DOCS/tutorial/classes.html

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang="" xml:lang="">
  <head>
    <meta charset="utf-8" />
    <meta name="generator" content="pandoc" />
    <meta
      name="viewport"
      content="width=device-width, initial-scale=1.0, user-scalable=yes"
    />
    <title>classes</title>
    <style type="text/css">
      code {
        white-space: pre-wrap;
      }
      span.smallcaps {
        font-variant: small-caps;
      }
      span.underline {
        text-decoration: underline;
      }
      div.column {
        display: inline-block;
        vertical-align: top;
        width: 50%;
      }
    </style>
  </head>
  <body>
    <h3 id="navigation">Navigation</h3>
    <ul>
      <li>
        <a href="https://docs.python.org/3/genindex.html" title="General Index"
          >index</a
        >
      </li>
      <li>
        <a
          href="https://docs.python.org/3/py-modindex.html"
          title="Python Module Index"
          >modules</a
        >
        |
      </li>
      <li>
        <a href="stdlib.html" title="10. Brief Tour of the Standard Library"
          >next</a
        >
        |
      </li>
      <li>
        <a href="errors.html" title="8. Errors and Exceptions">previous</a> |
      </li>
      <li><img src="../_static/py.png" /></li>
      <li><a href="https://www.python.org/">Python</a> »</li>
      <li>
        <a href="https://docs.python.org/3/index.html">3.9.5 Documentation</a> »
      </li>
      <li><a href="index.html">The Python Tutorial</a> »</li>
      <li></li>
    </ul>
    <p><span id="tut-classes"></span></p>
    <h1 id="classes">
      <span class="section-number">9. </span>Classes<a
        href="#classes"
        class="headerlink"
        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 href="#tut-private" class="reference internal"
        ><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>
    <p><span id="tut-object"></span></p>
    <h2 id="a-word-about-names-and-objects">
      <span class="section-number">9.1. </span>A Word About Names and Objects<a
        href="#a-word-about-names-and-objects"
        class="headerlink"
        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>
    <p><span id="tut-scopes"></span></p>
    <h2 id="python-scopes-and-namespaces">
      <span class="section-number">9.2. </span>Python Scopes and Namespaces<a
        href="#python-scopes-and-namespaces"
        class="headerlink"
        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
        href="https://docs.python.org/3/library/functions.html#abs"
        class="reference internal"
        title="abs"
        ><code class="sourceCode python"><span class="bu">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>maximize</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>z.real</code>, <code>real</code> is
      an attribute of the object <code>z</code>. Strictly speaking, references
      to names in modules are attribute references: in the expression
      <code>modname.funcname</code>, <code>modname</code> is a module object and
      <code>funcname</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 href="#id2" id="id1" class="footnote-reference brackets">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>modname.the_answer = 42</code>. Writable attributes may also be
      deleted with the
      <a
        href="https://docs.python.org/3/reference/simple_stmts.html#del"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >del</code
        ></a
      >
      statement. For example, <code>del modname.the_answer</code> will remove
      the attribute <code>the_answer</code> from the object named by
      <code>modname</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
        href="https://docs.python.org/3/library/__main__.html#module-__main__"
        class="reference internal"
        title="__main__: The environment where the top-level script is run."
        ><code class="sourceCode python">main</code></a
      >, so they have their own global namespace. (The built-in names actually
      also live in a module; this is called
      <a
        href="https://docs.python.org/3/library/builtins.html#module-builtins"
        class="reference internal"
        title="builtins: The module that provides the built-in namespace."
        ><code class="sourceCode python">builtins</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 3 or 4 nested scopes whose namespaces
      are directly accessible:
    </p>
    <ul>
      <li>
        <p>
          the innermost scope, which is searched first, contains the local names
        </p>
      </li>
      <li>
        <p>
          the scopes of any enclosing functions, which are searched starting
          with the nearest enclosing scope, contains non-local, but also
          non-global names
        </p>
      </li>
      <li>
        <p>the next-to-last scope contains the current module’s global names</p>
      </li>
      <li>
        <p>
          the outermost scope (searched last) is the namespace containing
          built-in names
        </p>
      </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
        href="https://docs.python.org/3/reference/simple_stmts.html#nonlocal"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >nonlocal</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
        href="https://docs.python.org/3/reference/simple_stmts.html#global"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >global</code
        ></a
      >
      or
      <a
        href="https://docs.python.org/3/reference/simple_stmts.html#nonlocal"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >nonlocal</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>del x</code> removes the
      binding of <code>x</code> from the namespace referenced by the local
      scope. In fact, all operations that introduce new names use the local
      scope: in particular,
      <a
        href="https://docs.python.org/3/reference/simple_stmts.html#import"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >import</code
        ></a
      >
      statements and function definitions bind the module or function name in
      the local scope.
    </p>
    <p>
      The
      <a
        href="https://docs.python.org/3/reference/simple_stmts.html#global"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >global</code
        ></a
      >
      statement can be used to indicate that particular variables live in the
      global scope and should be rebound there; the
      <a
        href="https://docs.python.org/3/reference/simple_stmts.html#nonlocal"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >nonlocal</code
        ></a
      >
      statement indicates that particular variables live in an enclosing scope
      and should be rebound there.
    </p>
    <p><span id="tut-scopeexample"></span></p>
    <h3 id="scopes-and-namespaces-example">
      <span class="section-number">9.2.1. </span>Scopes and Namespaces Example<a
        href="#scopes-and-namespaces-example"
        class="headerlink"
        title="Permalink to this headline"
        >¶</a
      >
    </h3>
    <p>
      This is an example demonstrating how to reference the different scopes and
      namespaces, and how
      <a
        href="https://docs.python.org/3/reference/simple_stmts.html#global"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >global</code
        ></a
      >
      and
      <a
        href="https://docs.python.org/3/reference/simple_stmts.html#nonlocal"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >nonlocal</code
        ></a
      >
      affect variable binding:
    </p>
    <pre><code>def scope_test():
    def do_local():
        spam = &quot;local spam&quot;

