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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>9. Classes — 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="10. Brief Tour of the Standard Library" href="stdlib.html" /> <link rel="prev" title="8. Errors and Exceptions" href="errors.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/3/tutorial/classes.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="stdlib.html" title="10. Brief Tour of the Standard Library" accesskey="N">next</a> |</li> <li class="right" > <a href="errors.html" title="8. Errors and Exceptions" 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 Tutorial</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="classes"> <span id="tut-classes"></span><h1>9. Classes<a class="headerlink" href="#classes" title="Permalink to this headline">¶</a></h1> <p>Classes provide a means of bundling data and functionality together. Creating a new class creates a new <em>type</em> of object, allowing new <em>instances</em> of that type to be made. Each class instance can have attributes attached to it for maintaining its state. Class instances can also have methods (defined by its class) for modifying its state.</p> <p>Compared with other programming languages, Python’s class mechanism adds classes with a minimum of new syntax and semantics. It is a mixture of the class mechanisms found in C++ and Modula-3. Python classes provide all the standard features of Object Oriented Programming: the class inheritance mechanism allows multiple base classes, a derived class can override any methods of its base class or classes, and a method can call the method of a base class with the same name. Objects can contain arbitrary amounts and kinds of data. As is true for modules, classes partake of the dynamic nature of Python: they are created at runtime, and can be modified further after creation.</p> <p>In C++ terminology, normally class members (including the data members) are <em>public</em> (except see below <a class="reference internal" href="#tut-private"><span class="std std-ref">Private Variables</span></a>), and all member functions are <em>virtual</em>. As in Modula-3, there are no shorthands for referencing the object’s members from its methods: the method function is declared with an explicit first argument representing the object, which is provided implicitly by the call. As in Smalltalk, classes themselves are objects. This provides semantics for importing and renaming. Unlike C++ and Modula-3, built-in types can be used as base classes for extension by the user. Also, like in C++, most built-in operators with special syntax (arithmetic operators, subscripting etc.) can be redefined for class instances.</p> <p>(Lacking universally accepted terminology to talk about classes, I will make occasional use of Smalltalk and C++ terms. I would use Modula-3 terms, since its object-oriented semantics are closer to those of Python than C++, but I expect that few readers have heard of it.)</p> <div class="section" id="a-word-about-names-and-objects"> <span id="tut-object"></span><h2>9.1. A Word About Names and Objects<a class="headerlink" href="#a-word-about-names-and-objects" title="Permalink to this headline">¶</a></h2> <p>Objects have individuality, and multiple names (in multiple scopes) can be bound to the same object. This is known as aliasing in other languages. This is usually not appreciated on a first glance at Python, and can be safely ignored when dealing with immutable basic types (numbers, strings, tuples). However, aliasing has a possibly surprising effect on the semantics of Python code involving mutable objects such as lists, dictionaries, and most other types. This is usually used to the benefit of the program, since aliases behave like pointers in some respects. For example, passing an object is cheap since only a pointer is passed by the implementation; and if a function modifies an object passed as an argument, the caller will see the change — this eliminates the need for two different argument passing mechanisms as in Pascal.</p> </div> <div class="section" id="python-scopes-and-namespaces"> <span id="tut-scopes"></span><h2>9.2. Python Scopes and Namespaces<a class="headerlink" href="#python-scopes-and-namespaces" title="Permalink to this headline">¶</a></h2> <p>Before introducing classes, I first have to tell you something about Python’s scope rules. Class definitions play some neat tricks with namespaces, and you need to know how scopes and namespaces work to fully understand what’s going on. Incidentally, knowledge about this subject is useful for any advanced Python programmer.</p> <p>Let’s begin with some definitions.</p> <p>A <em>namespace</em> is a mapping from names to objects. Most namespaces are currently implemented as Python dictionaries, but that’s normally not noticeable in any way (except for performance), and it may change in the future. Examples of namespaces are: the set of built-in names (containing functions such as <a class="reference internal" href="../library/functions.html#abs" title="abs"><code class="xref py py-func docutils literal notranslate"><span class="pre">abs()</span></code></a>, and built-in exception names); the global names in a module; and the local names in a function invocation. In a sense the set of attributes of an object also form a namespace. The important thing to know about namespaces is that there is absolutely no relation between names in different namespaces; for instance, two different modules may both define a function <code class="docutils literal notranslate"><span class="pre">maximize</span></code> without confusion — users of the modules must prefix it with the module name.</p> <p>By the way, I use the word <em>attribute</em> for any name following a dot — for example, in the expression <code class="docutils literal notranslate"><span class="pre">z.real</span></code>, <code class="docutils literal notranslate"><span class="pre">real</span></code> is an attribute of the object <code class="docutils literal notranslate"><span class="pre">z</span></code>. Strictly speaking, references to names in modules are attribute references: in the expression <code class="docutils literal notranslate"><span class="pre">modname.funcname</span></code>, <code class="docutils literal notranslate"><span class="pre">modname</span></code> is a module object and <code class="docutils literal notranslate"><span class="pre">funcname</span></code> is an attribute of it. In this case there happens to be a straightforward mapping between the module’s attributes and the global names defined in the module: they share the same namespace! <a class="footnote-reference" href="#id2" id="id1">[1]</a></p> <p>Attributes may be read-only or writable. In the latter case, assignment to attributes is possible. Module attributes are writable: you can write <code class="docutils literal notranslate"><span class="pre">modname.the_answer</span> <span class="pre">=</span> <span class="pre">42</span></code>. Writable attributes may also be deleted with the <a class="reference internal" href="../reference/simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> statement. For example, <code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">modname.the_answer</span></code> will remove the attribute <code class="xref py py-attr docutils literal notranslate"><span class="pre">the_answer</span></code> from the object named by <code class="docutils literal notranslate"><span class="pre">modname</span></code>.</p> <p>Namespaces are created at different moments and have different lifetimes. The namespace containing the built-in names is created when the Python interpreter starts up, and is never deleted. The global namespace for a module is created when the module definition is read in; normally, module namespaces also last until the interpreter quits. The statements executed by the top-level invocation of the interpreter, either read from a script file or interactively, are considered part of a module called <a class="reference internal" href="../library/__main__.html#module-__main__" title="__main__: The environment where the top-level script is run."><code class="xref py py-mod docutils literal notranslate"><span class="pre">__main__</span></code></a>, so they have their own global namespace. (The built-in names actually also live in a module; this is called <a class="reference internal" href="../library/builtins.html#module-builtins" title="builtins: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal notranslate"><span class="pre">builtins</span></code></a>.)</p> <p>The local namespace for a function is created when the function is called, and deleted when the function returns or raises an exception that is not handled within the function. (Actually, forgetting would be a better way to describe what actually happens.) Of course, recursive invocations each have their own local namespace.</p> <p>A <em>scope</em> is a textual region of a Python program where a namespace is directly accessible. “Directly accessible” here means that an unqualified reference to a name attempts to find the name in the namespace.