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<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>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"><em>Private Variables</em></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"><tt class="xref py py-func docutils literal"><span class="pre">abs()</span></tt></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 <tt class="docutils literal"><span class="pre">maximize</span></tt> 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 <tt class="docutils literal"><span class="pre">z.real</span></tt>, <tt class="docutils literal"><span class="pre">real</span></tt> is an attribute of the object
<tt class="docutils literal"><span class="pre">z</span></tt>. Strictly speaking, references to names in modules are attribute
references: in the expression <tt class="docutils literal"><span class="pre">modname.funcname</span></tt>, <tt class="docutils literal"><span class="pre">modname</span></tt> is a module
object and <tt class="docutils literal"><span class="pre">funcname</span></tt> 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
<tt class="docutils literal"><span class="pre">modname.the_answer</span> <span class="pre">=</span> <span class="pre">42</span></tt>. Writable attributes may also be deleted with the
<a class="reference internal" href="../reference/simple_stmts.html#del"><tt class="xref std std-keyword docutils literal"><span class="pre">del</span></tt></a> statement. For example, <tt class="docutils literal"><span class="pre">del</span> <span class="pre">modname.the_answer</span></tt> will remove
the attribute <tt class="xref py py-attr docutils literal"><span class="pre">the_answer</span></tt> from the object named by <tt class="docutils literal"><span class="pre">modname</span></tt>.</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."><tt class="xref py py-mod docutils literal"><span class="pre">__main__</span></tt></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."><tt class="xref py py-mod docutils literal"><span class="pre">builtins</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">nonlocal</span></tt></a> statement can be
used; if not declared nonlocal, those variable 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"><tt class="xref std std-keyword docutils literal"><span class="pre">global</span></tt></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 <tt class="docutils literal"><span class="pre">del</span> <span class="pre">x</span></tt> removes the binding of <tt class="docutils literal"><span class="pre">x</span></tt> 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"><tt class="xref std std-keyword docutils literal"><span class="pre">import</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">global</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">nonlocal</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">global</span></tt></a> and <a class="reference internal" href="../reference/simple_stmts.html#nonlocal"><tt class="xref std std-keyword docutils literal"><span class="pre">nonlocal</span></tt></a> affect variable
binding:</p>
<div class="highlight-python3"><div class="highlight"><pre><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="s">"local spam"</span>
<span class="k">def</span> <span class="nf">do_nonlocal</span><span class="p">():</span>
<span class="n">nonlocal</span> <span class="n">spam</span>
<span class="n">spam</span> <span class="o">=</span> <span class="s">"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="s">"global spam"</span>
<span class="n">spam</span> <span class="o">=</span> <span class="s">"test spam"</span>
<span class="n">do_local</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"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="s">"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="s">"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="s">"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-python3"><div class="highlight"><pre><span class="n">After</span> <span class="n">local</span> <span class="n">assignment</span><span class="p">:</span> <span class="n">test</span> <span class="n">spam</span>
<span class="n">After</span> <span class="n">nonlocal</span> <span class="n">assignment</span><span class="p">:</span> <span class="n">nonlocal</span> <span class="n">spam</span>
<span class="n">After</span> <span class="k">global</span> <span class="n">assignment</span><span class="p">:</span> <span class="n">nonlocal</span> <span class="n">spam</span>
<span class="n">In</span> <span class="k">global</span> <span class="n">scope</span><span class="p">:</span> <span class="k">global</span> <span class="n">spam</span>
</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"><tt class="xref std std-keyword docutils literal"><span class="pre">nonlocal</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">global</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">global</span></tt></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"><div class="highlight"><pre><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"><tt class="xref std std-keyword docutils literal"><span class="pre">def</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">if</span></tt></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 (<tt class="xref py py-class docutils literal"><span class="pre">ClassName</span></tt> 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: <tt class="docutils literal"><span class="pre">obj.name</span></tt>. 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"><div class="highlight"><pre><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="s">'hello world'</span>
</pre></div>
</div>
<p>then <tt class="docutils literal"><span class="pre">MyClass.i</span></tt> and <tt class="docutils literal"><span class="pre">MyClass.f</span></tt> 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 <tt class="docutils literal"><span class="pre">MyClass.i</span></tt> by assignment.
