary.html#term-hashable"><span class="xref std std-term">hashable</span></a>, which excludes all mutable objects.) Clashes between duplicate keys are not detected; the last datum (textually rightmost in the display) stored for a given key value prevails.</p> </div> <div class="section" id="generator-expressions"> <span id="genexpr"></span><h3>6.2.8. Generator expressions<a class="headerlink" href="#generator-expressions" title="Permalink to this headline">¶</a></h3> <p id="index-18">A generator expression is a compact generator notation in parentheses:</p> <pre> <strong id="grammar-token-generator_expression">generator_expression</strong> ::= "(" <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> <a class="reference internal" href="#grammar-token-comp_for"><code class="xref docutils literal notranslate"><span class="pre">comp_for</span></code></a> ")" </pre> <p>A generator expression yields a new generator object. Its syntax is the same as for comprehensions, except that it is enclosed in parentheses instead of brackets or curly braces.</p> <p>Variables used in the generator expression are evaluated lazily when the <a class="reference internal" href="#generator.__next__" title="generator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code></a> method is called for the generator object (in the same fashion as normal generators). However, the leftmost <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> clause is immediately evaluated, so that an error produced by it can be seen before any other possible error in the code that handles the generator expression. Subsequent <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> clauses cannot be evaluated immediately since they may depend on the previous <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> loop. For example: <code class="docutils literal notranslate"><span class="pre">(x*y</span> <span class="pre">for</span> <span class="pre">x</span> <span class="pre">in</span> <span class="pre">range(10)</span> <span class="pre">for</span> <span class="pre">y</span> <span class="pre">in</span> <span class="pre">bar(x))</span></code>.</p> <p>The parentheses can be omitted on calls with only one argument. See section <a class="reference internal" href="#calls"><span class="std std-ref">Calls</span></a> for details.</p> <p>Since Python 3.6, if the generator appears in an <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> function, then <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> clauses and <a class="reference internal" href="#await"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">await</span></code></a> expressions are permitted as with an asynchronous comprehension. If a generator expression contains either <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> clauses or <a class="reference internal" href="#await"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">await</span></code></a> expressions it is called an <em class="dfn">asynchronous generator expression</em>. An asynchronous generator expression yields a new asynchronous generator object, which is an asynchronous iterator (see <a class="reference internal" href="datamodel.html#async-iterators"><span class="std std-ref">Asynchronous Iterators</span></a>).</p> </div> <div class="section" id="yield-expressions"> <span id="yieldexpr"></span><h3>6.2.9. Yield expressions<a class="headerlink" href="#yield-expressions" title="Permalink to this headline">¶</a></h3> <pre id="index-19"> <strong id="grammar-token-yield_atom">yield_atom </strong> ::= "(" <a class="reference internal" href="#grammar-token-yield_expression"><code class="xref docutils literal notranslate"><span class="pre">yield_expression</span></code></a> ")" <strong id="grammar-token-yield_expression">yield_expression</strong> ::= "yield" [<a class="reference internal" href="#grammar-token-expression_list"><code class="xref docutils literal notranslate"><span class="pre">expression_list</span></code></a> | "from" <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>] </pre> <p>The yield expression is used when defining a <a class="reference internal" href="../glossary.html#term-generator"><span class="xref std std-term">generator</span></a> function or an <a class="reference internal" href="../glossary.html#term-asynchronous-generator"><span class="xref std std-term">asynchronous generator</span></a> function and thus can only be used in the body of a function definition. Using a yield expression in a function’s body causes that function to be a generator, and using it in an <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> function’s body causes that coroutine function to be an asynchronous generator. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">gen</span><span class="p">():</span> <span class="c1"># defines a generator function</span> <span class="k">yield</span> <span class="mi">123</span> <span class="k">async</span> <span class="k">def</span> <span class="nf">agen</span><span class="p">():</span> <span class="c1"># defines an asynchronous generator function (PEP 525)</span> <span class="k">yield</span> <span class="mi">123</span> </pre></div> </div> <p>Generator functions are described below, while asynchronous generator functions are described separately in section <a class="reference internal" href="#asynchronous-generator-functions"><span class="std std-ref">Asynchronous generator functions</span></a>.</p> <p>When a generator function is called, it returns an iterator known as a generator. That generator then controls the execution of the generator function. The execution starts when one of the generator’s methods is called. At that time, the execution proceeds to the first yield expression, where it is suspended again, returning the value of <a class="reference internal" href="#grammar-token-expression_list"><code class="xref std std-token docutils literal notranslate"><span class="pre">expression_list</span></code></a> to the generator’s caller. By suspended, we mean that all local state is retained, including the current bindings of local variables, the instruction pointer, the internal evaluation stack, and the state of any exception handling. When the execution is resumed by calling one of the generator’s methods, the function can proceed exactly as if the yield expression were just another external call. The value of the yield expression after resuming depends on the method which resumed the execution. If <a class="reference internal" href="#generator.__next__" title="generator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code></a> is used (typically via either a <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> or the <a class="reference internal" href="../library/functions.html#next" title="next"><code class="xref py py-func docutils literal notranslate"><span class="pre">next()</span></code></a> builtin) then the result is <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>. Otherwise, if <a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code></a> is used, then the result will be the value passed in to that method.</p> <p id="index-20">All of this makes generator functions quite similar to coroutines; they yield multiple times, they have more than one entry point and their execution can be suspended. The only difference is that a generator function cannot control where the execution should continue after it yields; the control is always transferred to the generator’s caller.</p> <p>Yield expressions are allowed anywhere 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> construct. If the generator is not resumed before it is finalized (by reaching a zero reference count or by being garbage collected), the generator-iterator’s <a class="reference internal" href="#generator.close" title="generator.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code></a> method will be called, allowing any pending <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> clauses to execute.</p> <p>When <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span> <span class="pre"><expr></span></code> is used, it treats the supplied expression as a subiterator. All values produced by that subiterator are passed directly to the caller of the current generator’s methods. Any values passed in with <a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code></a> and any exceptions passed in with <a class="reference internal" href="#generator.throw" title="generator.throw"><code class="xref py py-meth docutils literal notranslate"><span class="pre">throw()</span></code></a> are passed to the underlying iterator if it has the appropriate methods. If this is not the case, then <a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code></a> will raise <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>, while <a class="reference internal" href="#generator.throw" title="generator.throw"><code class="xref py py-meth docutils literal notranslate"><span class="pre">throw()</span></code></a> will just raise the passed in exception immediately.</p> <p>When the underlying iterator is complete, the <code class="xref py py-attr docutils literal notranslate"><span class="pre">value</span></code> attribute of the raised <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> instance becomes the value of the yield expression. It can be either set explicitly when raising <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 automatically when the sub-iterator is a generator (by returning a value from the sub-generator).</p> <blockquote> <div><div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.3: </span>Added <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span> <span class="pre"><expr></span></code> to delegate control flow to a subiterator.</p> </div> </div></blockquote> <p>The parentheses may be omitted when the yield expression is the sole expression on the right hand side of an assignment statement.</p> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><span class="target" id="index-21"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0255"><strong>PEP 255</strong></a> - Simple Generators</dt> <dd>The proposal for adding generators and 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 to Python.</dd> <dt><span class="target" id="index-22"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0342"><strong>PEP 342</strong></a> - Coroutines via Enhanced Generators</dt> <dd>The proposal to enhance the API and syntax of generators, making them usable as simple coroutines.</dd> <dt><span class="target" id="index-23"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0380"><strong>PEP 380</strong></a> - Syntax for Delegating to a Subgenerator</dt> <dd>The proposal to introduce the <code class="xref std std-token docutils literal notranslate"><span class="pre">yield_from</span></code> syntax, making delegation to sub-generators easy.</dd> </dl> </div> <div class="section" id="generator-iterator-methods"> <span id="generator-methods"></span><span id="index-24"></span><h4>6.2.9.1. Generator-iterator methods<a class="headerlink" href="#generator-iterator-methods" title="Permalink to this headline">¶</a></h4> <p>This subsection describes the methods of a generator iterator. They can be used to control the execution of a generator function.</p> <p>Note that calling any of the generator methods below when the generator is already executing raises a <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> exception.</p> <span class="target" id="index-25"></span><dl class="method"> <dt id="generator.__next__"> <code class="descclassname">generator.</code><code class="descname">__next__</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#generator.__next__" title="Permalink to this definition">¶</a></dt> <dd><p>Starts the execution of a generator function or resumes it at the last executed yield expression. When a generator function is resumed with a <a class="reference internal" href="#generator.__next__" title="generator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code></a> method, the current yield expression always evaluates to <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>. The execution then continues to the next yield expression, where the generator is suspended again, and the value of the <a class="reference internal" href="#grammar-token-expression_list"><code class="xref std std-token docutils literal notranslate"><span class="pre">expression_list</span></code></a> is returned to <a class="reference internal" href="#generator.__next__" title="generator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code></a>’s caller. If the generator exits without yielding another value, 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.</p> <p>This method is normally called implicitly, e.g. by a <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> loop, or by the built-in <a class="reference internal" href="../library/functions.html#next" title="next"><code class="xref py py-func docutils literal notranslate"><span class="pre">next()</span></code></a> function.</p> </dd></dl> <dl class="method"> <dt id="generator.send"> <code class="descclassname">generator.</code><code class="descname">send</code><span class="sig-paren">(</span><em>value</em><span class="sig-paren">)</span><a class="headerlink" href="#generator.send" title="Permalink to this definition">¶</a></dt> <dd><p>Resumes the execution and “sends” a value into the generator function. The <em>value</em> argument becomes the result of the current yield expression. The <a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code></a> method returns the next value yielded by the generator, or 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> if the generator exits without yielding another value. When <a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code></a> is called to start the generator, it must be called with <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> as the argument, because there is no yield expression that could receive the value.</p> </dd></dl> <dl class="method"> <dt id="generator.throw"> <code class="descclassname">generator.</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="#generator.throw" title="Permalink to this definition">¶</a></dt> <dd><p>Raises an exception of type <code class="docutils literal notranslate"><span class="pre">type</span></code> at the point where the generator was paused, and returns the next value yielded by the generator function. If the generator exits without yielding another value, 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. If the generator function does not catch the passed-in exception, or raises a different exception, then that exception propagates to the caller.</p> </dd></dl> <span class="target" id="index-26"></span><dl class="method"> <dt id="generator.close"> <code class="descclassname">generator.