</p> <p>Although scopes are determined statically, they are used dynamically. At any time during execution, there are at least three nested scopes whose namespaces are directly accessible:</p> <ul class="simple"> <li>the innermost scope, which is searched first, contains the local names</li> <li>the scopes of any enclosing functions, which are searched starting with the nearest enclosing scope, contains non-local, but also non-global names</li> <li>the next-to-last scope contains the current module’s global names</li> <li>the outermost scope (searched last) is the namespace containing built-in names</li> </ul> <p>If a name is declared global, then all references and assignments go directly to the middle scope containing the module’s global names. To rebind variables found outside of the innermost scope, the <a class="reference internal" href="../reference/simple_stmts.html#nonlocal"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code></a> statement can be used; if not declared nonlocal, those variables are read-only (an attempt to write to such a variable will simply create a <em>new</em> local variable in the innermost scope, leaving the identically named outer variable unchanged).</p> <p>Usually, the local scope references the local names of the (textually) current function. Outside functions, the local scope references the same namespace as the global scope: the module’s namespace. Class definitions place yet another namespace in the local scope.</p> <p>It is important to realize that scopes are determined textually: the global scope of a function defined in a module is that module’s namespace, no matter from where or by what alias the function is called. On the other hand, the actual search for names is done dynamically, at run time — however, the language definition is evolving towards static name resolution, at “compile” time, so don’t rely on dynamic name resolution! (In fact, local variables are already determined statically.)</p> <p>A special quirk of Python is that – if no <a class="reference internal" href="../reference/simple_stmts.html#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement is in effect – assignments to names always go into the innermost scope. Assignments do not copy data — they just bind names to objects. The same is true for deletions: the statement <code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">x</span></code> removes the binding of <code class="docutils literal notranslate"><span class="pre">x</span></code> from the namespace referenced by the local scope. In fact, all operations that introduce new names use the local scope: in particular, <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> statements and function definitions bind the module or function name in the local scope.</p> <p>The <a class="reference internal" href="../reference/simple_stmts.html#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement can be used to indicate that particular variables live in the global scope and should be rebound there; the <a class="reference internal" href="../reference/simple_stmts.html#nonlocal"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code></a> statement indicates that particular variables live in an enclosing scope and should be rebound there.</p> <div class="section" id="scopes-and-namespaces-example"> <span id="tut-scopeexample"></span><h3>9.2.1. Scopes and Namespaces Example<a class="headerlink" href="#scopes-and-namespaces-example" title="Permalink to this headline">¶</a></h3> <p>This is an example demonstrating how to reference the different scopes and namespaces, and how <a class="reference internal" href="../reference/simple_stmts.html#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> and <a class="reference internal" href="../reference/simple_stmts.html#nonlocal"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code></a> affect variable binding:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">scope_test</span><span class="p">():</span> <span class="k">def</span> <span class="nf">do_local</span><span class="p">():</span> <span class="n">spam</span> <span class="o">=</span> <span class="s2">"local spam"</span> <span class="k">def</span> <span class="nf">do_nonlocal</span><span class="p">():</span> <span class="k">nonlocal</span> <span class="n">spam</span> <span class="n">spam</span> <span class="o">=</span> <span class="s2">"nonlocal spam"</span> <span class="k">def</span> <span class="nf">do_global</span><span class="p">():</span> <span class="k">global</span> <span class="n">spam</span> <span class="n">spam</span> <span class="o">=</span> <span class="s2">"global spam"</span> <span class="n">spam</span> <span class="o">=</span> <span class="s2">"test spam"</span> <span class="n">do_local</span><span class="p">()</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"After local assignment:"</span><span class="p">,</span> <span class="n">spam</span><span class="p">)</span> <span class="n">do_nonlocal</span><span class="p">()</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"After nonlocal assignment:"</span><span class="p">,</span> <span class="n">spam</span><span class="p">)</span> <span class="n">do_global</span><span class="p">()</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"After global assignment:"</span><span class="p">,</span> <span class="n">spam</span><span class="p">)</span> <span class="n">scope_test</span><span class="p">()</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"In global scope:"</span><span class="p">,</span> <span class="n">spam</span><span class="p">)</span> </pre></div> </div> <p>The output of the example code is:</p> <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>After local assignment: test spam After nonlocal assignment: nonlocal spam After global assignment: nonlocal spam In global scope: global spam </pre></div> </div> <p>Note how the <em>local</em> assignment (which is default) didn’t change <em>scope_test</em>’s binding of <em>spam</em>. The <a class="reference internal" href="../reference/simple_stmts.html#nonlocal"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code></a> assignment changed <em>scope_test</em>’s binding of <em>spam</em>, and the <a class="reference internal" href="../reference/simple_stmts.html#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> assignment changed the module-level binding.</p> <p>You can also see that there was no previous binding for <em>spam</em> before the <a class="reference internal" href="../reference/simple_stmts.html#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> assignment.</p> </div> </div> <div class="section" id="a-first-look-at-classes"> <span id="tut-firstclasses"></span><h2>9.3. A First Look at Classes<a class="headerlink" href="#a-first-look-at-classes" title="Permalink to this headline">¶</a></h2> <p>Classes introduce a little bit of new syntax, three new object types, and some new semantics.</p> <div class="section" id="class-definition-syntax"> <span id="tut-classdefinition"></span><h3>9.3.1. Class Definition Syntax<a class="headerlink" href="#class-definition-syntax" title="Permalink to this headline">¶</a></h3> <p>The simplest form of class definition looks like this:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">ClassName</span><span class="p">:</span> <span class="o"><</span><span class="n">statement</span><span class="o">-</span><span class="mi">1</span><span class="o">></span> <span class="o">.</span> <span class="o">.</span> <span class="o">.</span> <span class="o"><</span><span class="n">statement</span><span class="o">-</span><span class="n">N</span><span class="o">></span> </pre></div> </div> <p>Class definitions, like function definitions (<a class="reference internal" href="../reference/compound_stmts.html#def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">def</span></code></a> statements) must be executed before they have any effect. (You could conceivably place a class definition in a branch of an <a class="reference internal" href="../reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> statement, or inside a function.)</p> <p>In practice, the statements inside a class definition will usually be function definitions, but other statements are allowed, and sometimes useful — we’ll come back to this later. The function definitions inside a class normally have a peculiar form of argument list, dictated by the calling conventions for methods — again, this is explained later.</p> <p>When a class definition is entered, a new namespace is created, and used as the local scope — thus, all assignments to local variables go into this new namespace. In particular, function definitions bind the name of the new function here.