<tt class="xref py py-attr docutils literal"><span class="pre">__doc__</span></tt> is also a valid attribute, returning the docstring belonging to
the class: <tt class="docutils literal"><span class="pre">"A</span> <span class="pre">simple</span> <span class="pre">example</span> <span class="pre">class"</span></tt>.</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"><div class="highlight"><pre><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 <tt class="docutils literal"><span class="pre">x</span></tt>.</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__"><tt class="xref py py-meth docutils literal"><span class="pre">__init__()</span></tt></a>, like this:</p>
<div class="highlight-python3"><div class="highlight"><pre><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__"><tt class="xref py py-meth docutils literal"><span class="pre">__init__()</span></tt></a> method, class instantiation
automatically invokes <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><tt class="xref py py-meth docutils literal"><span class="pre">__init__()</span></tt></a> for the newly-created class instance. So
in this example, a new, initialized instance can be obtained by:</p>
<div class="highlight-python3"><div class="highlight"><pre><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__"><tt class="xref py py-meth docutils literal"><span class="pre">__init__()</span></tt></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__"><tt class="xref py py-meth docutils literal"><span class="pre">__init__()</span></tt></a>. For example,</p>
<div class="highlight-python3"><div class="highlight"><pre><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
<tt class="docutils literal"><span class="pre">x</span></tt> is the instance of <tt class="xref py py-class docutils literal"><span class="pre">MyClass</span></tt> created above, the following piece of
code will print the value <tt class="docutils literal"><span class="pre">16</span></tt>, without leaving a trace:</p>
<div class="highlight-python3"><div class="highlight"><pre><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, <tt class="docutils literal"><span class="pre">x.f</span></tt> is a valid method
reference, since <tt class="docutils literal"><span class="pre">MyClass.f</span></tt> is a function, but <tt class="docutils literal"><span class="pre">x.i</span></tt> is not, since
<tt class="docutils literal"><span class="pre">MyClass.i</span></tt> is not. But <tt class="docutils literal"><span class="pre">x.f</span></tt> is not the same thing as <tt class="docutils literal"><span class="pre">MyClass.f</span></tt> — 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"><div class="highlight"><pre><span class="n">x</span><span class="o">.</span><span class="n">f</span><span class="p">()</span>
</pre></div>
</div>
<p>In the <tt class="xref py py-class docutils literal"><span class="pre">MyClass</span></tt> example, this will return the string <tt class="docutils literal"><span class="pre">'hello</span> <span class="pre">world'</span></tt>.
However, it is not necessary to call a method right away: <tt class="docutils literal"><span class="pre">x.f</span></tt> is a method
object, and can be stored away and called at a later time. For example:</p>
<div class="highlight-python3"><div class="highlight"><pre><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="k">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 <tt class="docutils literal"><span class="pre">hello</span> <span class="pre">world</span></tt> until the end of time.</p>
<p>What exactly happens when a method is called? You may have noticed that
<tt class="docutils literal"><span class="pre">x.f()</span></tt> was called without an argument above, even though the function
definition for <tt class="xref py py-meth docutils literal"><span class="pre">f()</span></tt> 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 object is passed as the first argument of the function. In our
example, the call <tt class="docutils literal"><span class="pre">x.f()</span></tt> is exactly equivalent to <tt class="docutils literal"><span class="pre">MyClass.f(x)</span></tt>. 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 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 an instance attribute is referenced that isn’t a
data attribute, its 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>
<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 <tt class="docutils literal"><span class="pre">self</span></tt>. This is nothing more
than a convention: the name <tt class="docutils literal"><span class="pre">self</span></tt> 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"><div class="highlight"><pre><span class="c"># 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="s">'hello world'</span>
<span class="n">h</span> <span class="o">=</span> <span class="n">g</span>
</pre></div>
</div>
<p>Now <tt class="docutils literal"><span class="pre">f</span></tt>, <tt class="docutils literal"><span class="pre">g</span></tt> and <tt class="docutils literal"><span class="pre">h</span></tt> are all attributes of class <tt class="xref py py-class docutils literal"><span class="pre">C</span></tt> that refer to
function objects, and consequently they are all methods of instances of
<tt class="xref py py-class docutils literal"><span class="pre">C</span></tt> — <tt class="docutils literal"><span class="pre">h</span></tt> being exactly equivalent to <tt class="docutils literal"><span class="pre">g</span></tt>. 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 <tt class="docutils literal"><span class="pre">self</span></tt>