</code><code class="descname">close</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#generator.close" title="Permalink to this definition">¶</a></dt> <dd><p>Raises a <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 point where the generator function was paused. If the generator function then exits gracefully, is already closed, or 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> (by not catching the exception), close returns to its caller. If the generator yields a value, 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> is raised. If the generator raises any other exception, it is propagated to the caller. <a class="reference internal" href="#generator.close" title="generator.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code></a> does nothing if the generator has already exited due to an exception or normal exit.</p> </dd></dl> </div> <div class="section" id="examples"> <span id="index-27"></span><h4>6.2.9.2. Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h4> <p>Here is a simple example that demonstrates the behavior of generators and generator functions:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">echo</span><span class="p">(</span><span class="n">value</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Execution starts when 'next()' is called for the first time."</span><span class="p">)</span> <span class="gp">... </span> <span class="k">try</span><span class="p">:</span> <span class="gp">... </span> <span class="k">while</span> <span class="kc">True</span><span class="p">:</span> <span class="gp">... </span> <span class="k">try</span><span class="p">:</span> <span class="gp">... </span> <span class="n">value</span> <span class="o">=</span> <span class="p">(</span><span class="k">yield</span> <span class="n">value</span><span class="p">)</span> <span class="gp">... </span> <span class="k">except</span> <span class="ne">Exception</span> <span class="k">as</span> <span class="n">e</span><span class="p">:</span> <span class="gp">... </span> <span class="n">value</span> <span class="o">=</span> <span class="n">e</span> <span class="gp">... </span> <span class="k">finally</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Don't forget to clean up when 'close()' is called."</span><span class="p">)</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">generator</span> <span class="o">=</span> <span class="n">echo</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="nb">next</span><span class="p">(</span><span class="n">generator</span><span class="p">))</span> <span class="go">Execution starts when 'next()' is called for the first time.</span> <span class="go">1</span> <span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="nb">next</span><span class="p">(</span><span class="n">generator</span><span class="p">))</span> <span class="go">None</span> <span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">generator</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="mi">2</span><span class="p">))</span> <span class="go">2</span> <span class="gp">>>> </span><span class="n">generator</span><span class="o">.</span><span class="n">throw</span><span class="p">(</span><span class="ne">TypeError</span><span class="p">,</span> <span class="s2">"spam"</span><span class="p">)</span> <span class="go">TypeError('spam',)</span> <span class="gp">>>> </span><span class="n">generator</span><span class="o">.</span><span class="n">close</span><span class="p">()</span> <span class="go">Don't forget to clean up when 'close()' is called.</span> </pre></div> </div> <p>For examples using <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span></code>, see <a class="reference internal" href="../whatsnew/3.3.html#pep-380"><span class="std std-ref">PEP 380: Syntax for Delegating to a Subgenerator</span></a> in “What’s New in Python.”</p> </div> <div class="section" id="asynchronous-generator-functions"> <span id="id3"></span><h4>6.2.9.3. Asynchronous generator functions<a class="headerlink" href="#asynchronous-generator-functions" title="Permalink to this headline">¶</a></h4> <p>The presence of a yield expression in a function or method 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> further defines the function as a <a class="reference internal" href="../glossary.html#term-asynchronous-generator"><span class="xref std std-term">asynchronous generator</span></a> function.</p> <p>When an asynchronous generator function is called, it returns an asynchronous iterator known as an asynchronous generator object. That object then controls the execution of the generator function. An asynchronous generator object is typically 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 in a coroutine function analogously to how a generator object would be used in a <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> statement.</p> <p>Calling one of the asynchronous generator’s methods returns an <a class="reference internal" href="../glossary.html#term-awaitable"><span class="xref std std-term">awaitable</span></a> object, and the execution starts when this object is awaited on. At that time, the execution proceeds to the first yield expression, where it is suspended again, returning the value of <a class="reference internal" href="#grammar-token-expression_list"><code class="xref std std-token docutils literal notranslate"><span class="pre">expression_list</span></code></a> to the awaiting coroutine. As with a generator, suspension means that all local state is retained, including the current bindings of local variables, the instruction pointer, the internal evaluation stack, and the state of any exception handling. When the execution is resumed by awaiting on the next object returned by the asynchronous generator’s methods, the function can proceed exactly as if the yield expression were just another external call. The value of the yield expression after resuming depends on the method which resumed the execution. If <a class="reference internal" href="#agen.__anext__" title="agen.__anext__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__anext__()</span></code></a> is used then the result is <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>. Otherwise, if <a class="reference internal" href="#agen.asend" title="agen.asend"><code class="xref py py-meth docutils literal notranslate"><span class="pre">asend()</span></code></a> is used, then the result will be the value passed in to that method.</p> <p>In an asynchronous generator function, yield expressions are allowed anywhere 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> construct. However, if an asynchronous generator is not resumed before it is finalized (by reaching a zero reference count or by being garbage collected), then a yield expression within 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> construct could result in a failure to execute pending <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> clauses. In this case, it is the responsibility of the event loop or scheduler running the asynchronous generator to call the asynchronous generator-iterator’s <a class="reference internal" href="#agen.aclose" title="agen.aclose"><code class="xref py py-meth docutils literal notranslate"><span class="pre">aclose()</span></code></a> method and run the resulting coroutine object, thus allowing any pending <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> clauses to execute.</p> <p>To take care of finalization, an event loop should define a <em>finalizer</em> function which takes an asynchronous generator-iterator and presumably calls <a class="reference internal" href="#agen.aclose" title="agen.aclose"><code class="xref py py-meth docutils literal notranslate"><span class="pre">aclose()</span></code></a> and executes the coroutine. This <em>finalizer</em> may be registered by calling <a class="reference internal" href="../library/sys.html#sys.set_asyncgen_hooks" title="sys.set_asyncgen_hooks"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.set_asyncgen_hooks()</span></code></a>. When first iterated over, an asynchronous generator-iterator will store the registered <em>finalizer</em> to be called upon finalization. For a reference example of a <em>finalizer</em> method see the implementation of <code class="docutils literal notranslate"><span class="pre">asyncio.Loop.shutdown_asyncgens</span></code> in <a class="reference external" href="https://github.com/python/cpython/tree/3.6/Lib/asyncio/base_events.py">Lib/asyncio/base_events.py</a>.</p> <p>The expression <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span> <span class="pre"><expr></span></code> is a syntax error when used in an asynchronous generator function.</p> </div> <div class="section" id="asynchronous-generator-iterator-methods"> <span id="asynchronous-generator-methods"></span><span id="index-28"></span><h4>6.2.9.4. Asynchronous generator-iterator methods<a class="headerlink" href="#asynchronous-generator-iterator-methods" title="Permalink to this headline">¶</a></h4> <p>This subsection describes the methods of an asynchronous generator iterator, which are used to control the execution of a generator function.</p> <span class="target" id="index-29"></span><dl class="method"> <dt id="agen.__anext__"> <em class="property">coroutine </em><code class="descclassname">agen.</code><code class="descname">__anext__</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#agen.__anext__" title="Permalink to this definition">¶</a></dt> <dd><p>Returns an awaitable which when run starts to execute the asynchronous generator or resumes it at the last executed yield expression. When an asynchronous generator function is resumed with a <a class="reference internal" href="#agen.__anext__" title="agen.__anext__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__anext__()</span></code></a> method, the current yield expression always evaluates to <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> in the returned awaitable, which when run will continue to the next yield expression. The value of the <a class="reference internal" href="#grammar-token-expression_list"><code class="xref std std-token docutils literal notranslate"><span class="pre">expression_list</span></code></a> of the yield expression is the value of the <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 raised by the completing coroutine. If the asynchronous generator exits without yielding another value, the awaitable instead raises an <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, signalling that the asynchronous iteration has completed.</p> <p>This method is normally called implicitly by a <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> loop.</p> </dd></dl> <dl class="method"> <dt id="agen.asend"> <em class="property">coroutine </em><code class="descclassname">agen.</code><code class="descname">asend</code><span class="sig-paren">(</span><em>value</em><span class="sig-paren">)</span><a class="headerlink" href="#agen.asend" title="Permalink to this definition">¶</a></dt> <dd><p>Returns an awaitable which when run resumes the execution of the asynchronous generator. As with the <a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code></a> method for a generator, this “sends” a value into the asynchronous generator function, and the <em>value</em> argument becomes the result of the current yield expression. The awaitable returned by the <a class="reference internal" href="#agen.asend" title="agen.asend"><code class="xref py py-meth docutils literal notranslate"><span class="pre">asend()</span></code></a> method will return the next value yielded by the generator as the value of the raised <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 raises <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> if the asynchronous generator exits without yielding another value. When <a class="reference internal" href="#agen.asend" title="agen.asend"><code class="xref py py-meth docutils literal notranslate"><span class="pre">asend()</span></code></a> is called to start the asynchronous generator, it must be called with <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> as the argument, because there is no yield expression that could receive the value.</p> </dd></dl> <dl class="method"> <dt id="agen.athrow"> <em class="property">coroutine </em><code class="descclassname">agen.</code><code class="descname">athrow</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="#agen.athrow" title="Permalink to this definition">¶</a></dt> <dd><p>Returns an awaitable that raises an exception of type <code class="docutils literal notranslate"><span class="pre">type</span></code> at the point where the asynchronous generator was paused, and returns the next value yielded by the generator function as the value of the raised <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. If the asynchronous generator exits without yielding another value, an <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 by the awaitable. If the generator function does not catch the passed-in exception, or raises a different exception, then when the awaitable is run that exception propagates to the caller of the awaitable.</p> </dd></dl> <span class="target" id="index-30"></span><dl class="method"> <dt id="agen.aclose"> <em class="property">coroutine </em><code class="descclassname">agen.</code><code class="descname">aclose</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#agen.aclose" title="Permalink to this definition">¶</a></dt> <dd><p>Returns an awaitable that when run will throw a <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> into the asynchronous generator function at the point where it was paused. If the asynchronous generator function then exits gracefully, is already closed, or 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> (by not catching the exception), then the returned awaitable will raise 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. Any further awaitables returned by subsequent calls to the asynchronous generator will 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> exception. If the asynchronous generator yields a value, 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> is raised by the awaitable. If the asynchronous generator raises any other exception, it is propagated to the caller of the awaitable. If the asynchronous generator has already exited due to an exception or normal exit, then further calls to <a class="reference internal" href="#agen.aclose" title="agen.aclose"><code class="xref py py-meth docutils literal notranslate"><span class="pre">aclose()</span></code></a> will return an awaitable that does nothing.