    def do_nonlocal():
        nonlocal spam
        spam = &quot;nonlocal spam&quot;

    def do_global():
        global spam
        spam = &quot;global spam&quot;

    spam = &quot;test spam&quot;
    do_local()
    print(&quot;After local assignment:&quot;, spam)
    do_nonlocal()
    print(&quot;After nonlocal assignment:&quot;, spam)
    do_global()
    print(&quot;After global assignment:&quot;, spam)

scope_test()
print(&quot;In global scope:&quot;, spam)</code></pre>
    <p>The output of the example code is:</p>
    <pre><code>After local assignment: test spam
After nonlocal assignment: nonlocal spam
After global assignment: nonlocal spam
In global scope: global spam</code></pre>
    <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
        href="https://docs.python.org/3/reference/simple_stmts.html#nonlocal"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >nonlocal</code
        ></a
      >
      assignment changed <em>scope_test</em>’s binding of <em>spam</em>, and the
      <a
        href="https://docs.python.org/3/reference/simple_stmts.html#global"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >global</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
        href="https://docs.python.org/3/reference/simple_stmts.html#global"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >global</code
        ></a
      >
      assignment.
    </p>
    <p><span id="tut-firstclasses"></span></p>
    <h2 id="a-first-look-at-classes">
      <span class="section-number">9.3. </span>A First Look at Classes<a
        href="#a-first-look-at-classes"
        class="headerlink"
        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>
    <p><span id="tut-classdefinition"></span></p>
    <h3 id="class-definition-syntax">
      <span class="section-number">9.3.1. </span>Class Definition Syntax<a
        href="#class-definition-syntax"
        class="headerlink"
        title="Permalink to this headline"
        >¶</a
      >
    </h3>
    <p>The simplest form of class definition looks like this:</p>
    <pre><code>class ClassName:
    &lt;statement-1&gt;
    .
    .
    .
    &lt;statement-N&gt;</code></pre>
    <p>
      Class definitions, like function definitions (<a
        href="https://docs.python.org/3/reference/compound_stmts.html#def"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >def</code
        ></a
      >
      statements) must be executed before they have any effect. (You could
      conceivably place a class definition in a branch of an
      <a
        href="https://docs.python.org/3/reference/compound_stmts.html#if"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >if</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>ClassName</code> in the example).
    </p>
    <p><span id="tut-classobjects"></span></p>
    <h3 id="class-objects">
      <span class="section-number">9.3.2. </span>Class Objects<a
        href="#class-objects"
        class="headerlink"
        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>obj.name</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>
    <pre><code>class MyClass:
    &quot;&quot;&quot;A simple example class&quot;&quot;&quot;
    i = 12345