argument:</p>
<div class="highlight-python3"><div class="highlight"><pre><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 the class
definition. (The class itself 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 <tt class="docutils literal"><span class="pre">object.__class__</span></tt>.</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"><div class="highlight"><pre><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 <tt class="xref py py-class docutils literal"><span class="pre">BaseClassName</span></tt> 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"><div class="highlight"><pre><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:
<tt class="docutils literal"><span class="pre">DerivedClassName()</span></tt> 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 <tt class="docutils literal"><span class="pre">virtual</span></tt>.)</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 <tt class="docutils literal"><span class="pre">BaseClassName.methodname(self,</span>
<span class="pre">arguments)</span></tt>. This is occasionally useful to clients as well. (Note that this
only works if the base class is accessible as <tt class="docutils literal"><span class="pre">BaseClassName</span></tt> 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"><tt class="xref py py-func docutils literal"><span class="pre">isinstance()</span></tt></a> to check an instance’s type: <tt class="docutils literal"><span class="pre">isinstance(obj,</span> <span class="pre">int)</span></tt>
will be <tt class="xref docutils literal"><span class="pre">True</span></tt> only if <tt class="docutils literal"><span class="pre">obj.__class__</span></tt> is <a class="reference internal" href="../library/functions.html#int" title="int"><tt class="xref py py-class docutils literal"><span class="pre">int</span></tt></a> or some class
derived from <a class="reference internal" href="../library/functions.html#int" title="int"><tt class="xref py py-class docutils literal"><span class="pre">int</span></tt></a>.</li>
<li>Use <a class="reference internal" href="../library/functions.html#issubclass" title="issubclass"><tt class="xref py py-func docutils literal"><span class="pre">issubclass()</span></tt></a> to check class inheritance: <tt class="docutils literal"><span class="pre">issubclass(bool,</span> <span class="pre">int)</span></tt>
is <tt class="xref docutils literal"><span class="pre">True</span></tt> since <a class="reference internal" href="../library/functions.html#bool" title="bool"><tt class="xref py py-class docutils literal"><span class="pre">bool</span></tt></a> is a subclass of <a class="reference internal" href="../library/functions.html#int" title="int"><tt class="xref py py-class docutils literal"><span class="pre">int</span></tt></a>. However,
<tt class="docutils literal"><span class="pre">issubclass(float,</span> <span class="pre">int)</span></tt> is <tt class="xref docutils literal"><span class="pre">False</span></tt> since <a class="reference internal" href="../library/functions.html#float" title="float"><tt class="xref py py-class docutils literal"><span class="pre">float</span></tt></a> is not a
subclass of <a class="reference internal" href="../library/functions.html#int" title="int"><tt class="xref py py-class docutils literal"><span class="pre">int</span></tt></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"><div class="highlight"><pre><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 <tt class="xref py py-class docutils literal"><span class="pre">DerivedClassName</span></tt>, it is searched
for in <tt class="xref py py-class docutils literal"><span class="pre">Base1</span></tt>, then (recursively) in the base classes of <tt class="xref py py-class docutils literal"><span class="pre">Base1</span></tt>,
and if it was not found there, it was searched for in <tt class="xref py py-class docutils literal"><span class="pre">Base2</span></tt>, 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"><tt class="xref py py-func docutils literal"><span class="pre">super()</span></tt></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"><tt class="xref py py-class docutils literal"><span class="pre">object</span></tt></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"><tt class="xref py py-class docutils literal"><span class="pre">object</span></tt></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="http://www.python.org/download/releases/2.3/mro/">http://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. <tt class="docutils literal"><span class="pre">_spam</span></tt>) 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
<tt class="docutils literal"><span class="pre">__spam</span></tt> (at least two leading underscores, at most one trailing underscore)
is textually replaced with <tt class="docutils literal"><span class="pre">_classname__spam</span></tt>, where <tt class="docutils literal"><span class="pre">classname</span></tt> 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"><div class="highlight"><pre><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="c"># 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="c"># provides new signature for update()</span>