</p> </dd></dl> </div> </div> </div> <div class="section" id="primaries"> <span id="id4"></span><h2>6.3. Primaries<a class="headerlink" href="#primaries" title="Permalink to this headline">¶</a></h2> <p id="index-31">Primaries represent the most tightly bound operations of the language. Their syntax is:</p> <pre> <strong id="grammar-token-primary">primary</strong> ::= <a class="reference internal" href="#grammar-token-atom"><code class="xref docutils literal notranslate"><span class="pre">atom</span></code></a> | <a class="reference internal" href="#grammar-token-attributeref"><code class="xref docutils literal notranslate"><span class="pre">attributeref</span></code></a> | <a class="reference internal" href="#grammar-token-subscription"><code class="xref docutils literal notranslate"><span class="pre">subscription</span></code></a> | <a class="reference internal" href="#grammar-token-slicing"><code class="xref docutils literal notranslate"><span class="pre">slicing</span></code></a> | <a class="reference internal" href="#grammar-token-call"><code class="xref docutils literal notranslate"><span class="pre">call</span></code></a> </pre> <div class="section" id="attribute-references"> <span id="id5"></span><h3>6.3.1. Attribute references<a class="headerlink" href="#attribute-references" title="Permalink to this headline">¶</a></h3> <p id="index-32">An attribute reference is a primary followed by a period and a name:</p> <pre> <strong id="grammar-token-attributeref">attributeref</strong> ::= <a class="reference internal" href="#grammar-token-primary"><code class="xref docutils literal notranslate"><span class="pre">primary</span></code></a> "." <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> </pre> <p id="index-33">The primary must evaluate to an object of a type that supports attribute references, which most objects do. This object is then asked to produce the attribute whose name is the identifier. This production can be customized by overriding the <a class="reference internal" href="datamodel.html#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattr__()</span></code></a> method. If this attribute is not available, the exception <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> is raised. Otherwise, the type and value of the object produced is determined by the object. Multiple evaluations of the same attribute reference may yield different objects.</p> </div> <div class="section" id="subscriptions"> <span id="id6"></span><h3>6.3.2. Subscriptions<a class="headerlink" href="#subscriptions" title="Permalink to this headline">¶</a></h3> <span class="target" id="index-34"></span><p id="index-35">A subscription selects an item of a sequence (string, tuple or list) or mapping (dictionary) object:</p> <pre> <strong id="grammar-token-subscription">subscription</strong> ::= <a class="reference internal" href="#grammar-token-primary"><code class="xref docutils literal notranslate"><span class="pre">primary</span></code></a> "[" <a class="reference internal" href="#grammar-token-expression_list"><code class="xref docutils literal notranslate"><span class="pre">expression_list</span></code></a> "]" </pre> <p>The primary must evaluate to an object that supports subscription (lists or dictionaries for example). User-defined objects can support subscription by defining a <a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> method.</p> <p>For built-in objects, there are two types of objects that support subscription:</p> <p>If the primary is a mapping, the expression list must evaluate to an object whose value is one of the keys of the mapping, and the subscription selects the value in the mapping that corresponds to that key. (The expression list is a tuple except if it has exactly one item.)</p> <p>If the primary is a sequence, the expression list must evaluate to an integer or a slice (as discussed in the following section).</p> <p>The formal syntax makes no special provision for negative indices in sequences; however, built-in sequences all provide a <a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> method that interprets negative indices by adding the length of the sequence to the index (so that <code class="docutils literal notranslate"><span class="pre">x[-1]</span></code> selects the last item of <code class="docutils literal notranslate"><span class="pre">x</span></code>). The resulting value must be a nonnegative integer less than the number of items in the sequence, and the subscription selects the item whose index is that value (counting from zero). Since the support for negative indices and slicing occurs in the object’s <a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> method, subclasses overriding this method will need to explicitly add that support.</p> <p id="index-36">A string’s items are characters. A character is not a separate data type but a string of exactly one character.</p> </div> <div class="section" id="slicings"> <span id="id7"></span><h3>6.3.3. Slicings<a class="headerlink" href="#slicings" title="Permalink to this headline">¶</a></h3> <span class="target" id="index-37"></span><p id="index-38">A slicing selects a range of items in a sequence object (e.g., a string, tuple or list). Slicings may be used as expressions or as targets in assignment 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 syntax for a slicing:</p> <pre> <strong id="grammar-token-slicing">slicing </strong> ::= <a class="reference internal" href="#grammar-token-primary"><code class="xref docutils literal notranslate"><span class="pre">primary</span></code></a> "[" <a class="reference internal" href="#grammar-token-slice_list"><code class="xref docutils literal notranslate"><span class="pre">slice_list</span></code></a> "]" <strong id="grammar-token-slice_list">slice_list </strong> ::= <a class="reference internal" href="#grammar-token-slice_item"><code class="xref docutils literal notranslate"><span class="pre">slice_item</span></code></a> ("," <a class="reference internal" href="#grammar-token-slice_item"><code class="xref docutils literal notranslate"><span class="pre">slice_item</span></code></a>)* [","] <strong id="grammar-token-slice_item">slice_item </strong> ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> | <a class="reference internal" href="#grammar-token-proper_slice"><code class="xref docutils literal notranslate"><span class="pre">proper_slice</span></code></a> <strong id="grammar-token-proper_slice">proper_slice</strong> ::= [<a class="reference internal" href="#grammar-token-lower_bound"><code class="xref docutils literal notranslate"><span class="pre">lower_bound</span></code></a>] ":" [<a class="reference internal" href="#grammar-token-upper_bound"><code class="xref docutils literal notranslate"><span class="pre">upper_bound</span></code></a>] [ ":" [<a class="reference internal" href="#grammar-token-stride"><code class="xref docutils literal notranslate"><span class="pre">stride</span></code></a>] ] <strong id="grammar-token-lower_bound">lower_bound </strong> ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> <strong id="grammar-token-upper_bound">upper_bound </strong> ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> <strong id="grammar-token-stride">stride </strong> ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> </pre> <p>There is ambiguity in the formal syntax here: anything that looks like an expression list also looks like a slice list, so any subscription can be interpreted as a slicing. Rather than further complicating the syntax, this is disambiguated by defining that in this case the interpretation as a subscription takes priority over the interpretation as a slicing (this is the case if the slice list contains no proper slice).</p> <p id="index-39">The semantics for a slicing are as follows. The primary is indexed (using the same <a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> method as normal subscription) with a key that is constructed from the slice list, as follows. If the slice list contains at least one comma, the key is a tuple containing the conversion of the slice items; otherwise, the conversion of the lone slice item is the key. The conversion of a slice item that is an expression is that expression. The conversion of a proper slice is a slice object (see section <a class="reference internal" href="datamodel.html#types"><span class="std std-ref">The standard type hierarchy</span></a>) whose <code class="xref py py-attr docutils literal notranslate"><span class="pre">start</span></code>, <code class="xref py py-attr docutils literal notranslate"><span class="pre">stop</span></code> and <code class="xref py py-attr docutils literal notranslate"><span class="pre">step</span></code> attributes are the values of the expressions given as lower bound, upper bound and stride, respectively, substituting <code class="docutils literal notranslate"><span class="pre">None</span></code> for missing expressions.</p> </div> <div class="section" id="calls"> <span id="index-40"></span><span id="id8"></span><h3>6.3.4. Calls<a class="headerlink" href="#calls" title="Permalink to this headline">¶</a></h3> <p>A call calls a callable object (e.g., a <a class="reference internal" href="../glossary.html#term-function"><span class="xref std std-term">function</span></a>) with a possibly empty series of <a class="reference internal" href="../glossary.html#term-argument"><span class="xref std std-term">arguments</span></a>:</p> <pre> <strong id="grammar-token-call">call </strong> ::= <a class="reference internal" href="#grammar-token-primary"><code class="xref docutils literal notranslate"><span class="pre">primary</span></code></a> "(" [<a class="reference internal" href="#grammar-token-argument_list"><code class="xref docutils literal notranslate"><span class="pre">argument_list</span></code></a> [","] | <a class="reference internal" href="#grammar-token-comprehension"><code class="xref docutils literal notranslate"><span class="pre">comprehension</span></code></a>] ")" <strong id="grammar-token-argument_list">argument_list </strong> ::= <a class="reference internal" href="#grammar-token-positional_arguments"><code class="xref docutils literal notranslate"><span class="pre">positional_arguments</span></code></a> ["," <a class="reference internal" href="#grammar-token-starred_and_keywords"><code class="xref docutils literal notranslate"><span class="pre">starred_and_keywords</span></code></a>] ["," <a class="reference internal" href="#grammar-token-keywords_arguments"><code class="xref docutils literal notranslate"><span class="pre">keywords_arguments</span></code></a>] | <a class="reference internal" href="#grammar-token-starred_and_keywords"><code class="xref docutils literal notranslate"><span class="pre">starred_and_keywords</span></code></a> ["," <a class="reference internal" href="#grammar-token-keywords_arguments"><code class="xref docutils literal notranslate"><span class="pre">keywords_arguments</span></code></a>] | <a class="reference internal" href="#grammar-token-keywords_arguments"><code class="xref docutils literal notranslate"><span class="pre">keywords_arguments</span></code></a> <strong id="grammar-token-positional_arguments">positional_arguments</strong> ::= ["*"] <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> ("," ["*"] <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>)* <strong id="grammar-token-starred_and_keywords">starred_and_keywords</strong> ::= ("*" <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> | <a class="reference internal" href="#grammar-token-keyword_item"><code class="xref docutils literal notranslate"><span class="pre">keyword_item</span></code></a>) ("," "*" <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> | "," <a class="reference internal" href="#grammar-token-keyword_item"><code class="xref docutils literal notranslate"><span class="pre">keyword_item</span></code></a>)* <strong id="grammar-token-keywords_arguments">keywords_arguments </strong> ::= (<a class="reference internal" href="#grammar-token-keyword_item"><code class="xref docutils literal notranslate"><span class="pre">keyword_item</span></code></a> | "**" <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>) ("," <a class="reference internal" href="#grammar-token-keyword_item"><code class="xref docutils literal notranslate"><span class="pre">keyword_item</span></code></a> | "," "**" <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>)* <strong id="grammar-token-keyword_item">keyword_item </strong> ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> "=" <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> </pre> <p>An optional trailing comma may be present after the positional and keyword arguments but does not affect the semantics.</p> <p id="index-41">The primary must evaluate to a callable object (user-defined functions, built-in functions, methods of built-in objects, class objects, methods of class instances, and all objects having a <a class="reference internal" href="datamodel.html#object.__call__" title="object.__call__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__call__()</span></code></a> method are callable). All argument expressions are evaluated before the call is attempted. Please refer to section <a class="reference internal" href="compound_stmts.html#function"><span class="std std-ref">Function definitions</span></a> for the syntax of formal <a class="reference internal" href="../glossary.html#term-parameter"><span class="xref std std-term">parameter</span></a> lists.</p> <p>If keyword arguments are present, they are first converted to positional arguments, as follows. First, a list of unfilled slots is created for the formal parameters. If there are N positional arguments, they are placed in the first N slots. Next, for each keyword argument, the identifier is used to determine the corresponding slot (if the identifier is the same as the first formal parameter name, the first slot is used, and so on). If the slot is already filled, 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> exception is raised. Otherwise, the value of the argument is placed in the slot, filling it (even if the expression is <code class="docutils literal notranslate"><span class="pre">None</span></code>, it fills the slot). When all arguments have been processed, the slots that are still unfilled are filled with the corresponding default value from the function definition. (Default values are calculated, once, when the function is defined; thus, a mutable object such as a list or dictionary used as default value will be shared by all calls that don’t specify an argument value for the corresponding slot; this should usually be avoided.) If there are any unfilled slots for which no default value is specified, 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> exception is raised. Otherwise, the list of filled slots is used as the argument list for the call.</p> <div class="impl-detail compound"> <p><strong>CPython implementation detail:</strong> An implementation may provide built-in functions whose positional parameters do not have names, even if they are ‘named’ for the purpose of documentation, and which therefore cannot be supplied by keyword. In CPython, this is the case for functions implemented in C that use <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a> to parse their arguments.</p> </div> <p>If there are more positional arguments than there are formal parameter slots, 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> exception is raised, unless a formal parameter using the syntax <code class="docutils literal notranslate"><span class="pre">*identifier</span></code> is present; in this case, that formal parameter receives a tuple containing the excess positional arguments (or an empty tuple if there were no excess positional arguments).