    def f(self):
        return &#39;hello world&#39;</code></pre>
    <p>
      then <code>MyClass.i</code> and <code>MyClass.f</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>MyClass.i</code> by assignment. <code>__doc__</code> is also a valid
      attribute, returning the docstring belonging to the class:
      <code>"A simple example class"</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>
    <pre><code>x = MyClass()</code></pre>
    <p>
      creates a new <em>instance</em> of the class and assigns this object to
      the local variable <code>x</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
        href="https://docs.python.org/3/reference/datamodel.html#object.__init__"
        class="reference internal"
        title="object.__init__"
        ><code class="sourceCode python">init()</code></a
      >, like this:
    </p>
    <pre><code>def __init__(self):
    self.data = []</code></pre>
    <p>
      When a class defines an
      <a
        href="https://docs.python.org/3/reference/datamodel.html#object.__init__"
        class="reference internal"
        title="object.__init__"
        ><code class="sourceCode python">init()</code></a
      >
      method, class instantiation automatically invokes
      <a
        href="https://docs.python.org/3/reference/datamodel.html#object.__init__"
        class="reference internal"
        title="object.__init__"
        ><code class="sourceCode python">init()</code></a
      >
      for the newly-created class instance. So in this example, a new,
      initialized instance can be obtained by:
    </p>
    <pre><code>x = MyClass()</code></pre>
    <p>
      Of course, the
      <a
        href="https://docs.python.org/3/reference/datamodel.html#object.__init__"
        class="reference internal"
        title="object.__init__"
        ><code class="sourceCode python">init()</code></a
      >
      method may have arguments for greater flexibility. In that case, arguments
      given to the class instantiation operator are passed on to
      <a
        href="https://docs.python.org/3/reference/datamodel.html#object.__init__"
        class="reference internal"
        title="object.__init__"
        ><code class="sourceCode python">init()</code></a
      >. For example,
    </p>
    <pre><code>&gt;&gt;&gt; class Complex:
...     def __init__(self, realpart, imagpart):
...         self.r = realpart
...         self.i = imagpart
...
&gt;&gt;&gt; x = Complex(3.0, -4.5)
&gt;&gt;&gt; x.r, x.i
(3.0, -4.5)</code></pre>
    <p><span id="tut-instanceobjects"></span></p>
    <h3 id="instance-objects">
      <span class="section-number">9.3.3. </span>Instance Objects<a
        href="#instance-objects"
        class="headerlink"
        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>x</code> is the instance of
      <code>MyClass</code> created above, the following piece of code will print
      the value <code>16</code>, without leaving a trace:
    </p>
    <pre><code>x.counter = 1
while x.counter &lt; 10:
    x.counter = x.counter * 2
print(x.counter)
del x.counter</code></pre>
    <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>
      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>x.f</code> is a valid method reference, since
      <code>MyClass.f</code> is a function, but <code>x.i</code> is not, since
      <code>MyClass.i</code> is not. But <code>x.f</code> is not the same thing
      as <code>MyClass.f</code> — it is a <em>method object</em>, not a function
      object.
    </p>
    <p><span id="tut-methodobjects"></span></p>
    <h3 id="method-objects">
      <span class="section-number">9.3.4. </span>Method Objects<a
        href="#method-objects"
        class="headerlink"
        title="Permalink to this headline"
        >¶</a
      >
    </h3>
    <p>Usually, a method is called right after it is bound:</p>
    <pre><code>x.f()</code></pre>
    <p>
      In the <code>MyClass</code> example, this will return the string
      <code>'hello world'</code>. However, it is not necessary to call a method
      right away: <code>x.f</code> is a method object, and can be stored away
      and called at a later time. For example:
    </p>
    <pre><code>xf = x.f
while True:
    print(xf())</code></pre>
    <p>
      will continue to print <code>hello world</code> until the end of time.
    </p>
    <p>
      What exactly happens when a method is called? You may have noticed that
      <code>x.f()</code> was called without an argument above, even though the
      function definition for <code>f()</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>x.f()</code> is exactly
      equivalent to <code>MyClass.f(x)</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>
    <p><span id="tut-class-and-instance-variables"></span></p>
    <h3 id="class-and-instance-variables">
      <span class="section-number">9.3.5. </span>Class and Instance Variables<a
        href="#class-and-instance-variables"
        class="headerlink"
        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>
    <pre><code>class Dog:

    kind = &#39;canine&#39;         # class variable shared by all instances

    def __init__(self, name):
        self.name = name    # instance variable unique to each instance

&gt;&gt;&gt; d = Dog(&#39;Fido&#39;)
&gt;&gt;&gt; e = Dog(&#39;Buddy&#39;)
&gt;&gt;&gt; d.kind                  # shared by all dogs
&#39;canine&#39;
&gt;&gt;&gt; e.kind                  # shared by all dogs
&#39;canine&#39;
&gt;&gt;&gt; d.name                  # unique to d
&#39;Fido&#39;
&gt;&gt;&gt; e.name                  # unique to e
&#39;Buddy&#39;</code></pre>
    <p>
      As discussed in
      <a href="#tut-object" class="reference internal"
        ><span class="std std-ref">A Word About Names and Objects</span></a
      >, shared data can have possibly surprising effects with involving
      <a
        href="https://docs.python.org/3/glossary.html#term-mutable"
        class="reference internal"
        ><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>
    <pre><code>class Dog:

    tricks = []             # mistaken use of a class variable

    def __init__(self, name):
        self.name = name

    def add_trick(self, trick):
        self.tricks.append(trick)