<span class="c"># 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 <tt class="docutils literal"><span class="pre">exec()</span></tt> or <tt class="docutils literal"><span class="pre">eval()</span></tt> does not consider the
classname of the invoking class to be the current class; this is similar to the
effect of the <tt class="docutils literal"><span class="pre">global</span></tt> statement, the effect of which is likewise restricted
to code that is byte-compiled together. The same restriction applies to
<tt class="docutils literal"><span class="pre">getattr()</span></tt>, <tt class="docutils literal"><span class="pre">setattr()</span></tt> and <tt class="docutils literal"><span class="pre">delattr()</span></tt>, as well as when referencing
<tt class="docutils literal"><span class="pre">__dict__</span></tt> 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"><div class="highlight"><pre><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="c"># Create an empty employee record</span>
<span class="c"># 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="s">'John Doe'</span>
<span class="n">john</span><span class="o">.</span><span class="n">dept</span> <span class="o">=</span> <span class="s">'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 <tt class="xref py py-meth docutils literal"><span class="pre">read()</span></tt> and <a class="reference internal" href="../library/readline.html#module-readline" title="readline: GNU readline support for Python. (Unix)"><tt class="xref py py-meth docutils literal"><span class="pre">readline()</span></tt></a> that get the
data from a string buffer instead, and pass it as an argument.</p>
<p>Instance method objects have attributes, too: <tt class="docutils literal"><span class="pre">m.__self__</span></tt> is the instance
object with the method <tt class="xref py py-meth docutils literal"><span class="pre">m()</span></tt>, and <tt class="docutils literal"><span class="pre">m.__func__</span></tt> is the function object
corresponding to the method.</p>
</div>
<div class="section" id="exceptions-are-classes-too">
<span id="tut-exceptionclasses"></span><h2>9.8. Exceptions Are Classes Too<a class="headerlink" href="#exceptions-are-classes-too" title="Permalink to this headline">¶</a></h2>
<p>User-defined exceptions are identified by classes as well. Using this mechanism
it is possible to create extensible hierarchies of exceptions.</p>
<p>There are two new valid (semantic) forms for the <a class="reference internal" href="../reference/simple_stmts.html#raise"><tt class="xref std std-keyword docutils literal"><span class="pre">raise</span></tt></a> statement:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="k">raise</span> <span class="n">Class</span>
<span class="k">raise</span> <span class="n">Instance</span>
</pre></div>
</div>
<p>In the first form, <tt class="docutils literal"><span class="pre">Class</span></tt> must be an instance of <a class="reference internal" href="../library/functions.html#type" title="type"><tt class="xref py py-class docutils literal"><span class="pre">type</span></tt></a> or of a
class derived from it. The first form is a shorthand for:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="k">raise</span> <span class="n">Class</span><span class="p">()</span>
</pre></div>
</div>
<p>A class in an <a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></a> clause is compatible with an exception if it is
the same class or a base class thereof (but not the other way around — an
except clause listing a derived class is not compatible with a base class). For
example, the following code will print B, C, D in that order:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="k">class</span> <span class="nc">B</span><span class="p">(</span><span class="ne">Exception</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="n">B</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">D</span><span class="p">(</span><span class="n">C</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">for</span> <span class="n">c</span> <span class="ow">in</span> <span class="p">[</span><span class="n">B</span><span class="p">,</span> <span class="n">C</span><span class="p">,</span> <span class="n">D</span><span class="p">]:</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">raise</span> <span class="n">c</span><span class="p">()</span>
<span class="k">except</span> <span class="n">D</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"D"</span><span class="p">)</span>
<span class="k">except</span> <span class="n">C</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"C"</span><span class="p">)</span>
<span class="k">except</span> <span class="n">B</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"B"</span><span class="p">)</span>
</pre></div>
</div>
<p>Note that if the except clauses were reversed (with <tt class="docutils literal"><span class="pre">except</span> <span class="pre">B</span></tt> first), it
would have printed B, B, B — the first matching except clause is triggered.</p>
<p>When an error message is printed for an unhandled exception, the exception’s
class name is printed, then a colon and a space, and finally the instance
converted to a string using the built-in function <a class="reference internal" href="../library/functions.html#str" title="str"><tt class="xref py py-func docutils literal"><span class="pre">str()</span></tt></a>.</p>
</div>
<div class="section" id="iterators">