</p> <p>When a class definition is left normally (via the end), a <em>class object</em> is created. This is basically a wrapper around the contents of the namespace created by the class definition; we’ll learn more about class objects in the next section. The original local scope (the one in effect just before the class definition was entered) is reinstated, and the class object is bound here to the class name given in the class definition header (<code class="xref py py-class docutils literal notranslate"><span class="pre">ClassName</span></code> in the example).</p> </div> <div class="section" id="class-objects"> <span id="tut-classobjects"></span><h3>9.3.2. Class Objects<a class="headerlink" href="#class-objects" title="Permalink to this headline">¶</a></h3> <p>Class objects support two kinds of operations: attribute references and instantiation.</p> <p><em>Attribute references</em> use the standard syntax used for all attribute references in Python: <code class="docutils literal notranslate"><span class="pre">obj.name</span></code>. Valid attribute names are all the names that were in the class’s namespace when the class object was created. So, if the class definition looked like this:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">MyClass</span><span class="p">:</span> <span class="sd">"""A simple example class"""</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">12345</span> <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">return</span> <span class="s1">'hello world'</span> </pre></div> </div> <p>then <code class="docutils literal notranslate"><span class="pre">MyClass.i</span></code> and <code class="docutils literal notranslate"><span class="pre">MyClass.f</span></code> are valid attribute references, returning an integer and a function object, respectively. Class attributes can also be assigned to, so you can change the value of <code class="docutils literal notranslate"><span class="pre">MyClass.i</span></code> by assignment. <code class="xref py py-attr docutils literal notranslate"><span class="pre">__doc__</span></code> is also a valid attribute, returning the docstring belonging to the class: <code class="docutils literal notranslate"><span class="pre">"A</span> <span class="pre">simple</span> <span class="pre">example</span> <span class="pre">class"</span></code>.</p> <p>Class <em>instantiation</em> uses function notation. Just pretend that the class object is a parameterless function that returns a new instance of the class. For example (assuming the above class):</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">x</span> <span class="o">=</span> <span class="n">MyClass</span><span class="p">()</span> </pre></div> </div> <p>creates a new <em>instance</em> of the class and assigns this object to the local variable <code class="docutils literal notranslate"><span class="pre">x</span></code>.</p> <p>The instantiation operation (“calling” a class object) creates an empty object. Many classes like to create objects with instances customized to a specific initial state. Therefore a class may define a special method named <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a>, like this:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">data</span> <span class="o">=</span> <span class="p">[]</span> </pre></div> </div> <p>When a class defines an <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method, class instantiation automatically invokes <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> for the newly-created class instance. So in this example, a new, initialized instance can be obtained by:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">x</span> <span class="o">=</span> <span class="n">MyClass</span><span class="p">()</span> </pre></div> </div> <p>Of course, the <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method may have arguments for greater flexibility. In that case, arguments given to the class instantiation operator are passed on to <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a>. For example,</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">Complex</span><span class="p">:</span> <span class="gp">... </span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">realpart</span><span class="p">,</span> <span class="n">imagpart</span><span class="p">):</span> <span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">r</span> <span class="o">=</span> <span class="n">realpart</span> <span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">i</span> <span class="o">=</span> <span class="n">imagpart</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="n">Complex</span><span class="p">(</span><span class="mf">3.0</span><span class="p">,</span> <span class="o">-</span><span class="mf">4.5</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">x</span><span class="o">.</span><span class="n">r</span><span class="p">,</span> <span class="n">x</span><span class="o">.</span><span class="n">i</span> <span class="go">(3.0, -4.5)</span> </pre></div> </div> </div> <div class="section" id="instance-objects"> <span id="tut-instanceobjects"></span><h3>9.3.3. Instance Objects<a class="headerlink" href="#instance-objects" title="Permalink to this headline">¶</a></h3> <p>Now what can we do with instance objects? The only operations understood by instance objects are attribute references. There are two kinds of valid attribute names, data attributes and methods.</p> <p><em>data attributes</em> correspond to “instance variables” in Smalltalk, and to “data members” in C++. Data attributes need not be declared; like local variables, they spring into existence when they are first assigned to. For example, if <code class="docutils literal notranslate"><span class="pre">x</span></code> is the instance of <code class="xref py py-class docutils literal notranslate"><span class="pre">MyClass</span></code> created above, the following piece of code will print the value <code class="docutils literal notranslate"><span class="pre">16</span></code>, without leaving a trace:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">x</span><span class="o">.</span><span class="n">counter</span> <span class="o">=</span> <span class="mi">1</span> <span class="k">while</span> <span class="n">x</span><span class="o">.</span><span class="n">counter</span> <span class="o"><</span> <span class="mi">10</span><span class="p">:</span> <span class="n">x</span><span class="o">.</span><span class="n">counter</span> <span class="o">=</span> <span class="n">x</span><span class="o">.</span><span class="n">counter</span> <span class="o">*</span> <span class="mi">2</span> <span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="o">.</span><span class="n">counter</span><span class="p">)</span> <span class="k">del</span> <span class="n">x</span><span class="o">.</span><span class="n">counter</span> </pre></div> </div> <p>The other kind of instance attribute reference is a <em>method</em>. A method is a function that “belongs to” an object. (In Python, the term method is not unique to class instances: other object types can have methods as well. For example, list objects have methods called append, insert, remove, sort, and so on. However, in the following discussion, we’ll use the term method exclusively to mean methods of class instance objects, unless explicitly stated otherwise.)</p> <p id="index-0">Valid method names of an instance object depend on its class. By definition, all attributes of a class that are function objects define corresponding methods of its instances. So in our example, <code class="docutils literal notranslate"><span class="pre">x.f</span></code> is a valid method reference, since <code class="docutils literal notranslate"><span class="pre">MyClass.f</span></code> is a function, but <code class="docutils literal notranslate"><span class="pre">x.i</span></code> is not, since <code class="docutils literal notranslate"><span class="pre">MyClass.i</span></code> is not. But <code class="docutils literal notranslate"><span class="pre">x.f</span></code> is not the same thing as <code class="docutils literal notranslate"><span class="pre">MyClass.f</span></code> — it is a <em>method object</em>, not a function object.</p> </div> <div class="section" id="method-objects"> <span id="tut-methodobjects"></span><h3>9.3.4. Method Objects<a class="headerlink" href="#method-objects" title="Permalink to this headline">¶</a></h3> <p>Usually, a method is called right after it is bound:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">x</span><span class="o">.</span><span class="n">f</span><span class="p">()</span> </pre></div> </div> <p>In the <code class="xref py py-class docutils literal notranslate"><span class="pre">MyClass</span></code> example, this will return the string <code class="docutils literal notranslate"><span class="pre">'hello</span> <span class="pre">world'</span></code>. However, it is not necessary to call a method right away: <code class="docutils literal notranslate"><span class="pre">x.f</span></code> is a method object, and can be stored away and called at a later time. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">xf</span> <span class="o">=</span> <span class="n">x</span><span class="o">.