</p> <p>If any keyword argument does not correspond to a formal parameter name, 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> exception is raised, unless a formal parameter using the syntax <code class="docutils literal notranslate"><span class="pre">**identifier</span></code> is present; in this case, that formal parameter receives a dictionary containing the excess keyword arguments (using the keywords as keys and the argument values as corresponding values), or a (new) empty dictionary if there were no excess keyword arguments.</p> <p id="index-42">If the syntax <code class="docutils literal notranslate"><span class="pre">*expression</span></code> appears in the function call, <code class="docutils literal notranslate"><span class="pre">expression</span></code> must evaluate to an <a class="reference internal" href="../glossary.html#term-iterable"><span class="xref std std-term">iterable</span></a>. Elements from these iterables are treated as if they were additional positional arguments. For the call <code class="docutils literal notranslate"><span class="pre">f(x1,</span> <span class="pre">x2,</span> <span class="pre">*y,</span> <span class="pre">x3,</span> <span class="pre">x4)</span></code>, if <em>y</em> evaluates to a sequence <em>y1</em>, …, <em>yM</em>, this is equivalent to a call with M+4 positional arguments <em>x1</em>, <em>x2</em>, <em>y1</em>, …, <em>yM</em>, <em>x3</em>, <em>x4</em>.</p> <p>A consequence of this is that although the <code class="docutils literal notranslate"><span class="pre">*expression</span></code> syntax may appear <em>after</em> explicit keyword arguments, it is processed <em>before</em> the keyword arguments (and any <code class="docutils literal notranslate"><span class="pre">**expression</span></code> arguments – see below). So:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">):</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">)</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">f</span><span class="p">(</span><span class="n">b</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="o">*</span><span class="p">(</span><span class="mi">2</span><span class="p">,))</span> <span class="go">2 1</span> <span class="gp">>>> </span><span class="n">f</span><span class="p">(</span><span class="n">a</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="o">*</span><span class="p">(</span><span class="mi">2</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">f() got multiple values for keyword argument 'a'</span> <span class="gp">>>> </span><span class="n">f</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="o">*</span><span class="p">(</span><span class="mi">2</span><span class="p">,))</span> <span class="go">1 2</span> </pre></div> </div> <p>It is unusual for both keyword arguments and the <code class="docutils literal notranslate"><span class="pre">*expression</span></code> syntax to be used in the same call, so in practice this confusion does not arise.</p> <p id="index-43">If the syntax <code class="docutils literal notranslate"><span class="pre">**expression</span></code> appears in the function call, <code class="docutils literal notranslate"><span class="pre">expression</span></code> must evaluate to a <a class="reference internal" href="../glossary.html#term-mapping"><span class="xref std std-term">mapping</span></a>, the contents of which are treated as additional keyword arguments. If a keyword is already present (as an explicit keyword argument, or from another unpacking), 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> exception is raised.</p> <p>Formal parameters using the syntax <code class="docutils literal notranslate"><span class="pre">*identifier</span></code> or <code class="docutils literal notranslate"><span class="pre">**identifier</span></code> cannot be used as positional argument slots or as keyword argument names.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.5: </span>Function calls accept any number of <code class="docutils literal notranslate"><span class="pre">*</span></code> and <code class="docutils literal notranslate"><span class="pre">**</span></code> unpackings, positional arguments may follow iterable unpackings (<code class="docutils literal notranslate"><span class="pre">*</span></code>), and keyword arguments may follow dictionary unpackings (<code class="docutils literal notranslate"><span class="pre">**</span></code>). Originally proposed by <span class="target" id="index-44"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0448"><strong>PEP 448</strong></a>.</p> </div> <p>A call always returns some value, possibly <code class="docutils literal notranslate"><span class="pre">None</span></code>, unless it raises an exception. How this value is computed depends on the type of the callable object.</p> <p>If it is—</p> <dl class="docutils"> <dt>a user-defined function:</dt> <dd><p class="first last" id="index-45">The code block for the function is executed, passing it the argument list. The first thing the code block will do is bind the formal parameters to the arguments; this is described in section <a class="reference internal" href="compound_stmts.html#function"><span class="std std-ref">Function definitions</span></a>. When the code block 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, this specifies the return value of the function call.</p> </dd> <dt>a built-in function or method:</dt> <dd><p class="first last" id="index-46">The result is up to the interpreter; see <a class="reference internal" href="../library/functions.html#built-in-funcs"><span class="std std-ref">Built-in Functions</span></a> for the descriptions of built-in functions and methods.</p> </dd> <dt>a class object:</dt> <dd><p class="first last" id="index-47">A new instance of that class is returned.</p> </dd> <dt>a class instance method:</dt> <dd><p class="first last" id="index-48">The corresponding user-defined function is called, with an argument list that is one longer than the argument list of the call: the instance becomes the first argument.</p> </dd> <dt>a class instance:</dt> <dd><p class="first last" id="index-49">The class must define a <a class="reference internal" href="datamodel.html#object.__call__" title="object.__call__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__call__()</span></code></a> method; the effect is then the same as if that method was called.</p> </dd> </dl> </div> </div> <div class="section" id="await-expression"> <span id="await"></span><h2>6.4. Await expression<a class="headerlink" href="#await-expression" title="Permalink to this headline">¶</a></h2> <p>Suspend the execution of <a class="reference internal" href="../glossary.html#term-coroutine"><span class="xref std std-term">coroutine</span></a> on an <a class="reference internal" href="../glossary.html#term-awaitable"><span class="xref std std-term">awaitable</span></a> object. Can only be used inside a <a class="reference internal" href="../glossary.html#term-coroutine-function"><span class="xref std std-term">coroutine function</span></a>.</p> <pre> <strong id="grammar-token-await_expr">await_expr</strong> ::= "await" <a class="reference internal" href="#grammar-token-primary"><code class="xref docutils literal notranslate"><span class="pre">primary</span></code></a> </pre> <div class="versionadded"> <p><span class="versionmodified">New in version 3.5.</span></p> </div> </div> <div class="section" id="the-power-operator"> <span id="power"></span><h2>6.5. The power operator<a class="headerlink" href="#the-power-operator" title="Permalink to this headline">¶</a></h2> <p>The power operator binds more tightly than unary operators on its left; it binds less tightly than unary operators on its right. The syntax is:</p> <pre> <strong id="grammar-token-power">power</strong> ::= (<a class="reference internal" href="#grammar-token-await_expr"><code class="xref docutils literal notranslate"><span class="pre">await_expr</span></code></a> | <a class="reference internal" href="#grammar-token-primary"><code class="xref docutils literal notranslate"><span class="pre">primary</span></code></a>) ["**" <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a>] </pre> <p>Thus, in an unparenthesized sequence of power and unary operators, the operators are evaluated from right to left (this does not constrain the evaluation order for the operands): <code class="docutils literal notranslate"><span class="pre">-1**2</span></code> results in <code class="docutils literal notranslate"><span class="pre">-1</span></code>.</p> <p>The power operator has the same semantics as 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, when called with two arguments: it yields its left argument raised to the power of its right argument. The numeric arguments are first converted to a common type, and the result is of that type.</p> <p>For int operands, the result has the same type as the operands unless the second argument is negative; in that case, all arguments are converted to float and a float result is delivered. For example, <code class="docutils literal notranslate"><span class="pre">10**2</span></code> returns <code class="docutils literal notranslate"><span class="pre">100</span></code>, but <code class="docutils literal notranslate"><span class="pre">10**-2</span></code> returns <code class="docutils literal notranslate"><span class="pre">0.01</span></code>.</p> <p>Raising <code class="docutils literal notranslate"><span class="pre">0.0</span></code> to a negative power results in a <a class="reference internal" href="../library/exceptions.html#ZeroDivisionError" title="ZeroDivisionError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ZeroDivisionError</span></code></a>. Raising a negative number to a fractional power results in 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> number. (In earlier versions it raised a <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a>.)</p> </div> <div class="section" id="unary-arithmetic-and-bitwise-operations"> <span id="unary"></span><h2>6.6. Unary arithmetic and bitwise operations<a class="headerlink" href="#unary-arithmetic-and-bitwise-operations" title="Permalink to this headline">¶</a></h2> <p id="index-50">All unary arithmetic and bitwise operations have the same priority:</p> <pre> <strong id="grammar-token-u_expr">u_expr</strong> ::= <a class="reference internal" href="#grammar-token-power"><code class="xref docutils literal notranslate"><span class="pre">power</span></code></a> | "-" <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a> | "+" <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a> | "~" <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a> </pre> <p id="index-51">The unary <code class="docutils literal notranslate"><span class="pre">-</span></code> (minus) operator yields the negation of its numeric argument.</p> <p id="index-52">The unary <code class="docutils literal notranslate"><span class="pre">+</span></code> (plus) operator yields its numeric argument unchanged.</p> <p id="index-53">The unary <code class="docutils literal notranslate"><span class="pre">~</span></code> (invert) operator yields the bitwise inversion of its integer argument. The bitwise inversion of <code class="docutils literal notranslate"><span class="pre">x</span></code> is defined as <code class="docutils literal notranslate"><span class="pre">-(x+1)</span></code>. It only applies to integral numbers.</p> <p id="index-54">In all three cases, if the argument does not have the proper type, 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> exception is raised.</p> </div> <div class="section" id="binary-arithmetic-operations"> <span id="binary"></span><h2>6.7. Binary arithmetic operations<a class="headerlink" href="#binary-arithmetic-operations" title="Permalink to this headline">¶</a></h2> <p id="index-55">The binary arithmetic operations have the conventional priority levels. Note that some of these operations also apply to certain non-numeric types. Apart from the power operator, there are only two levels, one for multiplicative operators and one for additive operators:</p> <pre> <strong id="grammar-token-m_expr">m_expr</strong> ::= <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a> | <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> "*" <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a> | <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> "@" <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> | <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> "//" <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a> | <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> "/" <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a> | <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> "%" <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a> <strong id="grammar-token-a_expr">a_expr</strong> ::= <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> | <a class="reference internal" href="#grammar-token-a_expr"><code class="xref docutils literal notranslate"><span class="pre">a_expr</span></code></a> "+" <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> | <a class="reference internal" href="#grammar-token-a_expr"><code class="xref docutils literal notranslate"><span class="pre">a_expr</span></code></a> "-" <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> </pre> <p id="index-56">The <code class="docutils literal notranslate"><span class="pre">*</span></code> (multiplication) operator yields the product of its arguments. The arguments must either both be numbers, or one argument must be an integer and the other must be a sequence. In the former case, the numbers are converted to a common type and then multiplied together. In the latter case, sequence repetition is performed; a negative repetition factor yields an empty sequence.</p> <p id="index-57">The <code class="docutils literal notranslate"><span class="pre">@</span></code> (at) operator is intended to be used for matrix multiplication. No builtin Python types implement this operator.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 3.5.</span></p> </div> <p id="index-58">The <code class="docutils literal notranslate"><span class="pre">/</span></code> (division) and <code class="docutils literal notranslate"><span class="pre">//</span></code> (floor division) operators yield the quotient of their arguments. The numeric arguments are first converted to a common type. Division of integers yields a float, while floor division of integers results in an integer; the result is that of mathematical division with the ‘floor’ function applied to the result. Division by zero raises the <a class="reference internal" href="../library/exceptions.html#ZeroDivisionError" title="ZeroDivisionError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ZeroDivisionError</span></code></a> exception.</p> <p id="index-59">The <code class="docutils literal notranslate"><span class="pre">%</span></code> (modulo) operator yields the remainder from the division of the first argument by the second. The numeric arguments are first converted to a common type. A zero right argument raises the <a class="reference internal" href="../library/exceptions.html#ZeroDivisionError" title="ZeroDivisionError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ZeroDivisionError</span></code></a> exception. The arguments may be floating point numbers, e.g., <code class="docutils literal notranslate"><span class="pre">3.14%0.7</span></code> equals <code class="docutils literal notranslate"><span class="pre">0.34</span></code> (since <code class="docutils literal notranslate"><span class="pre">3.14</span></code> equals <code class="docutils literal notranslate"><span class="pre">4*0.7</span> <span class="pre">+</span> <span class="pre">0.34</span></code>.) The modulo operator always yields a result with the same sign as its second operand (or zero); the absolute value of the result is strictly smaller than the absolute value of the second operand <a class="footnote-reference" href="#id17" id="id9">[1]</a>.