&gt;&gt;&gt; d = Dog(&#39;Fido&#39;)
&gt;&gt;&gt; e = Dog(&#39;Buddy&#39;)
&gt;&gt;&gt; d.add_trick(&#39;roll over&#39;)
&gt;&gt;&gt; e.add_trick(&#39;play dead&#39;)
&gt;&gt;&gt; d.tricks                # unexpectedly shared by all dogs
[&#39;roll over&#39;, &#39;play dead&#39;]</code></pre>
    <p>Correct design of the class should use an instance variable instead:</p>
    <pre><code>class Dog:

    def __init__(self, name):
        self.name = name
        self.tricks = []    # creates a new empty list for each dog

    def add_trick(self, trick):
        self.tricks.append(trick)

&gt;&gt;&gt; d = Dog(&#39;Fido&#39;)
&gt;&gt;&gt; e = Dog(&#39;Buddy&#39;)
&gt;&gt;&gt; d.add_trick(&#39;roll over&#39;)
&gt;&gt;&gt; e.add_trick(&#39;play dead&#39;)
&gt;&gt;&gt; d.tricks
[&#39;roll over&#39;]
&gt;&gt;&gt; e.tricks
[&#39;play dead&#39;]</code></pre>
    <p><span id="tut-remarks"></span></p>
    <h2 id="random-remarks">
      <span class="section-number">9.4. </span>Random Remarks<a
        href="#random-remarks"
        class="headerlink"
        title="Permalink to this headline"
        >¶</a
      >
    </h2>
    <p>
      If the same attribute name occurs in both an instance and in a class, then
      attribute lookup prioritizes the instance:
    </p>
    <pre><code>&gt;&gt;&gt; class Warehouse:
        purpose = &#39;storage&#39;
        region = &#39;west&#39;

&gt;&gt;&gt; w1 = Warehouse()
&gt;&gt;&gt; print(w1.purpose, w1.region)
storage west
&gt;&gt;&gt; w2 = Warehouse()
&gt;&gt;&gt; w2.region = &#39;east&#39;
&gt;&gt;&gt; print(w2.purpose, w2.region)
storage east</code></pre>
    <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>self</code>. This is
      nothing more than a convention: the name <code>self</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>
    <pre><code># Function defined outside the class
def f1(self, x, y):
    return min(x, x+y)

class C:
    f = f1

    def g(self):
        return &#39;hello world&#39;

    h = g</code></pre>
    <p>
      Now <code>f</code>, <code>g</code> and <code>h</code> are all attributes
      of class <code>C</code> that refer to function objects, and consequently
      they are all methods of instances of <code>C</code> — <code>h</code> being
      exactly equivalent to <code>g</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>self</code> argument:
    </p>
    <pre><code>class Bag:
    def __init__(self):
        self.data = []

    def add(self, x):
        self.data.append(x)