</span><span class="n">f</span> <span class="k">while</span> <span class="kc">True</span><span class="p">:</span> <span class="nb">print</span><span class="p">(</span><span class="n">xf</span><span class="p">())</span> </pre></div> </div> <p>will continue to print <code class="docutils literal notranslate"><span class="pre">hello</span> <span class="pre">world</span></code> until the end of time.</p> <p>What exactly happens when a method is called? You may have noticed that <code class="docutils literal notranslate"><span class="pre">x.f()</span></code> was called without an argument above, even though the function definition for <code class="xref py py-meth docutils literal notranslate"><span class="pre">f()</span></code> specified an argument. What happened to the argument? Surely Python raises an exception when a function that requires an argument is called without any — even if the argument isn’t actually used…</p> <p>Actually, you may have guessed the answer: the special thing about methods is that the instance object is passed as the first argument of the function. In our example, the call <code class="docutils literal notranslate"><span class="pre">x.f()</span></code> is exactly equivalent to <code class="docutils literal notranslate"><span class="pre">MyClass.f(x)</span></code>. In general, calling a method with a list of <em>n</em> arguments is equivalent to calling the corresponding function with an argument list that is created by inserting the method’s instance object before the first argument.</p> <p>If you still don’t understand how methods work, a look at the implementation can perhaps clarify matters. When a non-data attribute of an instance is referenced, the instance’s class is searched. If the name denotes a valid class attribute that is a function object, a method object is created by packing (pointers to) the instance object and the function object just found together in an abstract object: this is the method object. When the method object is called with an argument list, a new argument list is constructed from the instance object and the argument list, and the function object is called with this new argument list.</p> </div> <div class="section" id="class-and-instance-variables"> <span id="tut-class-and-instance-variables"></span><h3>9.3.5. Class and Instance Variables<a class="headerlink" href="#class-and-instance-variables" title="Permalink to this headline">¶</a></h3> <p>Generally speaking, instance variables are for data unique to each instance and class variables are for attributes and methods shared by all instances of the class:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Dog</span><span class="p">:</span> <span class="n">kind</span> <span class="o">=</span> <span class="s1">'canine'</span> <span class="c1"># class variable shared by all instances</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span> <span class="c1"># instance variable unique to each instance</span> <span class="o">>>></span> <span class="n">d</span> <span class="o">=</span> <span class="n">Dog</span><span class="p">(</span><span class="s1">'Fido'</span><span class="p">)</span> <span class="o">>>></span> <span class="n">e</span> <span class="o">=</span> <span class="n">Dog</span><span class="p">(</span><span class="s1">'Buddy'</span><span class="p">)</span> <span class="o">>>></span> <span class="n">d</span><span class="o">.</span><span class="n">kind</span> <span class="c1"># shared by all dogs</span> <span class="s1">'canine'</span> <span class="o">>>></span> <span class="n">e</span><span class="o">.</span><span class="n">kind</span> <span class="c1"># shared by all dogs</span> <span class="s1">'canine'</span> <span class="o">>>></span> <span class="n">d</span><span class="o">.</span><span class="n">name</span> <span class="c1"># unique to d</span> <span class="s1">'Fido'</span> <span class="o">>>></span> <span class="n">e</span><span class="o">.</span><span class="n">name</span> <span class="c1"># unique to e</span> <span class="s1">'Buddy'</span> </pre></div> </div> <p>As discussed in <a class="reference internal" href="#tut-object"><span class="std std-ref">A Word About Names and Objects</span></a>, shared data can have possibly surprising effects with involving <a class="reference internal" href="../glossary.html#term-mutable"><span class="xref std std-term">mutable</span></a> objects such as lists and dictionaries. For example, the <em>tricks</em> list in the following code should not be used as a class variable because just a single list would be shared by all <em>Dog</em> instances:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Dog</span><span class="p">:</span> <span class="n">tricks</span> <span class="o">=</span> <span class="p">[]</span> <span class="c1"># mistaken use of a class variable</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span> <span class="k">def</span> <span class="nf">add_trick</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">trick</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">tricks</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">trick</span><span class="p">)</span> <span class="o">>>></span> <span class="n">d</span> <span class="o">=</span> <span class="n">Dog</span><span class="p">(</span><span class="s1">'Fido'</span><span class="p">)</span> <span class="o">>>></span> <span class="n">e</span> <span class="o">=</span> <span class="n">Dog</span><span class="p">(</span><span class="s1">'Buddy'</span><span class="p">)</span> <span class="o">>>></span> <span class="n">d</span><span class="o">.</span><span class="n">add_trick</span><span class="p">(</span><span class="s1">'roll over'</span><span class="p">)</span> <span class="o">>>></span> <span class="n">e</span><span class="o">.</span><span class="n">add_trick</span><span class="p">(</span><span class="s1">'play dead'</span><span class="p">)</span> <span class="o">>>></span> <span class="n">d</span><span class="o">.</span><span class="n">tricks</span> <span class="c1"># unexpectedly shared by all dogs</span> <span class="p">[</span><span class="s1">'roll over'</span><span class="p">,</span> <span class="s1">'play dead'</span><span class="p">]</span> </pre></div> </div> <p>Correct design of the class should use an instance variable instead:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Dog</span><span class="p">:</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span> <span class="bp">self</span><span class="o">.</span><span class="n">tricks</span> <span class="o">=</span> <span class="p">[]</span> <span class="c1"># creates a new empty list for each dog</span> <span class="k">def</span> <span class="nf">add_trick</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">trick</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">tricks</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">trick</span><span class="p">)</span> <span class="o">>>></span> <span class="n">d</span> <span class="o">=</span> <span class="n">Dog</span><span class="p">(</span><span class="s1">'Fido'</span><span class="p">)</span> <span class="o">>>></span> <span class="n">e</span> <span class="o">=</span> <span class="n">Dog</span><span class="p">(</span><span class="s1">'Buddy'</span><span class="p">)</span> <span class="o">>>></span> <span class="n">d</span><span class="o">.</span><span class="n">add_trick</span><span class="p">(</span><span class="s1">'roll over'</span><span class="p">)</span> <span class="o">>>></span> <span class="n">e</span><span class="o">.</span><span class="n">add_trick</span><span class="p">(</span><span class="s1">'play dead'</span><span class="p">)</span> <span class="o">>>></span> <span class="n">d</span><span class="o">.</span><span class="n">tricks</span> <span class="p">[</span><span class="s1">'roll over'</span><span class="p">]</span> <span class="o">>>></span> <span class="n">e</span><span class="o">.</span><span class="n">tricks</span> <span class="p">[</span><span class="s1">'play dead'</span><span class="p">]</span> </pre></div> </div> </div> </div> <div class="section" id="random-remarks"> <span id="tut-remarks"></span><h2>9.4. Random Remarks<a class="headerlink" href="#random-remarks" title="Permalink to this headline">¶</a></h2> <p>Data attributes override method attributes with the same name; to avoid accidental name conflicts, which may cause hard-to-find bugs in large programs, it is wise to use some kind of convention that minimizes the chance of conflicts. Possible conventions include capitalizing method names, prefixing data attribute names with a small unique string (perhaps just an underscore), or using verbs for methods and nouns for data attributes.</p> <p>Data attributes may be referenced by methods as well as by ordinary users (“clients”) of an object. In other words, classes are not usable to implement pure abstract data types. In fact, nothing in Python makes it possible to enforce data hiding — it is all based upon convention. (On the other hand, the Python implementation, written in C, can completely hide implementation details and control access to an object if necessary; this can be used by extensions to Python written in C.)