</p> <p>The floor division and modulo operators are connected by the following identity: <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">(x//y)*y</span> <span class="pre">+</span> <span class="pre">(x%y)</span></code>. Floor division and modulo are also connected with the built-in function <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>: <code class="docutils literal notranslate"><span class="pre">divmod(x,</span> <span class="pre">y)</span> <span class="pre">==</span> <span class="pre">(x//y,</span> <span class="pre">x%y)</span></code>. <a class="footnote-reference" href="#id18" id="id10">[2]</a>.</p> <p>In addition to performing the modulo operation on numbers, the <code class="docutils literal notranslate"><span class="pre">%</span></code> operator is also overloaded by string objects to perform old-style string formatting (also known as interpolation). The syntax for string formatting is described in the Python Library Reference, section <a class="reference internal" href="../library/stdtypes.html#old-string-formatting"><span class="std std-ref">printf-style String Formatting</span></a>.</p> <p>The floor division operator, the modulo operator, and the <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> function are not defined for complex numbers. Instead, convert to a floating point number using the <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> function if appropriate.</p> <p id="index-60">The <code class="docutils literal notranslate"><span class="pre">+</span></code> (addition) operator yields the sum of its arguments. The arguments must either both be numbers or both be sequences of the same type. In the former case, the numbers are converted to a common type and then added together. In the latter case, the sequences are concatenated.</p> <p id="index-61">The <code class="docutils literal notranslate"><span class="pre">-</span></code> (subtraction) operator yields the difference of its arguments. The numeric arguments are first converted to a common type.</p> </div> <div class="section" id="shifting-operations"> <span id="shifting"></span><h2>6.8. Shifting operations<a class="headerlink" href="#shifting-operations" title="Permalink to this headline">¶</a></h2> <p id="index-62">The shifting operations have lower priority than the arithmetic operations:</p> <pre> <strong id="grammar-token-shift_expr">shift_expr</strong> ::= <a class="reference internal" href="#grammar-token-a_expr"><code class="xref docutils literal notranslate"><span class="pre">a_expr</span></code></a> | <a class="reference internal" href="#grammar-token-shift_expr"><code class="xref docutils literal notranslate"><span class="pre">shift_expr</span></code></a> ("<<" | ">>") <a class="reference internal" href="#grammar-token-a_expr"><code class="xref docutils literal notranslate"><span class="pre">a_expr</span></code></a> </pre> <p>These operators accept integers as arguments. They shift the first argument to the left or right by the number of bits given by the second argument.</p> <p id="index-63">A right shift by <em>n</em> bits is defined as floor division by <code class="docutils literal notranslate"><span class="pre">pow(2,n)</span></code>. A left shift by <em>n</em> bits is defined as multiplication with <code class="docutils literal notranslate"><span class="pre">pow(2,n)</span></code>.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">In the current implementation, the right-hand operand 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 right-hand operand is larger than <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> an <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> exception is raised.</p> </div> </div> <div class="section" id="binary-bitwise-operations"> <span id="bitwise"></span><h2>6.9. Binary bitwise operations<a class="headerlink" href="#binary-bitwise-operations" title="Permalink to this headline">¶</a></h2> <p id="index-64">Each of the three bitwise operations has a different priority level:</p> <pre> <strong id="grammar-token-and_expr">and_expr</strong> ::= <a class="reference internal" href="#grammar-token-shift_expr"><code class="xref docutils literal notranslate"><span class="pre">shift_expr</span></code></a> | <a class="reference internal" href="#grammar-token-and_expr"><code class="xref docutils literal notranslate"><span class="pre">and_expr</span></code></a> "&" <a class="reference internal" href="#grammar-token-shift_expr"><code class="xref docutils literal notranslate"><span class="pre">shift_expr</span></code></a> <strong id="grammar-token-xor_expr">xor_expr</strong> ::= <a class="reference internal" href="#grammar-token-and_expr"><code class="xref docutils literal notranslate"><span class="pre">and_expr</span></code></a> | <a class="reference internal" href="#grammar-token-xor_expr"><code class="xref docutils literal notranslate"><span class="pre">xor_expr</span></code></a> "^" <a class="reference internal" href="#grammar-token-and_expr"><code class="xref docutils literal notranslate"><span class="pre">and_expr</span></code></a> <strong id="grammar-token-or_expr">or_expr </strong> ::= <a class="reference internal" href="#grammar-token-xor_expr"><code class="xref docutils literal notranslate"><span class="pre">xor_expr</span></code></a> | <a class="reference internal" href="#grammar-token-or_expr"><code class="xref docutils literal notranslate"><span class="pre">or_expr</span></code></a> "|" <a class="reference internal" href="#grammar-token-xor_expr"><code class="xref docutils literal notranslate"><span class="pre">xor_expr</span></code></a> </pre> <p id="index-65">The <code class="docutils literal notranslate"><span class="pre">&</span></code> operator yields the bitwise AND of its arguments, which must be integers.</p> <p id="index-66">The <code class="docutils literal notranslate"><span class="pre">^</span></code> operator yields the bitwise XOR (exclusive OR) of its arguments, which must be integers.</p> <p id="index-67">The <code class="docutils literal notranslate"><span class="pre">|</span></code> operator yields the bitwise (inclusive) OR of its arguments, which must be integers.</p> </div> <div class="section" id="comparisons"> <span id="id11"></span><h2>6.10. Comparisons<a class="headerlink" href="#comparisons" title="Permalink to this headline">¶</a></h2> <span class="target" id="index-68"></span><p id="index-69">Unlike C, all comparison operations in Python have the same priority, which is lower than that of any arithmetic, shifting or bitwise operation. Also unlike C, expressions like <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre"><</span> <span class="pre">b</span> <span class="pre"><</span> <span class="pre">c</span></code> have the interpretation that is conventional in mathematics:</p> <pre> <strong id="grammar-token-comparison">comparison </strong> ::= <a class="reference internal" href="#grammar-token-or_expr"><code class="xref docutils literal notranslate"><span class="pre">or_expr</span></code></a> (<a class="reference internal" href="#grammar-token-comp_operator"><code class="xref docutils literal notranslate"><span class="pre">comp_operator</span></code></a> <a class="reference internal" href="#grammar-token-or_expr"><code class="xref docutils literal notranslate"><span class="pre">or_expr</span></code></a>)* <strong id="grammar-token-comp_operator">comp_operator</strong> ::= "<" | ">" | "==" | ">=" | "<=" | "!=" | "is" ["not"] | ["not"] "in" </pre> <p>Comparisons yield boolean values: <code class="docutils literal notranslate"><span class="pre">True</span></code> or <code class="docutils literal notranslate"><span class="pre">False</span></code>.</p> <p id="index-70">Comparisons can be chained arbitrarily, e.g., <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre"><</span> <span class="pre">y</span> <span class="pre"><=</span> <span class="pre">z</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre"><</span> <span class="pre">y</span> <span class="pre">and</span> <span class="pre">y</span> <span class="pre"><=</span> <span class="pre">z</span></code>, except that <code class="docutils literal notranslate"><span class="pre">y</span></code> is evaluated only once (but in both cases <code class="docutils literal notranslate"><span class="pre">z</span></code> is not evaluated at all when <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre"><</span> <span class="pre">y</span></code> is found to be false).</p> <p>Formally, if <em>a</em>, <em>b</em>, <em>c</em>, …, <em>y</em>, <em>z</em> are expressions and <em>op1</em>, <em>op2</em>, …, <em>opN</em> are comparison operators, then <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">op1</span> <span class="pre">b</span> <span class="pre">op2</span> <span class="pre">c</span> <span class="pre">...</span> <span class="pre">y</span> <span class="pre">opN</span> <span class="pre">z</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">op1</span> <span class="pre">b</span> <span class="pre">and</span> <span class="pre">b</span> <span class="pre">op2</span> <span class="pre">c</span> <span class="pre">and</span> <span class="pre">...</span> <span class="pre">y</span> <span class="pre">opN</span> <span class="pre">z</span></code>, except that each expression is evaluated at most once.</p> <p>Note that <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">op1</span> <span class="pre">b</span> <span class="pre">op2</span> <span class="pre">c</span></code> doesn’t imply any kind of comparison between <em>a</em> and <em>c</em>, so that, e.g., <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre"><</span> <span class="pre">y</span> <span class="pre">></span> <span class="pre">z</span></code> is perfectly legal (though perhaps not pretty).</p> <div class="section" id="value-comparisons"> <h3>6.10.1. Value comparisons<a class="headerlink" href="#value-comparisons" title="Permalink to this headline">¶</a></h3> <p>The operators <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>, and <code class="docutils literal notranslate"><span class="pre">!=</span></code> compare the values of two objects. The objects do not need to have the same type.</p> <p>Chapter <a class="reference internal" href="datamodel.html#objects"><span class="std std-ref">Objects, values and types</span></a> states that objects have a value (in addition to type and identity). The value of an object is a rather abstract notion in Python: For example, there is no canonical access method for an object’s value. Also, there is no requirement that the value of an object should be constructed in a particular way, e.g. comprised of all its data attributes. Comparison operators implement a particular notion of what the value of an object is. One can think of them as defining the value of an object indirectly, by means of their comparison implementation.</p> <p>Because all types are (direct or indirect) subtypes of <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>, they inherit the default comparison behavior from <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>. Types can customize their comparison behavior by implementing <em class="dfn">rich comparison methods</em> like <a class="reference internal" href="datamodel.html#object.__lt__" title="object.__lt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__lt__()</span></code></a>, described in <a class="reference internal" href="datamodel.html#customization"><span class="std std-ref">Basic customization</span></a>.</p> <p>The default behavior for equality comparison (<code class="docutils literal notranslate"><span class="pre">==</span></code> and <code class="docutils literal notranslate"><span class="pre">!=</span></code>) is based on the identity of the objects. Hence, equality comparison of instances with the same identity results in equality, and equality comparison of instances with different identities results in inequality. A motivation for this default behavior is the desire that all objects should be reflexive (i.e. <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">is</span> <span class="pre">y</span></code> implies <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">y</span></code>).</p> <p>A default order comparison (<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>, and <code class="docutils literal notranslate"><span class="pre">>=</span></code>) is not provided; an attempt raises <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>. A motivation for this default behavior is the lack of a similar invariant as for equality.</p> <p>The behavior of the default equality comparison, that instances with different identities are always unequal, may be in contrast to what types will need that have a sensible definition of object value and value-based equality. Such types will need to customize their comparison behavior, and in fact, a number of built-in types have done that.</p> <p>The following list describes the comparison behavior of the most important built-in types.</p> <ul> <li><p class="first">Numbers of built-in numeric types (<a class="reference internal" href="../library/stdtypes.html#typesnumeric"><span class="std std-ref">Numeric Types — int, float, complex</span></a>) and of the standard library types <a class="reference internal" href="../library/fractions.html#fractions.Fraction" title="fractions.Fraction"><code class="xref py py-class docutils literal notranslate"><span class="pre">fractions.Fraction</span></code></a> and <a class="reference internal" href="../library/decimal.html#decimal.Decimal" title="decimal.Decimal"><code class="xref py py-class docutils literal notranslate"><span class="pre">decimal.Decimal</span></code></a> can be compared within and across their types, with the restriction that complex numbers do not support order comparison. Within the limits of the types involved, they compare mathematically (algorithmically) correct without loss of precision.</p> <p>The not-a-number values <code class="xref py py-const docutils literal notranslate"><span class="pre">float('NaN')</span></code> and <code class="xref py py-const docutils literal notranslate"><span class="pre">Decimal('NaN')</span></code> are special. They are identical to themselves (<code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">is</span> <span class="pre">x</span></code> is true) but are not equal to themselves (<code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">x</span></code> is false). Additionally, comparing any number to a not-a-number value will return <code class="docutils literal notranslate"><span class="pre">False</span></code>. For example, both <code class="docutils literal notranslate"><span class="pre">3</span> <span class="pre"><</span> <span class="pre">float('NaN')</span></code> and <code class="docutils literal notranslate"><span class="pre">float('NaN')</span> <span class="pre"><</span> <span class="pre">3</span></code> will return <code class="docutils literal notranslate"><span class="pre">False</span></code>.