    def addtwice(self, x):
        self.add(x)
        self.add(x)</code></pre>
    <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>object.__class__</code>.
    </p>
    <p><span id="tut-inheritance"></span></p>
    <h2 id="inheritance">
      <span class="section-number">9.5. </span>Inheritance<a
        href="#inheritance"
        class="headerlink"
        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>
    <pre><code>class DerivedClassName(BaseClassName):
    &lt;statement-1&gt;
    .
    .
    .
    &lt;statement-N&gt;</code></pre>
    <p>
      The name <code>BaseClassName</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>
    <pre><code>class DerivedClassName(modname.BaseClassName):</code></pre>
    <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>DerivedClassName()</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>virtual</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>BaseClassName.methodname(self, arguments)</code>. This is
      occasionally useful to clients as well. (Note that this only works if the
      base class is accessible as <code>BaseClassName</code> in the global
      scope.)
    </p>
    <p>Python has two built-in functions that work with inheritance:</p>
    <ul>
      <li>
        <p>
          Use
          <a
            href="https://docs.python.org/3/library/functions.html#isinstance"
            class="reference internal"
            title="isinstance"
            ><code class="sourceCode python"
              ><span class="bu">isinstance</span>()</code
            ></a
          >
          to check an instance’s type: <code>isinstance(obj, int)</code> will be
          <code>True</code> only if <code>obj.__class__</code> is
          <a
            href="https://docs.python.org/3/library/functions.html#int"
            class="reference internal"
            title="int"
            ><code class="sourceCode python"
              ><span class="bu">int</span></code
            ></a
          >
          or some class derived from
          <a
            href="https://docs.python.org/3/library/functions.html#int"
            class="reference internal"
            title="int"
            ><code class="sourceCode python"
              ><span class="bu">int</span></code
            ></a
          >.
        </p>
      </li>
      <li>
        <p>
          Use
          <a
            href="https://docs.python.org/3/library/functions.html#issubclass"
            class="reference internal"
            title="issubclass"
            ><code class="sourceCode python"
              ><span class="bu">issubclass</span>()</code
            ></a
          >
          to check class inheritance: <code>issubclass(bool, int)</code> is
          <code>True</code> since
          <a
            href="https://docs.python.org/3/library/functions.html#bool"
            class="reference internal"
            title="bool"
            ><code class="sourceCode python"
              ><span class="bu">bool</span></code
            ></a
          >
          is a subclass of
          <a
            href="https://docs.python.org/3/library/functions.html#int"
            class="reference internal"
            title="int"
            ><code class="sourceCode python"
              ><span class="bu">int</span></code
            ></a
          >. However, <code>issubclass(float, int)</code> is
          <code>False</code> since
          <a
            href="https://docs.python.org/3/library/functions.html#float"
            class="reference internal"
            title="float"
            ><code class="sourceCode python"
              ><span class="bu">float</span></code
            ></a
          >
          is not a subclass of
          <a
            href="https://docs.python.org/3/library/functions.html#int"
            class="reference internal"
            title="int"
            ><code class="sourceCode python"
              ><span class="bu">int</span></code
            ></a
          >.
        </p>
      </li>
    </ul>
    <p><span id="tut-multiple"></span></p>
    <h3 id="multiple-inheritance">
      <span class="section-number">9.5.1. </span>Multiple Inheritance<a
        href="#multiple-inheritance"
        class="headerlink"
        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>
    <pre><code>class DerivedClassName(Base1, Base2, Base3):
    &lt;statement-1&gt;
    .
    .
    .
    &lt;statement-N&gt;</code></pre>
    <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>DerivedClassName</code>, it is searched for in <code>Base1</code>,
      then (recursively) in the base classes of <code>Base1</code>, and if it
      was not found there, it was searched for in <code>Base2</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
        href="https://docs.python.org/3/library/functions.html#super"
        class="reference internal"
        title="super"
        ><code class="sourceCode python"
          ><span class="bu">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
        href="https://docs.python.org/3/library/functions.html#object"
        class="reference internal"
        title="object"
        ><code class="sourceCode python"
          ><span class="bu">object</span></code
        ></a
      >, so any case of multiple inheritance provides more than one path to
      reach
      <a
        href="https://docs.python.org/3/library/functions.html#object"
        class="reference internal"
        title="object"
        ><code class="sourceCode python"
          ><span class="bu">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
        href="https://www.python.org/download/releases/2.3/mro/"
        class="reference external"
        >https://www.python.org/download/releases/2.3/mro/</a
      >.
    </p>
    <p><span id="tut-private"></span></p>
    <h2 id="private-variables">
      <span class="section-number">9.6. </span>Private Variables<a
        href="#private-variables"
        class="headerlink"
        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>_spam</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>name mangling</em>. Any
      identifier of the form <code>__spam</code> (at least two leading
      underscores, at most one trailing underscore) is textually replaced with
      <code>_classname__spam</code>, where <code>classname</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>
    <pre><code>class Mapping:
    def __init__(self, iterable):
        self.items_list = []
        self.__update(iterable)

    def update(self, iterable):
        for item in iterable:
            self.items_list.append(item)

    __update = update   # private copy of original update() method

class MappingSubclass(Mapping):

    def update(self, keys, values):
        # provides new signature for update()
        # but does not break __init__()
        for item in zip(keys, values):
            self.items_list.append(item)</code></pre>
    <p>
      The above example would work even if <code>MappingSubclass</code> were to
      introduce a <code>__update</code> identifier since it is replaced with
      <code>_Mapping__update</code> in the <code>Mapping</code> class and
      <code>_MappingSubclass__update</code> in the
      <code>MappingSubclass</code> class respectively.
    </p>
    <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>exec()</code> or <code>eval()</code> does
      not consider the classname of the invoking class to be the current class;
      this is similar to the effect of the <code>global</code> statement, the
      effect of which is likewise restricted to code that is byte-compiled
      together. The same restriction applies to <code>getattr()</code>,
      <code>setattr()</code> and <code>delattr()</code>, as well as when
      referencing <code>__dict__</code> directly.
    </p>
    <p><span id="tut-odds"></span></p>
    <h2 id="odds-and-ends">
      <span class="section-number">9.7. </span>Odds and Ends<a
        href="#odds-and-ends"
        class="headerlink"
        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>
    <pre><code>class Employee:
    pass