</p> <p>Clients should use data attributes with care — clients may mess up invariants maintained by the methods by stamping on their data attributes. Note that clients may add data attributes of their own to an instance object without affecting the validity of the methods, as long as name conflicts are avoided — again, a naming convention can save a lot of headaches here.</p> <p>There is no shorthand for referencing data attributes (or other methods!) from within methods. I find that this actually increases the readability of methods: there is no chance of confusing local variables and instance variables when glancing through a method.</p> <p>Often, the first argument of a method is called <code class="docutils literal notranslate"><span class="pre">self</span></code>. This is nothing more than a convention: the name <code class="docutils literal notranslate"><span class="pre">self</span></code> has absolutely no special meaning to Python. Note, however, that by not following the convention your code may be less readable to other Python programmers, and it is also conceivable that a <em>class browser</em> program might be written that relies upon such a convention.</p> <p>Any function object that is a class attribute defines a method for instances of that class. It is not necessary that the function definition is textually enclosed in the class definition: assigning a function object to a local variable in the class is also ok. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Function defined outside the class</span> <span class="k">def</span> <span class="nf">f1</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">):</span> <span class="k">return</span> <span class="nb">min</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">x</span><span class="o">+</span><span class="n">y</span><span class="p">)</span> <span class="k">class</span> <span class="nc">C</span><span class="p">:</span> <span class="n">f</span> <span class="o">=</span> <span class="n">f1</span> <span class="k">def</span> <span class="nf">g</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">return</span> <span class="s1">'hello world'</span> <span class="n">h</span> <span class="o">=</span> <span class="n">g</span> </pre></div> </div> <p>Now <code class="docutils literal notranslate"><span class="pre">f</span></code>, <code class="docutils literal notranslate"><span class="pre">g</span></code> and <code class="docutils literal notranslate"><span class="pre">h</span></code> are all attributes of class <code class="xref py py-class docutils literal notranslate"><span class="pre">C</span></code> that refer to function objects, and consequently they are all methods of instances of <code class="xref py py-class docutils literal notranslate"><span class="pre">C</span></code> — <code class="docutils literal notranslate"><span class="pre">h</span></code> being exactly equivalent to <code class="docutils literal notranslate"><span class="pre">g</span></code>. Note that this practice usually only serves to confuse the reader of a program.</p> <p>Methods may call other methods by using method attributes of the <code class="docutils literal notranslate"><span class="pre">self</span></code> argument:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Bag</span><span class="p">:</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">data</span> <span class="o">=</span> <span class="p">[]</span> <span class="k">def</span> <span class="nf">add</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">data</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="k">def</span> <span class="nf">addtwice</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="bp">self</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> </pre></div> </div> <p>Methods may reference global names in the same way as ordinary functions. The global scope associated with a method is the module containing its definition. (A class is never used as a global scope.) While one rarely encounters a good reason for using global data in a method, there are many legitimate uses of the global scope: for one thing, functions and modules imported into the global scope can be used by methods, as well as functions and classes defined in it. Usually, the class containing the method is itself defined in this global scope, and in the next section we’ll find some good reasons why a method would want to reference its own class.</p> <p>Each value is an object, and therefore has a <em>class</em> (also called its <em>type</em>). It is stored as <code class="docutils literal notranslate"><span class="pre">object.__class__</span></code>.</p> </div> <div class="section" id="inheritance"> <span id="tut-inheritance"></span><h2>9.5. Inheritance<a class="headerlink" href="#inheritance" title="Permalink to this headline">¶</a></h2> <p>Of course, a language feature would not be worthy of the name “class” without supporting inheritance. The syntax for a derived class definition looks like this:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">DerivedClassName</span><span class="p">(</span><span class="n">BaseClassName</span><span class="p">):</span> <span class="o"><</span><span class="n">statement</span><span class="o">-</span><span class="mi">1</span><span class="o">></span> <span class="o">.</span> <span class="o">.</span> <span class="o">.</span> <span class="o"><</span><span class="n">statement</span><span class="o">-</span><span class="n">N</span><span class="o">></span> </pre></div> </div> <p>The name <code class="xref py py-class docutils literal notranslate"><span class="pre">BaseClassName</span></code> must be defined in a scope containing the derived class definition. In place of a base class name, other arbitrary expressions are also allowed. This can be useful, for example, when the base class is defined in another module:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">DerivedClassName</span><span class="p">(</span><span class="n">modname</span><span class="o">.</span><span class="n">BaseClassName</span><span class="p">):</span> </pre></div> </div> <p>Execution of a derived class definition proceeds the same as for a base class. When the class object is constructed, the base class is remembered. This is used for resolving attribute references: if a requested attribute is not found in the class, the search proceeds to look in the base class. This rule is applied recursively if the base class itself is derived from some other class.</p> <p>There’s nothing special about instantiation of derived classes: <code class="docutils literal notranslate"><span class="pre">DerivedClassName()</span></code> creates a new instance of the class. Method references are resolved as follows: the corresponding class attribute is searched, descending down the chain of base classes if necessary, and the method reference is valid if this yields a function object.</p> <p>Derived classes may override methods of their base classes. Because methods have no special privileges when calling other methods of the same object, a method of a base class that calls another method defined in the same base class may end up calling a method of a derived class that overrides it. (For C++ programmers: all methods in Python are effectively <code class="docutils literal notranslate"><span class="pre">virtual</span></code>.)</p> <p>An overriding method in a derived class may in fact want to extend rather than simply replace the base class method of the same name. There is a simple way to call the base class method directly: just call <code class="docutils literal notranslate"><span class="pre">BaseClassName.methodname(self,</span> <span class="pre">arguments)</span></code>. This is occasionally useful to clients as well. (Note that this only works if the base class is accessible as <code class="docutils literal notranslate"><span class="pre">BaseClassName</span></code> in the global scope.)</p> <p>Python has two built-in functions that work with inheritance:</p> <ul class="simple"> <li>Use <a class="reference internal" href="../library/functions.html#isinstance" title="isinstance"><code class="xref py py-func docutils literal notranslate"><span class="pre">isinstance()</span></code></a> to check an instance’s type: <code class="docutils literal notranslate"><span class="pre">isinstance(obj,</span> <span class="pre">int)</span></code> will be <code class="docutils literal notranslate"><span class="pre">True</span></code> only if <code class="docutils literal notranslate"><span class="pre">obj.__class__</span></code> is <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a> or some class derived from <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a>.