<span id="tut-iterators"></span><h2>9.9. 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"><tt class="xref std std-keyword docutils literal"><span class="pre">for</span></tt></a> statement:</p>
<div class="highlight-python3"><div class="highlight"><pre><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="s">'one'</span><span class="p">:</span><span class="mi">1</span><span class="p">,</span> <span class="s">'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="s">"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="s">"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>
</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"><tt class="xref std std-keyword docutils literal"><span class="pre">for</span></tt></a> statement
calls <a class="reference internal" href="../library/functions.html#iter" title="iter"><tt class="xref py py-func docutils literal"><span class="pre">iter()</span></tt></a> on the container object. The function returns an iterator
object that defines the method <tt class="xref py py-meth docutils literal"><span class="pre">__next__()</span></tt> which accesses elements in the
container one at a time. When there are no more elements, <tt class="xref py py-meth docutils literal"><span class="pre">__next__()</span></tt>
raises a <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><tt class="xref py py-exc docutils literal"><span class="pre">StopIteration</span></tt></a> exception which tells the <a class="reference internal" href="../reference/compound_stmts.html#for"><tt class="xref std std-keyword docutils literal"><span class="pre">for</span></tt></a> loop to
terminate. You can call the <tt class="xref py py-meth docutils literal"><span class="pre">__next__()</span></tt> method using the <a class="reference internal" href="../library/functions.html#next" title="next"><tt class="xref py py-func docutils literal"><span class="pre">next()</span></tt></a>
built-in function; this example shows how it all works:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">s</span> <span class="o">=</span> <span class="s">'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-Identifier">?</span>
<span class="nb">next</span><span class="p">(</span><span class="n">it</span><span class="p">)</span>
<span class="nc">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__"><tt class="xref py py-meth docutils literal"><span class="pre">__iter__()</span></tt></a> method which
returns an object with a <tt class="xref py py-meth docutils literal"><span class="pre">__next__()</span></tt> method. If the class defines
<tt class="xref py py-meth docutils literal"><span class="pre">__next__()</span></tt>, then <a class="reference internal" href="../reference/datamodel.html#object.__iter__" title="object.__iter__"><tt class="xref py py-meth docutils literal"><span class="pre">__iter__()</span></tt></a> can just return <tt class="docutils literal"><span class="pre">self</span></tt>:</p>
<div class="highlight-python3"><div class="highlight"><pre><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"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">rev</span> <span class="o">=</span> <span class="n">Reverse</span><span class="p">(</span><span class="s">'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>9.10. Generators<a class="headerlink" href="#generators" title="Permalink to this headline">¶</a></h2>
<p><a class="reference internal" href="../glossary.html#term-generator"><em class="xref std std-term">Generator</em></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"><tt class="xref std std-keyword docutils literal"><span class="pre">yield</span></tt></a> statement
whenever they want to return data. Each time <a class="reference internal" href="../library/functions.html#next" title="next"><tt class="xref py py-func docutils literal"><span class="pre">next()</span></tt></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"><div class="highlight"><pre><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"><div class="highlight"><pre><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="s">'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__"><tt class="xref py py-meth docutils literal"><span class="pre">__iter__()</span></tt></a> and <tt class="xref py py-meth docutils literal"><span class="pre">__next__()</span></tt> 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 <tt class="docutils literal"><span class="pre">self.index</span></tt>
and <tt class="docutils literal"><span class="pre">self.data</span></tt>.</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"><tt class="xref py py-exc docutils literal"><span class="pre">StopIteration</span></tt></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>9.11. 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 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"><div class="highlight"><pre><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="c"># 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="c"># 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="s">'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
<tt class="xref py py-attr docutils literal"><span class="pre">__dict__</span></tt> which returns the dictionary used to implement the module’s
namespace; the name <tt class="xref py py-attr docutils literal"><span class="pre">__dict__</span></tt> 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">
<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>
</ul>
</li>
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