john = Employee()  # Create an empty employee record

# Fill the fields of the record
john.name = &#39;John Doe&#39;
john.dept = &#39;computer lab&#39;
john.salary = 1000</code></pre>
    <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>read()</code> and
      <code>readline()</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>m.__self__</code> is
      the instance object with the method <code>m()</code>, and
      <code>m.__func__</code> is the function object corresponding to the
      method.
    </p>
    <p><span id="tut-iterators"></span></p>
    <h2 id="iterators">
      <span class="section-number">9.8. </span>Iterators<a
        href="#iterators"
        class="headerlink"
        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
        href="https://docs.python.org/3/reference/compound_stmts.html#for"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >for</code
        ></a
      >
      statement:
    </p>
    <pre><code>for element in [1, 2, 3]:
    print(element)
for element in (1, 2, 3):
    print(element)
for key in {&#39;one&#39;:1, &#39;two&#39;:2}:
    print(key)
for char in &quot;123&quot;:
    print(char)
for line in open(&quot;myfile.txt&quot;):
    print(line, end=&#39;&#39;)</code></pre>
    <p>
      This style of access is clear, concise, and convenient. The use of
      iterators pervades and unifies Python. Behind the scenes, the
      <a
        href="https://docs.python.org/3/reference/compound_stmts.html#for"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >for</code
        ></a
      >
      statement calls
      <a
        href="https://docs.python.org/3/library/functions.html#iter"
        class="reference internal"
        title="iter"
        ><code class="sourceCode python"
          ><span class="bu">iter</span>()</code
        ></a
      >
      on the container object. The function returns an iterator object that
      defines the method
      <a
        href="https://docs.python.org/3/library/stdtypes.html#iterator.__next__"
        class="reference internal"
        title="iterator.__next__"
        ><code class="sourceCode python"
          ><span class="bu">next</span>()</code
        ></a
      >
      which accesses elements in the container one at a time. When there are no
      more elements,
      <a
        href="https://docs.python.org/3/library/stdtypes.html#iterator.__next__"
        class="reference internal"
        title="iterator.__next__"
        ><code class="sourceCode python"
          ><span class="bu">next</span>()</code
        ></a
      >
      raises a
      <a
        href="https://docs.python.org/3/library/exceptions.html#StopIteration"
        class="reference internal"
        title="StopIteration"
        ><code class="sourceCode python"
          ><span class="pp">StopIteration</span></code
        ></a
      >
      exception which tells the <code>for</code> loop to terminate. You can call
      the
      <a
        href="https://docs.python.org/3/library/stdtypes.html#iterator.__next__"
        class="reference internal"
        title="iterator.__next__"
        ><code class="sourceCode python"
          ><span class="bu">next</span>()</code
        ></a
      >
      method using the
      <a
        href="https://docs.python.org/3/library/functions.html#next"
        class="reference internal"
        title="next"
        ><code class="sourceCode python"
          ><span class="bu">next</span>()</code
        ></a
      >
      built-in function; this example shows how it all works:
    </p>
    <pre><code>&gt;&gt;&gt; s = &#39;abc&#39;
&gt;&gt;&gt; it = iter(s)
&gt;&gt;&gt; it
&lt;iterator object at 0x00A1DB50&gt;
&gt;&gt;&gt; next(it)
&#39;a&#39;
&gt;&gt;&gt; next(it)
&#39;b&#39;
&gt;&gt;&gt; next(it)
&#39;c&#39;
&gt;&gt;&gt; next(it)
Traceback (most recent call last):
  File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
    next(it)
StopIteration</code></pre>
    <p>
      Having seen the mechanics behind the iterator protocol, it is easy to add
      iterator behavior to your classes. Define an
      <a
        href="https://docs.python.org/3/reference/datamodel.html#object.__iter__"
        class="reference internal"
        title="object.__iter__"
        ><code class="sourceCode python"
          ><span class="bu">iter</span>()</code
        ></a
      >
      method which returns an object with a
      <a
        href="https://docs.python.org/3/library/stdtypes.html#iterator.__next__"
        class="reference internal"
        title="iterator.__next__"
        ><code class="sourceCode python"
          ><span class="bu">next</span>()</code
        ></a
      >
      method. If the class defines <code>__next__()</code>, then
      <a
        href="https://docs.python.org/3/reference/datamodel.html#object.__iter__"
        class="reference internal"
        title="object.__iter__"
        ><code class="sourceCode python"
          ><span class="bu">iter</span>()</code
        ></a
      >
      can just return <code>self</code>:
    </p>
    <pre><code>class Reverse:
    &quot;&quot;&quot;Iterator for looping over a sequence backwards.&quot;&quot;&quot;
    def __init__(self, data):
        self.data = data
        self.index = len(data)

    def __iter__(self):
        return self

    def __next__(self):
        if self.index == 0:
            raise StopIteration
        self.index = self.index - 1
        return self.data[self.index]