</li> <li>Use <a class="reference internal" href="../library/functions.html#issubclass" title="issubclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">issubclass()</span></code></a> to check class inheritance: <code class="docutils literal notranslate"><span class="pre">issubclass(bool,</span> <span class="pre">int)</span></code> is <code class="docutils literal notranslate"><span class="pre">True</span></code> since <a class="reference internal" href="../library/functions.html#bool" title="bool"><code class="xref py py-class docutils literal notranslate"><span class="pre">bool</span></code></a> is a subclass of <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a>. However, <code class="docutils literal notranslate"><span class="pre">issubclass(float,</span> <span class="pre">int)</span></code> is <code class="docutils literal notranslate"><span class="pre">False</span></code> since <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate"><span class="pre">float</span></code></a> is not a subclass of <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a>.</li> </ul> <div class="section" id="multiple-inheritance"> <span id="tut-multiple"></span><h3>9.5.1. Multiple Inheritance<a class="headerlink" href="#multiple-inheritance" title="Permalink to this headline">¶</a></h3> <p>Python supports a form of multiple inheritance as well. A class definition with multiple base classes looks like this:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">DerivedClassName</span><span class="p">(</span><span class="n">Base1</span><span class="p">,</span> <span class="n">Base2</span><span class="p">,</span> <span class="n">Base3</span><span class="p">):</span> <span class="o"><</span><span class="n">statement</span><span class="o">-</span><span class="mi">1</span><span class="o">></span> <span class="o">.</span> <span class="o">.</span> <span class="o">.</span> <span class="o"><</span><span class="n">statement</span><span class="o">-</span><span class="n">N</span><span class="o">></span> </pre></div> </div> <p>For most purposes, in the simplest cases, you can think of the search for attributes inherited from a parent class as depth-first, left-to-right, not searching twice in the same class where there is an overlap in the hierarchy. Thus, if an attribute is not found in <code class="xref py py-class docutils literal notranslate"><span class="pre">DerivedClassName</span></code>, it is searched for in <code class="xref py py-class docutils literal notranslate"><span class="pre">Base1</span></code>, then (recursively) in the base classes of <code class="xref py py-class docutils literal notranslate"><span class="pre">Base1</span></code>, and if it was not found there, it was searched for in <code class="xref py py-class docutils literal notranslate"><span class="pre">Base2</span></code>, and so on.</p> <p>In fact, it is slightly more complex than that; the method resolution order changes dynamically to support cooperative calls to <a class="reference internal" href="../library/functions.html#super" title="super"><code class="xref py py-func docutils literal notranslate"><span class="pre">super()</span></code></a>. This approach is known in some other multiple-inheritance languages as call-next-method and is more powerful than the super call found in single-inheritance languages.</p> <p>Dynamic ordering is necessary because all cases of multiple inheritance exhibit one or more diamond relationships (where at least one of the parent classes can be accessed through multiple paths from the bottommost class). For example, all classes inherit 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>, so any case of multiple inheritance provides more than one path to reach <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>. To keep the base classes from being accessed more than once, the dynamic algorithm linearizes the search order in a way that preserves the left-to-right ordering specified in each class, that calls each parent only once, and that is monotonic (meaning that a class can be subclassed without affecting the precedence order of its parents). Taken together, these properties make it possible to design reliable and extensible classes with multiple inheritance. For more detail, see <a class="reference external" href="https://www.python.org/download/releases/2.3/mro/">https://www.python.org/download/releases/2.3/mro/</a>.</p> </div> </div> <div class="section" id="private-variables"> <span id="tut-private"></span><h2>9.6. Private Variables<a class="headerlink" href="#private-variables" title="Permalink to this headline">¶</a></h2> <p>“Private” instance variables that cannot be accessed except from inside an object don’t exist in Python. However, there is a convention that is followed by most Python code: a name prefixed with an underscore (e.g. <code class="docutils literal notranslate"><span class="pre">_spam</span></code>) should be treated as a non-public part of the API (whether it is a function, a method or a data member). It should be considered an implementation detail and subject to change without notice.</p> <p>Since there is a valid use-case for class-private members (namely to avoid name clashes of names with names defined by subclasses), there is limited support for such a mechanism, called <em class="dfn">name mangling</em>. Any identifier of the form <code class="docutils literal notranslate"><span class="pre">__spam</span></code> (at least two leading underscores, at most one trailing underscore) is textually replaced with <code class="docutils literal notranslate"><span class="pre">_classname__spam</span></code>, where <code class="docutils literal notranslate"><span class="pre">classname</span></code> is the current class name with leading underscore(s) stripped. This mangling is done without regard to the syntactic position of the identifier, as long as it occurs within the definition of a class.</p> <p>Name mangling is helpful for letting subclasses override methods without breaking intraclass method calls. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Mapping</span><span class="p">:</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">iterable</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">items_list</span> <span class="o">=</span> <span class="p">[]</span> <span class="bp">self</span><span class="o">.</span><span class="n">__update</span><span class="p">(</span><span class="n">iterable</span><span class="p">)</span> <span class="k">def</span> <span class="nf">update</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">iterable</span><span class="p">):</span> <span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="n">iterable</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">items_list</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">item</span><span class="p">)</span> <span class="n">__update</span> <span class="o">=</span> <span class="n">update</span> <span class="c1"># private copy of original update() method</span> <span class="k">class</span> <span class="nc">MappingSubclass</span><span class="p">(</span><span class="n">Mapping</span><span class="p">):</span> <span class="k">def</span> <span class="nf">update</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">keys</span><span class="p">,</span> <span class="n">values</span><span class="p">):</span> <span class="c1"># provides new signature for update()</span> <span class="c1"># but does not break __init__()</span> <span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="nb">zip</span><span class="p">(</span><span class="n">keys</span><span class="p">,</span> <span class="n">values</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">items_list</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">item</span><span class="p">)</span> </pre></div> </div> <p>Note that the mangling rules are designed mostly to avoid accidents; it still is possible to access or modify a variable that is considered private. This can even be useful in special circumstances, such as in the debugger.</p> <p>Notice that code passed to <code class="docutils literal notranslate"><span class="pre">exec()</span></code> or <code class="docutils literal notranslate"><span class="pre">eval()</span></code> does not consider the classname of the invoking class to be the current class; this is similar to the effect of the <code class="docutils literal notranslate"><span class="pre">global</span></code> statement, the effect of which is likewise restricted to code that is byte-compiled together. The same restriction applies to <code class="docutils literal notranslate"><span class="pre">getattr()</span></code>, <code class="docutils literal notranslate"><span class="pre">setattr()</span></code> and <code class="docutils literal notranslate"><span class="pre">delattr()</span></code>, as well as when referencing <code class="docutils literal notranslate"><span class="pre">__dict__</span></code> directly.</p> </div> <div class="section" id="odds-and-ends"> <span id="tut-odds"></span><h2>9.7. Odds and Ends<a class="headerlink" href="#odds-and-ends" title="Permalink to this headline">¶</a></h2> <p>Sometimes it is useful to have a data type similar to the Pascal “record” or C “struct”, bundling together a few named data items. An empty class definition will do nicely:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Employee</span><span class="p">:</span> <span class="k">pass</span> <span class="n">john</span> <span class="o">=</span> <span class="n">Employee</span><span class="p">()</span> <span class="c1"># Create an empty employee record</span> <span class="c1"># Fill the fields of the record</span> <span class="n">john</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="s1">'John Doe'</span> <span class="n">john</span><span class="o">.