</p> </li> <li><p class="first">Binary sequences (instances of <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> or <a class="reference internal" href="../library/stdtypes.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytearray</span></code></a>) can be compared within and across their types. They compare lexicographically using the numeric values of their elements.</p> </li> <li><p class="first">Strings (instances of <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>) compare lexicographically using the numerical Unicode code points (the result of 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>) of their characters. <a class="footnote-reference" href="#id19" id="id12">[3]</a></p> <p>Strings and binary sequences cannot be directly compared.</p> </li> <li><p class="first">Sequences (instances of <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>, <a class="reference internal" href="../library/stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate"><span class="pre">list</span></code></a>, or <a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-class docutils literal notranslate"><span class="pre">range</span></code></a>) can be compared only within each of their types, with the restriction that ranges do not support order comparison. Equality comparison across these types results in inequality, and ordering comparison across these types raises <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>Sequences compare lexicographically using comparison of corresponding elements, whereby reflexivity of the elements is enforced.</p> <p>In enforcing reflexivity of elements, the comparison of collections assumes that for a collection element <code class="docutils literal notranslate"><span class="pre">x</span></code>, <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">x</span></code> is always true. Based on that assumption, element identity is compared first, and element comparison is performed only for distinct elements. This approach yields the same result as a strict element comparison would, if the compared elements are reflexive. For non-reflexive elements, the result is different than for strict element comparison, and may be surprising: The non-reflexive not-a-number values for example result in the following comparison behavior when used in a list:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">nan</span> <span class="o">=</span> <span class="nb">float</span><span class="p">(</span><span class="s1">'NaN'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">nan</span> <span class="ow">is</span> <span class="n">nan</span> <span class="go">True</span> <span class="gp">>>> </span><span class="n">nan</span> <span class="o">==</span> <span class="n">nan</span> <span class="go">False <-- the defined non-reflexive behavior of NaN</span> <span class="gp">>>> </span><span class="p">[</span><span class="n">nan</span><span class="p">]</span> <span class="o">==</span> <span class="p">[</span><span class="n">nan</span><span class="p">]</span> <span class="go">True <-- list enforces reflexivity and tests identity first</span> </pre></div> </div> <p>Lexicographical comparison between built-in collections works as follows:</p> <ul class="simple"> <li>For two collections to compare equal, they must be of the same type, have the same length, and each pair of corresponding elements must compare equal (for example, <code class="docutils literal notranslate"><span class="pre">[1,2]</span> <span class="pre">==</span> <span class="pre">(1,2)</span></code> is false because the type is not the same).</li> <li>Collections that support order comparison are ordered the same as their first unequal elements (for example, <code class="docutils literal notranslate"><span class="pre">[1,2,x]</span> <span class="pre"><=</span> <span class="pre">[1,2,y]</span></code> has the same value as <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre"><=</span> <span class="pre">y</span></code>). If a corresponding element does not exist, the shorter collection is ordered first (for example, <code class="docutils literal notranslate"><span class="pre">[1,2]</span> <span class="pre"><</span> <span class="pre">[1,2,3]</span></code> is true).</li> </ul> </li> <li><p class="first">Mappings (instances of <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>) compare equal if and only if they have equal <cite>(key, value)</cite> pairs. Equality comparison of the keys and values enforces reflexivity.</p> <p>Order comparisons (<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>, and <code class="docutils literal notranslate"><span class="pre">>=</span></code>) 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>.</p> </li> <li><p class="first">Sets (instances of <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> or <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>) can be compared within and across their types.</p> <p>They define order comparison operators to mean subset and superset tests. Those relations do not define total orderings (for example, the two sets <code class="docutils literal notranslate"><span class="pre">{1,2}</span></code> and <code class="docutils literal notranslate"><span class="pre">{2,3}</span></code> are not equal, nor subsets of one another, nor supersets of one another). Accordingly, sets are not appropriate arguments for functions which depend on total ordering (for example, <a class="reference internal" href="../library/functions.html#min" title="min"><code class="xref py py-func docutils literal notranslate"><span class="pre">min()</span></code></a>, <a class="reference internal" href="../library/functions.html#max" title="max"><code class="xref py py-func docutils literal notranslate"><span class="pre">max()</span></code></a>, and <a class="reference internal" href="../library/functions.html#sorted" title="sorted"><code class="xref py py-func docutils literal notranslate"><span class="pre">sorted()</span></code></a> produce undefined results given a list of sets as inputs).</p> <p>Comparison of sets enforces reflexivity of its elements.</p> </li> <li><p class="first">Most other built-in types have no comparison methods implemented, so they inherit the default comparison behavior.</p> </li> </ul> <p>User-defined classes that customize their comparison behavior should follow some consistency rules, if possible:</p> <ul> <li><p class="first">Equality comparison should be reflexive. In other words, identical objects should compare equal:</p> <blockquote> <div><p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">is</span> <span class="pre">y</span></code> implies <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">y</span></code></p> </div></blockquote> </li> <li><p class="first">Comparison should be symmetric. In other words, the following expressions should have the same result:</p> <blockquote> <div><p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">y</span></code> and <code class="docutils literal notranslate"><span class="pre">y</span> <span class="pre">==</span> <span class="pre">x</span></code></p> <p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">!=</span> <span class="pre">y</span></code> and <code class="docutils literal notranslate"><span class="pre">y</span> <span class="pre">!=</span> <span class="pre">x</span></code></p> <p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre"><</span> <span class="pre">y</span></code> and <code class="docutils literal notranslate"><span class="pre">y</span> <span class="pre">></span> <span class="pre">x</span></code></p> <p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre"><=</span> <span class="pre">y</span></code> and <code class="docutils literal notranslate"><span class="pre">y</span> <span class="pre">>=</span> <span class="pre">x</span></code></p> </div></blockquote> </li> <li><p class="first">Comparison should be transitive. The following (non-exhaustive) examples illustrate that:</p> <blockquote> <div><p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">></span> <span class="pre">y</span> <span class="pre">and</span> <span class="pre">y</span> <span class="pre">></span> <span class="pre">z</span></code> implies <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">></span> <span class="pre">z</span></code></p> <p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre"><</span> <span class="pre">y</span> <span class="pre">and</span> <span class="pre">y</span> <span class="pre"><=</span> <span class="pre">z</span></code> implies <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre"><</span> <span class="pre">z</span></code></p> </div></blockquote> </li> <li><p class="first">Inverse comparison should result in the boolean negation. In other words, the following expressions should have the same result:</p> <blockquote> <div><p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">y</span></code> and <code class="docutils literal notranslate"><span class="pre">not</span> <span class="pre">x</span> <span class="pre">!=</span> <span class="pre">y</span></code></p> <p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre"><</span> <span class="pre">y</span></code> and <code class="docutils literal notranslate"><span class="pre">not</span> <span class="pre">x</span> <span class="pre">>=</span> <span class="pre">y</span></code> (for total ordering)</p> <p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">></span> <span class="pre">y</span></code> and <code class="docutils literal notranslate"><span class="pre">not</span> <span class="pre">x</span> <span class="pre"><=</span> <span class="pre">y</span></code> (for total ordering)</p> </div></blockquote> <p>The last two expressions apply to totally ordered collections (e.g. to sequences, but not to sets or mappings). See also the <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">total_ordering()</span></code></a> decorator.</p> </li> <li><p class="first">The <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> result should be consistent with equality. Objects that are equal should either have the same hash value, or be marked as unhashable.</p> </li> </ul> <p>Python does not enforce these consistency rules. In fact, the not-a-number values are an example for not following these rules.</p> </div> <div class="section" id="membership-test-operations"> <span id="membership-test-details"></span><span id="not-in"></span><span id="in"></span><h3>6.10.2. Membership test operations<a class="headerlink" href="#membership-test-operations" title="Permalink to this headline">¶</a></h3> <p>The operators <a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a> and <a class="reference internal" href="#not-in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">not</span> <span class="pre">in</span></code></a> test for membership. <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">s</span></code> evaluates to <code class="docutils literal notranslate"><span class="pre">True</span></code> if <em>x</em> is a member of <em>s</em>, and <code class="docutils literal notranslate"><span class="pre">False</span></code> otherwise. <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">not</span> <span class="pre">in</span> <span class="pre">s</span></code> returns the negation of <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">s</span></code>. All built-in sequences and set types support this as well as dictionary, for which <a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a> tests whether the dictionary has a given key. For container types such as list, tuple, set, frozenset, dict, or collections.deque, the expression <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">y</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">any(x</span> <span class="pre">is</span> <span class="pre">e</span> <span class="pre">or</span> <span class="pre">x</span> <span class="pre">==</span> <span class="pre">e</span> <span class="pre">for</span> <span class="pre">e</span> <span class="pre">in</span> <span class="pre">y)</span></code>.</p> <p>For the string and bytes types, <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">y</span></code> is <code class="docutils literal notranslate"><span class="pre">True</span></code> if and only if <em>x</em> is a substring of <em>y</em>. An equivalent test is <code class="docutils literal notranslate"><span class="pre">y.find(x)</span> <span class="pre">!=</span> <span class="pre">-1</span></code>. Empty strings are always considered to be a substring of any other string, so <code class="docutils literal notranslate"><span class="pre">""</span> <span class="pre">in</span> <span class="pre">"abc"</span></code> will return <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p> <p>For user-defined classes which define the <a class="reference internal" href="datamodel.html#object.__contains__" title="object.__contains__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__contains__()</span></code></a> method, <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">y</span></code> returns <code class="docutils literal notranslate"><span class="pre">True</span></code> if <code class="docutils literal notranslate"><span class="pre">y.__contains__(x)</span></code> returns a true value, and <code class="docutils literal notranslate"><span class="pre">False</span></code> otherwise.</p> <p>For user-defined classes which do not define <a class="reference internal" href="datamodel.html#object.__contains__" title="object.__contains__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__contains__()</span></code></a> but do define <a class="reference internal" href="datamodel.html#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a>, <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">y</span></code> is <code class="docutils literal notranslate"><span class="pre">True</span></code> if some value <code class="docutils literal notranslate"><span class="pre">z</span></code> with <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">z</span></code> is produced while iterating over <code class="docutils literal notranslate"><span class="pre">y</span></code>. If an exception is raised during the iteration, it is as if <a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a> raised that exception.</p> <p>Lastly, the old-style iteration protocol is tried: if a class defines <a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>, <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">y</span></code> is <code class="docutils literal notranslate"><span class="pre">True</span></code> if and only if there is a non-negative integer index <em>i</em> such that <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">y[i]</span></code>, and all lower integer indices do not raise <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> exception. (If any other exception is raised, it is as if <a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a> raised that exception).