&gt;&gt;&gt; rev = Reverse(&#39;spam&#39;)
&gt;&gt;&gt; iter(rev)
&lt;__main__.Reverse object at 0x00A1DB50&gt;
&gt;&gt;&gt; for char in rev:
...     print(char)
...
m
a
p
s</code></pre>
    <p><span id="tut-generators"></span></p>
    <h2 id="generators">
      <span class="section-number">9.9. </span>Generators<a
        href="#generators"
        class="headerlink"
        title="Permalink to this headline"
        >¶</a
      >
    </h2>
    <p>
      <a
        href="https://docs.python.org/3/glossary.html#term-generator"
        class="reference internal"
        ><span class="xref std std-term">Generators</span></a
      >
      are a simple and powerful tool for creating iterators. They are written
      like regular functions but use the
      <a
        href="https://docs.python.org/3/reference/simple_stmts.html#yield"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >yield</code
        ></a
      >
      statement whenever they want to return data. Each time
      <a
        href="https://docs.python.org/3/library/functions.html#next"
        class="reference internal"
        title="next"
        ><code class="sourceCode python"
          ><span class="bu">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>
    <pre><code>def reverse(data):
    for index in range(len(data)-1, -1, -1):
        yield data[index]

&gt;&gt;&gt; for char in reverse(&#39;golf&#39;):
...     print(char)
...
f
l
o
g</code></pre>
    <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
        href="https://docs.python.org/3/reference/datamodel.html#object.__iter__"
        class="reference internal"
        title="object.__iter__"
        ><code class="sourceCode python"
          ><span class="bu">iter</span>()</code
        ></a
      >
      and
      <a
        href="https://docs.python.org/3/reference/expressions.html#generator.__next__"
        class="reference internal"
        title="generator.__next__"
        ><code class="sourceCode python"
          ><span class="bu">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>self.index</code> and <code>self.data</code>.
    </p>
    <p>
      In addition to automatic method creation and saving program state, when
      generators terminate, they automatically raise
      <a
        href="https://docs.python.org/3/library/exceptions.html#StopIteration"
        class="reference internal"
        title="StopIteration"
        ><code class="sourceCode python"
          ><span class="pp">StopIteration</span></code
        ></a
      >. In combination, these features make it easy to create iterators with no
      more effort than writing a regular function.
    </p>
    <p><span id="tut-genexps"></span></p>
    <h2 id="generator-expressions">
      <span class="section-number">9.10. </span>Generator Expressions<a
        href="#generator-expressions"
        class="headerlink"
        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>
    <pre><code>&gt;&gt;&gt; sum(i*i for i in range(10))                 # sum of squares
285

&gt;&gt;&gt; xvec = [10, 20, 30]
&gt;&gt;&gt; yvec = [7, 5, 3]
&gt;&gt;&gt; sum(x*y for x,y in zip(xvec, yvec))         # dot product
260

&gt;&gt;&gt; unique_words = set(word for line in page  for word in line.split())

&gt;&gt;&gt; valedictorian = max((student.gpa, student.name) for student in graduates)