</span><span class="n">dept</span> <span class="o">=</span> <span class="s1">'computer lab'</span> <span class="n">john</span><span class="o">.</span><span class="n">salary</span> <span class="o">=</span> <span class="mi">1000</span> </pre></div> </div> <p>A piece of Python code that expects a particular abstract data type can often be passed a class that emulates the methods of that data type instead. For instance, if you have a function that formats some data from a file object, you can define a class with methods <code class="xref py py-meth docutils literal notranslate"><span class="pre">read()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">readline()</span></code> that get the data from a string buffer instead, and pass it as an argument.</p> <p>Instance method objects have attributes, too: <code class="docutils literal notranslate"><span class="pre">m.__self__</span></code> is the instance object with the method <code class="xref py py-meth docutils literal notranslate"><span class="pre">m()</span></code>, and <code class="docutils literal notranslate"><span class="pre">m.__func__</span></code> is the function object corresponding to the method.</p> </div> <div class="section" id="iterators"> <span id="tut-iterators"></span><h2>9.8. Iterators<a class="headerlink" href="#iterators" title="Permalink to this headline">¶</a></h2> <p>By now you have probably noticed that most container objects can be looped over using a <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> statement:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">element</span> <span class="ow">in</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]:</span> <span class="nb">print</span><span class="p">(</span><span class="n">element</span><span class="p">)</span> <span class="k">for</span> <span class="n">element</span> <span class="ow">in</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">):</span> <span class="nb">print</span><span class="p">(</span><span class="n">element</span><span class="p">)</span> <span class="k">for</span> <span class="n">key</span> <span class="ow">in</span> <span class="p">{</span><span class="s1">'one'</span><span class="p">:</span><span class="mi">1</span><span class="p">,</span> <span class="s1">'two'</span><span class="p">:</span><span class="mi">2</span><span class="p">}:</span> <span class="nb">print</span><span class="p">(</span><span class="n">key</span><span class="p">)</span> <span class="k">for</span> <span class="n">char</span> <span class="ow">in</span> <span class="s2">"123"</span><span class="p">:</span> <span class="nb">print</span><span class="p">(</span><span class="n">char</span><span class="p">)</span> <span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="nb">open</span><span class="p">(</span><span class="s2">"myfile.txt"</span><span class="p">):</span> <span class="nb">print</span><span class="p">(</span><span class="n">line</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">''</span><span class="p">)</span> </pre></div> </div> <p>This style of access is clear, concise, and convenient. The use of iterators pervades and unifies Python. Behind the scenes, the <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> statement calls <a class="reference internal" href="../library/functions.html#iter" title="iter"><code class="xref py py-func docutils literal notranslate"><span class="pre">iter()</span></code></a> on the container object. The function returns an iterator object that defines the method <a class="reference internal" href="../library/stdtypes.html#iterator.__next__" title="iterator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code></a> which accesses elements in the container one at a time. When there are no more elements, <a class="reference internal" href="../library/stdtypes.html#iterator.__next__" title="iterator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code></a> raises 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 which tells the <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> loop to terminate. You can call the <a class="reference internal" href="../library/stdtypes.html#iterator.__next__" title="iterator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code></a> method using 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> built-in function; this example shows how it all works:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">s</span> <span class="o">=</span> <span class="s1">'abc'</span> <span class="gp">>>> </span><span class="n">it</span> <span class="o">=</span> <span class="nb">iter</span><span class="p">(</span><span class="n">s</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">it</span> <span class="go"><iterator object at 0x00A1DB50></span> <span class="gp">>>> </span><span class="nb">next</span><span class="p">(</span><span class="n">it</span><span class="p">)</span> <span class="go">'a'</span> <span class="gp">>>> </span><span class="nb">next</span><span class="p">(</span><span class="n">it</span><span class="p">)</span> <span class="go">'b'</span> <span class="gp">>>> </span><span class="nb">next</span><span class="p">(</span><span class="n">it</span><span class="p">)</span> <span class="go">'c'</span> <span class="gp">>>> </span><span class="nb">next</span><span class="p">(</span><span class="n">it</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="nb">next</span><span class="p">(</span><span class="n">it</span><span class="p">)</span> <span class="gr">StopIteration</span> </pre></div> </div> <p>Having seen the mechanics behind the iterator protocol, it is easy to add iterator behavior to your classes. Define an <a class="reference internal" href="../reference/datamodel.html#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a> method which returns an object with a <a class="reference internal" href="../library/stdtypes.html#iterator.__next__" title="iterator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code></a> method. If the class defines <code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code>, then <a class="reference internal" href="../reference/datamodel.html#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a> can just return <code class="docutils literal notranslate"><span class="pre">self</span></code>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Reverse</span><span class="p">:</span> <span class="sd">"""Iterator for looping over a sequence backwards."""</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">data</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">data</span> <span class="o">=</span> <span class="n">data</span> <span class="bp">self</span><span class="o">.</span><span class="n">index</span> <span class="o">=</span> <span class="nb">len</span><span class="p">(</span><span class="n">data</span><span class="p">)</span> <span class="k">def</span> <span class="nf">__iter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">return</span> <span class="bp">self</span> <span class="k">def</span> <span class="nf">__next__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">index</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span> <span class="k">raise</span> <span class="ne">StopIteration</span> <span class="bp">self</span><span class="o">.</span><span class="n">index</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">index</span> <span class="o">-</span> <span class="mi">1</span> <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">data</span><span class="p">[</span><span class="bp">self</span><span class="o">.</span><span class="n">index</span><span class="p">]</span> </pre></div> </div> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">rev</span> <span class="o">=</span> <span class="n">Reverse</span><span class="p">(</span><span class="s1">'spam'</span><span class="p">)</span> <span class="gp">>>> </span><span class="nb">iter</span><span class="p">(</span><span class="n">rev</span><span class="p">)</span> <span class="go"><__main__.Reverse object at 0x00A1DB50></span> <span class="gp">>>> </span><span class="k">for</span> <span class="n">char</span> <span class="ow">in</span> <span class="n">rev</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">char</span><span class="p">)</span> <span class="gp">...