</p> <p id="index-71">The operator <a class="reference internal" href="#not-in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">not</span> <span class="pre">in</span></code></a> is defined to have the inverse true value of <a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a>.</p> </div> <div class="section" id="is-not"> <span id="is"></span><span id="index-72"></span><span id="identity-comparisons"></span><h3>6.10.3. Identity comparisons<a class="headerlink" href="#is-not" title="Permalink to this headline">¶</a></h3> <p>The operators <a class="reference internal" href="#is"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">is</span></code></a> and <a class="reference internal" href="#is-not"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">is</span> <span class="pre">not</span></code></a> test for object identity: <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">is</span> <span class="pre">y</span></code> is true if and only if <em>x</em> and <em>y</em> are the same object. Object identity is determined using the <a class="reference internal" href="../library/functions.html#id" title="id"><code class="xref py py-meth docutils literal notranslate"><span class="pre">id()</span></code></a> function. <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">is</span> <span class="pre">not</span> <span class="pre">y</span></code> yields the inverse truth value. <a class="footnote-reference" href="#id20" id="id13">[4]</a></p> </div> </div> <div class="section" id="boolean-operations"> <span id="not"></span><span id="or"></span><span id="and"></span><span id="booleans"></span><h2>6.11. Boolean operations<a class="headerlink" href="#boolean-operations" title="Permalink to this headline">¶</a></h2> <pre id="index-73"> <strong id="grammar-token-or_test">or_test </strong> ::= <a class="reference internal" href="#grammar-token-and_test"><code class="xref docutils literal notranslate"><span class="pre">and_test</span></code></a> | <a class="reference internal" href="#grammar-token-or_test"><code class="xref docutils literal notranslate"><span class="pre">or_test</span></code></a> "or" <a class="reference internal" href="#grammar-token-and_test"><code class="xref docutils literal notranslate"><span class="pre">and_test</span></code></a> <strong id="grammar-token-and_test">and_test</strong> ::= <a class="reference internal" href="#grammar-token-not_test"><code class="xref docutils literal notranslate"><span class="pre">not_test</span></code></a> | <a class="reference internal" href="#grammar-token-and_test"><code class="xref docutils literal notranslate"><span class="pre">and_test</span></code></a> "and" <a class="reference internal" href="#grammar-token-not_test"><code class="xref docutils literal notranslate"><span class="pre">not_test</span></code></a> <strong id="grammar-token-not_test">not_test</strong> ::= <a class="reference internal" href="#grammar-token-comparison"><code class="xref docutils literal notranslate"><span class="pre">comparison</span></code></a> | "not" <a class="reference internal" href="#grammar-token-not_test"><code class="xref docutils literal notranslate"><span class="pre">not_test</span></code></a> </pre> <p>In the context of Boolean operations, and also when expressions are used by control flow statements, the following values are interpreted as false: <code class="docutils literal notranslate"><span class="pre">False</span></code>, <code class="docutils literal notranslate"><span class="pre">None</span></code>, numeric zero of all types, and empty strings and containers (including strings, tuples, lists, dictionaries, sets and frozensets). All other values are interpreted as true. User-defined objects can customize their truth value by providing a <a class="reference internal" href="datamodel.html#object.__bool__" title="object.__bool__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__bool__()</span></code></a> method.</p> <p id="index-74">The operator <a class="reference internal" href="#not"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">not</span></code></a> yields <code class="docutils literal notranslate"><span class="pre">True</span></code> if its argument is false, <code class="docutils literal notranslate"><span class="pre">False</span></code> otherwise.</p> <p id="index-75">The expression <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">and</span> <span class="pre">y</span></code> first evaluates <em>x</em>; if <em>x</em> is false, its value is returned; otherwise, <em>y</em> is evaluated and the resulting value is returned.</p> <p id="index-76">The expression <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">or</span> <span class="pre">y</span></code> first evaluates <em>x</em>; if <em>x</em> is true, its value is returned; otherwise, <em>y</em> is evaluated and the resulting value is returned.</p> <p>(Note that neither <a class="reference internal" href="#and"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">and</span></code></a> nor <a class="reference internal" href="#or"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">or</span></code></a> restrict the value and type they return to <code class="docutils literal notranslate"><span class="pre">False</span></code> and <code class="docutils literal notranslate"><span class="pre">True</span></code>, but rather return the last evaluated argument. This is sometimes useful, e.g., if <code class="docutils literal notranslate"><span class="pre">s</span></code> is a string that should be replaced by a default value if it is empty, the expression <code class="docutils literal notranslate"><span class="pre">s</span> <span class="pre">or</span> <span class="pre">'foo'</span></code> yields the desired value. Because <a class="reference internal" href="#not"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">not</span></code></a> has to create a new value, it returns a boolean value regardless of the type of its argument (for example, <code class="docutils literal notranslate"><span class="pre">not</span> <span class="pre">'foo'</span></code> produces <code class="docutils literal notranslate"><span class="pre">False</span></code> rather than <code class="docutils literal notranslate"><span class="pre">''</span></code>.)</p> </div> <div class="section" id="conditional-expressions"> <h2>6.12. Conditional expressions<a class="headerlink" href="#conditional-expressions" title="Permalink to this headline">¶</a></h2> <pre id="index-77"> <strong id="grammar-token-conditional_expression">conditional_expression</strong> ::= <a class="reference internal" href="#grammar-token-or_test"><code class="xref docutils literal notranslate"><span class="pre">or_test</span></code></a> ["if" <a class="reference internal" href="#grammar-token-or_test"><code class="xref docutils literal notranslate"><span class="pre">or_test</span></code></a> "else" <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>] <strong id="grammar-token-expression">expression </strong> ::= <a class="reference internal" href="#grammar-token-conditional_expression"><code class="xref docutils literal notranslate"><span class="pre">conditional_expression</span></code></a> | <a class="reference internal" href="#grammar-token-lambda_expr"><code class="xref docutils literal notranslate"><span class="pre">lambda_expr</span></code></a> <strong id="grammar-token-expression_nocond">expression_nocond </strong> ::= <a class="reference internal" href="#grammar-token-or_test"><code class="xref docutils literal notranslate"><span class="pre">or_test</span></code></a> | <a class="reference internal" href="#grammar-token-lambda_expr_nocond"><code class="xref docutils literal notranslate"><span class="pre">lambda_expr_nocond</span></code></a> </pre> <p>Conditional expressions (sometimes called a “ternary operator”) have the lowest priority of all Python operations.</p> <p>The expression <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">if</span> <span class="pre">C</span> <span class="pre">else</span> <span class="pre">y</span></code> first evaluates the condition, <em>C</em> rather than <em>x</em>. If <em>C</em> is true, <em>x</em> is evaluated and its value is returned; otherwise, <em>y</em> is evaluated and its value is returned.</p> <p>See <span class="target" id="index-78"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0308"><strong>PEP 308</strong></a> for more details about conditional expressions.</p> </div> <div class="section" id="lambda"> <span id="lambdas"></span><span id="id14"></span><h2>6.13. Lambdas<a class="headerlink" href="#lambda" title="Permalink to this headline">¶</a></h2> <pre id="index-79"> <strong id="grammar-token-lambda_expr">lambda_expr </strong> ::= "lambda" [<a class="reference internal" href="compound_stmts.html#grammar-token-parameter_list"><code class="xref docutils literal notranslate"><span class="pre">parameter_list</span></code></a>]: <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> <strong id="grammar-token-lambda_expr_nocond">lambda_expr_nocond</strong> ::= "lambda" [<a class="reference internal" href="compound_stmts.html#grammar-token-parameter_list"><code class="xref docutils literal notranslate"><span class="pre">parameter_list</span></code></a>]: <a class="reference internal" href="#grammar-token-expression_nocond"><code class="xref docutils literal notranslate"><span class="pre">expression_nocond</span></code></a> </pre> <p>Lambda expressions (sometimes called lambda forms) are used to create anonymous functions. The expression <code class="docutils literal notranslate"><span class="pre">lambda</span> <span class="pre">parameters:</span> <span class="pre">expression</span></code> yields a function object. The unnamed object behaves like a function object defined with:</p> <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>def <lambda>(parameters): return expression </pre></div> </div> <p>See section <a class="reference internal" href="compound_stmts.html#function"><span class="std std-ref">Function definitions</span></a> for the syntax of parameter lists. Note that functions created with lambda expressions cannot contain statements or annotations.</p> </div> <div class="section" id="expression-lists"> <span id="exprlists"></span><h2>6.14. Expression lists<a class="headerlink" href="#expression-lists" title="Permalink to this headline">¶</a></h2> <pre id="index-80"> <strong id="grammar-token-expression_list">expression_list </strong> ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> ("," <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>)* [","] <strong id="grammar-token-starred_list">starred_list </strong> ::= <a class="reference internal" href="#grammar-token-starred_item"><code class="xref docutils literal notranslate"><span class="pre">starred_item</span></code></a> ("," <a class="reference internal" href="#grammar-token-starred_item"><code class="xref docutils literal notranslate"><span class="pre">starred_item</span></code></a>)* [","] <strong id="grammar-token-starred_expression">starred_expression</strong> ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> | (<a class="reference internal" href="#grammar-token-starred_item"><code class="xref docutils literal notranslate"><span class="pre">starred_item</span></code></a> ",")* [<a class="reference internal" href="#grammar-token-starred_item"><code class="xref docutils literal notranslate"><span class="pre">starred_item</span></code></a>] <strong id="grammar-token-starred_item">starred_item </strong> ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> | "*" <a class="reference internal" href="#grammar-token-or_expr"><code class="xref docutils literal notranslate"><span class="pre">or_expr</span></code></a> </pre> <p id="index-81">Except when part of a list or set display, an expression list containing at least one comma yields a tuple. The length of the tuple is the number of expressions in the list. The expressions are evaluated from left to right.</p> <p id="index-82">An asterisk <code class="docutils literal notranslate"><span class="pre">*</span></code> denotes <em class="dfn">iterable unpacking</em>. Its operand must be an <a class="reference internal" href="../glossary.html#term-iterable"><span class="xref std std-term">iterable</span></a>. The iterable is expanded into a sequence of items, which are included in the new tuple, list, or set, at the site of the unpacking.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 3.5: </span>Iterable unpacking in expression lists, originally proposed by <span class="target" id="index-83"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0448"><strong>PEP 448</strong></a>.</p> </div> <p id="index-84">The trailing comma is required only to create a single tuple (a.k.a. a <em>singleton</em>); it is optional in all other cases. A single expression without a trailing comma doesn’t create a tuple, but rather yields the value of that expression. (To create an empty tuple, use an empty pair of parentheses: <code class="docutils literal notranslate"><span class="pre">()</span></code>.)</p> </div> <div class="section" id="evaluation-order"> <span id="evalorder"></span><h2>6.15. Evaluation order<a class="headerlink" href="#evaluation-order" title="Permalink to this headline">¶</a></h2> <p id="index-85">Python evaluates expressions from left to right. Notice that while evaluating an assignment, the right-hand side is evaluated before the left-hand side.</p> <p>In the following lines, expressions will be evaluated in the arithmetic order of their suffixes:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">expr1</span><span class="p">,</span> <span class="n">expr2</span><span class="p">,</span> <span class="n">expr3</span><span class="p">,</span> <span class="n">expr4</span> <span class="p">(</span><span class="n">expr1</span><span class="p">,</span> <span class="n">expr2</span><span class="p">,</span> <span class="n">expr3</span><span class="p">,</span> <span class="n">expr4</span><span class="p">)</span> <span class="p">{</span><span class="n">expr1</span><span class="p">:</span> <span class="n">expr2</span><span class="p">,</span> <span class="n">expr3</span><span class="p">:</span> <span class="n">expr4</span><span class="p">}</span> <span class="n">expr1</span> <span class="o">+</span> <span class="n">expr2</span> <span class="o">*</span> <span class="p">(</span><span class="n">expr3</span> <span class="o">-</span> <span class="n">expr4</span><span class="p">)</span> <span class="n">expr1</span><span class="p">(</span><span class="n">expr2</span><span class="p">,</span> <span class="n">expr3</span><span class="p">,</span> <span class="o">*</span><span class="n">expr4</span><span class="p">,</span> <span class="o">**</span><span class="n">expr5</span><span class="p">)</span> <span class="n">expr3</span><span class="p">,</span> <span class="n">expr4</span> <span class="o">=</span> <span class="n">expr1</span><span class="p">,</span> <span class="n">expr2</span> </pre></div> </div> </div> <div class="section" id="operator-precedence"> <span id="operator-summary"></span><h2>6.16. Operator precedence<a class="headerlink" href="#operator-precedence" title="Permalink to this headline">¶</a></h2> <p id="index-86">The following table summarizes the operator precedence in Python, from lowest precedence (least binding) to highest precedence (most binding). Operators in the same box have the same precedence. Unless the syntax is explicitly given, operators are binary. Operators in the same box group left to right (except for exponentiation, which groups from right to left).