&gt;&gt;&gt; data = &#39;golf&#39;
&gt;&gt;&gt; list(data[i] for i in range(len(data)-1, -1, -1))
[&#39;f&#39;, &#39;l&#39;, &#39;o&#39;, &#39;g&#39;]</code></pre>
    <p>Footnotes</p>
    <p>
      <span class="brackets"><a href="#id1" class="fn-backref">1</a></span
      ><br />
      Except for one thing. Module objects have a secret read-only attribute
      called
      <a
        href="https://docs.python.org/3/library/stdtypes.html#object.__dict__"
        class="reference internal"
        title="object.__dict__"
        ><code class="sourceCode python"><span class="bu">dict</span></code></a
      >
      which returns the dictionary used to implement the module’s namespace; the
      name
      <a
        href="https://docs.python.org/3/library/stdtypes.html#object.__dict__"
        class="reference internal"
        title="object.__dict__"
        ><code class="sourceCode python"><span class="bu">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.
    </p>
    <h3 id="table-of-contents">
      <a href="https://docs.python.org/3/contents.html">Table of Contents</a>
    </h3>
    <ul>
      <li>
        <a href="#" class="reference internal">9. Classes</a>
        <ul>
          <li>
            <a href="#a-word-about-names-and-objects" class="reference internal"
              >9.1. A Word About Names and Objects</a
            >
          </li>
          <li>
            <a href="#python-scopes-and-namespaces" class="reference internal"
              >9.2. Python Scopes and Namespaces</a
            >
            <ul>
              <li>
                <a
                  href="#scopes-and-namespaces-example"
                  class="reference internal"
                  >9.2.1. Scopes and Namespaces Example</a
                >
              </li>
            </ul>
          </li>
          <li>
            <a href="#a-first-look-at-classes" class="reference internal"
              >9.3. A First Look at Classes</a
            >
            <ul>
              <li>
                <a href="#class-definition-syntax" class="reference internal"
                  >9.3.1. Class Definition Syntax</a
                >
              </li>
              <li>
                <a href="#class-objects" class="reference internal"
                  >9.3.2. Class Objects</a
                >
              </li>
              <li>
                <a href="#instance-objects" class="reference internal"
                  >9.3.3. Instance Objects</a
                >
              </li>
              <li>
                <a href="#method-objects" class="reference internal"
                  >9.3.4. Method Objects</a
                >
              </li>
              <li>
                <a
                  href="#class-and-instance-variables"
                  class="reference internal"
                  >9.3.5. Class and Instance Variables</a
                >
              </li>
            </ul>
          </li>
          <li>
            <a href="#random-remarks" class="reference internal"
              >9.4. Random Remarks</a
            >
          </li>
          <li>
            <a href="#inheritance" class="reference internal"
              >9.5. Inheritance</a
            >
            <ul>
              <li>
                <a href="#multiple-inheritance" class="reference internal"
                  >9.5.1. Multiple Inheritance</a
                >
              </li>
            </ul>
          </li>
          <li>
            <a href="#private-variables" class="reference internal"
              >9.6. Private Variables</a
            >
          </li>
          <li>
            <a href="#odds-and-ends" class="reference internal"
              >9.7. Odds and Ends</a
            >
          </li>
          <li>
            <a href="#iterators" class="reference internal">9.8. Iterators</a>
          </li>
          <li>
            <a href="#generators" class="reference internal">9.9. Generators</a>
          </li>
          <li>
            <a href="#generator-expressions" class="reference internal"
              >9.10. Generator Expressions</a
            >
          </li>
        </ul>
      </li>
    </ul>
    <h4 id="previous-topic">Previous topic</h4>
    <p>
      <a href="errors.html" title="previous chapter"
        ><span class="section-number">8. </span>Errors and Exceptions</a
      >
    </p>
    <h4 id="next-topic">Next topic</h4>
    <p>
      <a href="stdlib.html" title="next chapter"
        ><span class="section-number">10. </span>Brief Tour of the Standard
        Library</a
      >
    </p>
    <h3 id="this-page">This Page</h3>
    <ul>
      <li><a href="https://docs.python.org/3/bugs.html">Report a Bug</a></li>
      <li>
        <a
          href="https://github.com/python/cpython/blob/3.9/Doc/tutorial/classes.rst"
          >Show Source</a
        >
      </li>
    </ul>
    <h3 id="navigation-1">Navigation</h3>
    <ul>
      <li>
        <a href="https://docs.python.org/3/genindex.html" title="General Index"
          >index</a
        >
      </li>
      <li>
        <a
          href="https://docs.python.org/3/py-modindex.html"
          title="Python Module Index"
          >modules</a
        >
        |
      </li>
      <li>
        <a href="stdlib.html" title="10. Brief Tour of the Standard Library"
          >next</a
        >
        |
      </li>
      <li>
        <a href="errors.html" title="8. Errors and Exceptions">previous</a> |
      </li>
      <li><img src="../_static/py.png" /></li>
      <li><a href="https://www.python.org/">Python</a> »</li>
      <li>
        <a href="https://docs.python.org/3/index.html">3.9.5 Documentation</a> »
      </li>
      <li><a href="index.html">The Python Tutorial</a> »</li>
      <li></li>
    </ul>
    <p>
      ©
      <a href="https://docs.python.org/3/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>
    </p>
    <p>
      Last updated on May 30, 2021.
      <a href="https://docs.python.org/3/bugs.html">Found a bug</a>?<br />
      Created using <a href="https://www.sphinx-doc.org/">Sphinx</a> 2.4.4.
    </p>
  </body>
</html>