</span> <span class="go">m</span> <span class="go">a</span> <span class="go">p</span> <span class="go">s</span> </pre></div> </div> </div> <div class="section" id="generators"> <span id="tut-generators"></span><h2>9.9. Generators<a class="headerlink" href="#generators" title="Permalink to this headline">¶</a></h2> <p><a class="reference internal" href="../glossary.html#term-generator"><span class="xref std std-term">Generator</span></a>s are a simple and powerful tool for creating iterators. They are written like regular functions but use the <a class="reference internal" href="../reference/simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement whenever they want to return data. Each time <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> is called on it, the generator resumes where it left off (it remembers all the data values and which statement was last executed). An example shows that generators can be trivially easy to create:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">reverse</span><span class="p">(</span><span class="n">data</span><span class="p">):</span> <span class="k">for</span> <span class="n">index</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">data</span><span class="p">)</span><span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">):</span> <span class="k">yield</span> <span class="n">data</span><span class="p">[</span><span class="n">index</span><span class="p">]</span> </pre></div> </div> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">for</span> <span class="n">char</span> <span class="ow">in</span> <span class="n">reverse</span><span class="p">(</span><span class="s1">'golf'</span><span class="p">):</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">char</span><span class="p">)</span> <span class="gp">...</span> <span class="go">f</span> <span class="go">l</span> <span class="go">o</span> <span class="go">g</span> </pre></div> </div> <p>Anything that can be done with generators can also be done with class-based iterators as described in the previous section. What makes generators so compact is that the <a class="reference internal" href="../reference/datamodel.html#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a> and <a class="reference internal" href="../reference/expressions.html#generator.__next__" title="generator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code></a> methods are created automatically.</p> <p>Another key feature is that the local variables and execution state are automatically saved between calls. This made the function easier to write and much more clear than an approach using instance variables like <code class="docutils literal notranslate"><span class="pre">self.index</span></code> and <code class="docutils literal notranslate"><span class="pre">self.data</span></code>.</p> <p>In addition to automatic method creation and saving program state, when generators terminate, they automatically raise <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>. In combination, these features make it easy to create iterators with no more effort than writing a regular function.</p> </div> <div class="section" id="generator-expressions"> <span id="tut-genexps"></span><h2>9.10. Generator Expressions<a class="headerlink" href="#generator-expressions" title="Permalink to this headline">¶</a></h2> <p>Some simple generators can be coded succinctly as expressions using a syntax similar to list comprehensions but with parentheses instead of square brackets. These expressions are designed for situations where the generator is used right away by an enclosing function. Generator expressions are more compact but less versatile than full generator definitions and tend to be more memory friendly than equivalent list comprehensions.</p> <p>Examples:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">sum</span><span class="p">(</span><span class="n">i</span><span class="o">*</span><span class="n">i</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span> <span class="c1"># sum of squares</span> <span class="go">285</span> <span class="gp">>>> </span><span class="n">xvec</span> <span class="o">=</span> <span class="p">[</span><span class="mi">10</span><span class="p">,</span> <span class="mi">20</span><span class="p">,</span> <span class="mi">30</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">yvec</span> <span class="o">=</span> <span class="p">[</span><span class="mi">7</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span> <span class="gp">>>> </span><span class="nb">sum</span><span class="p">(</span><span class="n">x</span><span class="o">*</span><span class="n">y</span> <span class="k">for</span> <span class="n">x</span><span class="p">,</span><span class="n">y</span> <span class="ow">in</span> <span class="nb">zip</span><span class="p">(</span><span class="n">xvec</span><span class="p">,</span> <span class="n">yvec</span><span class="p">))</span> <span class="c1"># dot product</span> <span class="go">260</span> <span class="gp">>>> </span><span class="kn">from</span> <span class="nn">math</span> <span class="k">import</span> <span class="n">pi</span><span class="p">,</span> <span class="n">sin</span> <span class="gp">>>> </span><span class="n">sine_table</span> <span class="o">=</span> <span class="p">{</span><span class="n">x</span><span class="p">:</span> <span class="n">sin</span><span class="p">(</span><span class="n">x</span><span class="o">*</span><span class="n">pi</span><span class="o">/</span><span class="mi">180</span><span class="p">)</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">91</span><span class="p">)}</span> <span class="gp">>>> </span><span class="n">unique_words</span> <span class="o">=</span> <span class="nb">set</span><span class="p">(</span><span class="n">word</span> <span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="n">page</span> <span class="k">for</span> <span class="n">word</span> <span class="ow">in</span> <span class="n">line</span><span class="o">.</span><span class="n">split</span><span class="p">())</span> <span class="gp">>>> </span><span class="n">valedictorian</span> <span class="o">=</span> <span class="nb">max</span><span class="p">((</span><span class="n">student</span><span class="o">.</span><span class="n">gpa</span><span class="p">,</span> <span class="n">student</span><span class="o">.</span><span class="n">name</span><span class="p">)</span> <span class="k">for</span> <span class="n">student</span> <span class="ow">in</span> <span class="n">graduates</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">data</span> <span class="o">=</span> <span class="s1">'golf'</span> <span class="gp">>>> </span><span class="nb">list</span><span class="p">(</span><span class="n">data</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">data</span><span class="p">)</span><span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">))</span> <span class="go">['f', 'l', 'o', 'g']</span> </pre></div> </div> <p class="rubric">Footnotes</p> <table class="docutils footnote" frame="void" id="id2" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>Except for one thing. Module objects have a secret read-only attribute called <a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a> which returns the dictionary used to implement the module’s namespace; the name <a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a> is an attribute but not a global name. Obviously, using this violates the abstraction of namespace implementation, and should be restricted to things like post-mortem debuggers.</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="#">9. Classes</a><ul> <li><a class="reference internal" href="#a-word-about-names-and-objects">9.1. A Word About Names and Objects</a></li> <li><a class="reference internal" href="#python-scopes-and-namespaces">9.2. Python Scopes and Namespaces</a><ul> <li><a class="reference internal" href="#scopes-and-namespaces-example">9.2.1. Scopes and Namespaces Example</a></li> </ul> </li> <li><a class="reference internal" href="#a-first-look-at-classes">9.3. A First Look at Classes</a><ul> <li><a class="reference internal" href="#class-definition-syntax">9.3.1. Class Definition Syntax</a></li> <li><a class="reference internal" href="#class-objects">9.3.2. Class Objects</a></li> <li><a class="reference internal" href="#instance-objects">9.3.3. Instance Objects</a></li> <li><a class="reference internal" href="#method-objects">9.3.4. Method Objects</a></li> <li><a class="reference internal" href="#class-and-instance-variables">9.3.5. Class and Instance Variables</a></li> </ul> </li> <li><a class="reference internal" href="#random-remarks">9.4. Random Remarks</a></li> <li><a class="reference internal" href="#inheritance">9.5. Inheritance</a><ul> <li><a class="reference internal" href="#multiple-inheritance">9.5.1. Multiple Inheritance</a></li> </ul> </li> <li><a class="reference internal" href="#private-variables">9.6. Private Variables</a></li> <li><a class="reference internal" href="#odds-and-ends">9.7. Odds and Ends</a></li> <li><a class="reference internal" href="#iterators">9.8. Iterators</a></li> <li><a class="reference internal" href="#generators">9.9. Generators</a></li> <li><a class="reference internal" href="#generator-expressions">9.10. Generator Expressions</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="errors.html" title="previous chapter">8. Errors and Exceptions</a></p> <h4>Next topic</h4> <p class="topless"><a href="stdlib.html" title="next chapter">10. 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