</p> <p>Note that comparisons, membership tests, and identity tests, all have the same precedence and have a left-to-right chaining feature as described in the <a class="reference internal" href="#comparisons"><span class="std std-ref">Comparisons</span></a> section.</p> <table border="1" class="docutils"> <colgroup> <col width="56%" /> <col width="44%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Operator</th> <th class="head">Description</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><a class="reference internal" href="#lambda"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">lambda</span></code></a></td> <td>Lambda expression</td> </tr> <tr class="row-odd"><td><a class="reference internal" href="compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> – <a class="reference internal" href="compound_stmts.html#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a></td> <td>Conditional expression</td> </tr> <tr class="row-even"><td><a class="reference internal" href="#or"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">or</span></code></a></td> <td>Boolean OR</td> </tr> <tr class="row-odd"><td><a class="reference internal" href="#and"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">and</span></code></a></td> <td>Boolean AND</td> </tr> <tr class="row-even"><td><a class="reference internal" href="#not"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">not</span></code></a> <code class="docutils literal notranslate"><span class="pre">x</span></code></td> <td>Boolean NOT</td> </tr> <tr class="row-odd"><td><a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a>, <a class="reference internal" href="#not-in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">not</span> <span class="pre">in</span></code></a>, <a class="reference internal" href="#is"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">is</span></code></a>, <a class="reference internal" href="#is-not"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">is</span> <span class="pre">not</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></td> <td>Comparisons, including membership tests and identity tests</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">|</span></code></td> <td>Bitwise OR</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">^</span></code></td> <td>Bitwise XOR</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">&</span></code></td> <td>Bitwise AND</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre"><<</span></code>, <code class="docutils literal notranslate"><span class="pre">>></span></code></td> <td>Shifts</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">+</span></code>, <code class="docutils literal notranslate"><span class="pre">-</span></code></td> <td>Addition and subtraction</td> </tr> <tr class="row-odd"><td><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></td> <td>Multiplication, matrix multiplication, division, floor division, remainder <a class="footnote-reference" href="#id21" id="id15">[5]</a></td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">+x</span></code>, <code class="docutils literal notranslate"><span class="pre">-x</span></code>, <code class="docutils literal notranslate"><span class="pre">~x</span></code></td> <td>Positive, negative, bitwise NOT</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">**</span></code></td> <td>Exponentiation <a class="footnote-reference" href="#id22" id="id16">[6]</a></td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">await</span></code> <code class="docutils literal notranslate"><span class="pre">x</span></code></td> <td>Await expression</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">x[index]</span></code>, <code class="docutils literal notranslate"><span class="pre">x[index:index]</span></code>, <code class="docutils literal notranslate"><span class="pre">x(arguments...)</span></code>, <code class="docutils literal notranslate"><span class="pre">x.attribute</span></code></td> <td>Subscription, slicing, call, attribute reference</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">(expressions...)</span></code>, <code class="docutils literal notranslate"><span class="pre">[expressions...]</span></code>, <code class="docutils literal notranslate"><span class="pre">{key:</span> <span class="pre">value...}</span></code>, <code class="docutils literal notranslate"><span class="pre">{expressions...}</span></code></td> <td>Binding or tuple display, list display, dictionary display, set display</td> </tr> </tbody> </table> <p class="rubric">Footnotes</p> <table class="docutils footnote" frame="void" id="id17" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id9">[1]</a></td><td>While <code class="docutils literal notranslate"><span class="pre">abs(x%y)</span> <span class="pre"><</span> <span class="pre">abs(y)</span></code> is true mathematically, for floats it may not be true numerically due to roundoff. For example, and assuming a platform on which a Python float is an IEEE 754 double-precision number, in order that <code class="docutils literal notranslate"><span class="pre">-1e-100</span> <span class="pre">%</span> <span class="pre">1e100</span></code> have the same sign as <code class="docutils literal notranslate"><span class="pre">1e100</span></code>, the computed result is <code class="docutils literal notranslate"><span class="pre">-1e-100</span> <span class="pre">+</span> <span class="pre">1e100</span></code>, which is numerically exactly equal to <code class="docutils literal notranslate"><span class="pre">1e100</span></code>. The function <a class="reference internal" href="../library/math.html#math.fmod" title="math.fmod"><code class="xref py py-func docutils literal notranslate"><span class="pre">math.fmod()</span></code></a> returns a result whose sign matches the sign of the first argument instead, and so returns <code class="docutils literal notranslate"><span class="pre">-1e-100</span></code> in this case. Which approach is more appropriate depends on the application.</td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id18" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id10">[2]</a></td><td>If x is very close to an exact integer multiple of y, it’s possible for <code class="docutils literal notranslate"><span class="pre">x//y</span></code> to be one larger than <code class="docutils literal notranslate"><span class="pre">(x-x%y)//y</span></code> due to rounding. In such cases, Python returns the latter result, in order to preserve that <code class="docutils literal notranslate"><span class="pre">divmod(x,y)[0]</span> <span class="pre">*</span> <span class="pre">y</span> <span class="pre">+</span> <span class="pre">x</span> <span class="pre">%</span> <span class="pre">y</span></code> be very close to <code class="docutils literal notranslate"><span class="pre">x</span></code>.</td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id19" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id12">[3]</a></td><td><p class="first">The Unicode standard distinguishes between <em class="dfn">code points</em> (e.g. U+0041) and <em class="dfn">abstract characters</em> (e.g. “LATIN CAPITAL LETTER A”). While most abstract characters in Unicode are only represented using one code point, there is a number of abstract characters that can in addition be represented using a sequence of more than one code point. For example, the abstract character “LATIN CAPITAL LETTER C WITH CEDILLA” can be represented as a single <em class="dfn">precomposed character</em> at code position U+00C7, or as a sequence of a <em class="dfn">base character</em> at code position U+0043 (LATIN CAPITAL LETTER C), followed by a <em class="dfn">combining character</em> at code position U+0327 (COMBINING CEDILLA).</p> <p>The comparison operators on strings compare at the level of Unicode code points. This may be counter-intuitive to humans. For example, <code class="docutils literal notranslate"><span class="pre">"\u00C7"</span> <span class="pre">==</span> <span class="pre">"\u0043\u0327"</span></code> is <code class="docutils literal notranslate"><span class="pre">False</span></code>, even though both strings represent the same abstract character “LATIN CAPITAL LETTER C WITH CEDILLA”.</p> <p class="last">To compare strings at the level of abstract characters (that is, in a way intuitive to humans), use <a class="reference internal" href="../library/unicodedata.html#unicodedata.normalize" title="unicodedata.normalize"><code class="xref py py-func docutils literal notranslate"><span class="pre">unicodedata.normalize()</span></code></a>.</p> </td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id20" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id13">[4]</a></td><td>Due to automatic garbage-collection, free lists, and the dynamic nature of descriptors, you may notice seemingly unusual behaviour in certain uses of the <a class="reference internal" href="#is"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">is</span></code></a> operator, like those involving comparisons between instance methods, or constants. Check their documentation for more info.</td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id21" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id15">[5]</a></td><td>The <code class="docutils literal notranslate"><span class="pre">%</span></code> operator is also used for string formatting; the same precedence applies.</td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id22" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id16">[6]</a></td><td>The power operator <code class="docutils literal notranslate"><span class="pre">**</span></code> binds less tightly than an arithmetic or bitwise unary operator on its right, that is, <code class="docutils literal notranslate"><span class="pre">2**-1</span></code> is <code class="docutils literal notranslate"><span class="pre">0.5</span></code>.</td></tr> </tbody> </table> </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="#">6. Expressions</a><ul> <li><a class="reference internal" href="#arithmetic-conversions">6.1. Arithmetic conversions</a></li> <li><a class="reference internal" href="#atoms">6.2. Atoms</a><ul> <li><a class="reference internal" href="#atom-identifiers">6.2.1. Identifiers (Names)</a></li> <li><a class="reference internal" href="#literals">6.2.2. Literals</a></li> <li><a class="reference internal" href="#parenthesized-forms">6.2.3. Parenthesized forms</a></li> <li><a class="reference internal" href="#displays-for-lists-sets-and-dictionaries">6.2.4. Displays for lists, sets and dictionaries</a></li> <li><a class="reference internal" href="#list-displays">6.2.5. List displays</a></li> <li><a class="reference internal" href="#set-displays">6.2.6. Set displays</a></li> <li><a class="reference internal" href="#dictionary-displays">6.2.7. Dictionary displays</a></li> <li><a class="reference internal" href="#generator-expressions">6.2.8. Generator expressions</a></li> <li><a class="reference internal" href="#yield-expressions">6.2.9. Yield expressions</a><ul> <li><a class="reference internal" href="#generator-iterator-methods">6.2.9.1. Generator-iterator methods</a></li> <li><a class="reference internal" href="#examples">6.2.9.2. Examples</a></li> <li><a class="reference internal" href="#asynchronous-generator-functions">6.2.9.3. Asynchronous generator functions</a></li> <li><a class="reference internal" href="#asynchronous-generator-iterator-methods">6.2.9.4. Asynchronous generator-iterator methods</a></li> </ul> </li> </ul> </li> <li><a class="reference internal" href="#primaries">6.3. Primaries</a><ul> <li><a class="reference internal" href="#attribute-references">6.3.1. Attribute references</a></li> <li><a class="reference internal" href="#subscriptions">6.3.2. Subscriptions</a></li> <li><a class="reference internal" href="#slicings">6.3.3. Slicings</a></li> <li><a class="reference internal" href="#calls">6.3.4. Calls</a></li> </ul> </li> <li><a class="reference internal" href="#await-expression">6.4. Await expression</a></li> <li><a class="reference internal" href="#the-power-operator">6.5. The power operator</a></li> <li><a class="reference internal" href="#unary-arithmetic-and-bitwise-operations">6.6. Unary arithmetic and bitwise operations</a></li> <li><a class="reference internal" href="#binary-arithmetic-operations">6.7. Binary arithmetic operations</a></li> <li><a class="reference internal" href="#shifting-operations">6.8. Shifting operations</a></li> <li><a class="reference internal" href="#binary-bitwise-operations">6.9. Binary bitwise operations</a></li> <li><a class="reference internal" href="#comparisons">6.10. Comparisons</a><ul> <li><a class="reference internal" href="#value-comparisons">6.10.1. Value comparisons</a></li> <li><a class="reference internal" href="#membership-test-operations">6.10.2. Membership test operations</a></li> <li><a class="reference internal" href="#is-not">6.10.3. Identity comparisons</a></li> </ul> </li> <li><a class="reference internal" href="#boolean-operations">6.11. Boolean operations</a></li> <li><a class="reference internal" href="#conditional-expressions">6.12. Conditional expressions</a></li> <li><a class="reference internal" href="#lambda">6.13. Lambdas</a></li> <li><a class="reference internal" href="#expression-lists">6.14. Expression lists</a></li> <li><a class="reference internal" href="#evaluation-order">6.15. Evaluation order</a></li> <li><a class="reference internal" href="#operator-precedence">6.16. Operator precedence</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="import.html" title="previous chapter">5. The import system</a></p> <h4>Next topic</h4> <p class="topless"><a href="simple_stmts.html" title="next chapter">7. Simple statements</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li> <a href="https://github.com/python/cpython/blob/3.6/Doc/reference/expressions.rst" rel="nofollow">Show Source </a> </li> </ul> </div> </div> </div> <div class="clearer"></div> </div> <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" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="simple_stmts.html" title="7. Simple statements" >next</a> |</li> <li class="right" > <a href="import.html" title="5. The import system" >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" >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="footer"> © <a href="../copyright.html">Copyright</a> 2001-2021, Python Software Foundation. <br /> The Python Software Foundation is a non-profit corporation. <a href="https://www.python.org/psf/donations/">Please donate.</a> <br /> Last updated on Aug 25, 2021. <a href="../bugs.html">Found a bug</a>? <br /> Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 1.7.6. </div> </body> </html>