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<div class="section" id="module-abc">
<h1>27.8. <tt class="xref docutils literal"><span class="pre">abc</span></tt> — Abstract Base Classes<a class="headerlink" href="#module-abc" title="Permalink to this headline">¶</a></h1>
<p class="versionadded">
<span class="versionmodified">New in version 2.6.</span></p>
<p>This module provides the infrastructure for defining an <a class="reference external" href="../glossary.html#term-abstract-base-class"><em class="xref">abstract base
class</em></a> (ABCs) in Python, as outlined in <span class="target" id="index-220"></span><a class="reference external" href="http://www.python.org/dev/peps/pep-3119"><strong>PEP 3119</strong></a>; see the PEP for why this
was added to Python. (See also <span class="target" id="index-221"></span><a class="reference external" href="http://www.python.org/dev/peps/pep-3141"><strong>PEP 3141</strong></a> and the <a title="Numeric abstract base classes (Complex, Real, Integral, etc.)." class="reference external" href="numbers.html#module-numbers"><tt class="xref docutils literal"><span class="pre">numbers</span></tt></a> module
regarding a type hierarchy for numbers based on ABCs.)</p>
<p>The <a title="High-performance datatypes" class="reference external" href="collections.html#module-collections"><tt class="xref docutils literal"><span class="pre">collections</span></tt></a> module has some concrete classes that derive from
ABCs; these can, of course, be further derived. In addition the
<a title="High-performance datatypes" class="reference external" href="collections.html#module-collections"><tt class="xref docutils literal"><span class="pre">collections</span></tt></a> module has some ABCs that can be used to test whether
a class or instance provides a particular interface, for example, is it
hashable or a mapping.</p>
<p>This module provides the following class:</p>
<dl class="class">
<dt id="abc.ABCMeta">
<em class="property">class </em><tt class="descclassname">abc.</tt><tt class="descname">ABCMeta</tt><a class="headerlink" href="#abc.ABCMeta" title="Permalink to this definition">¶</a></dt>
<dd><p>Metaclass for defining Abstract Base Classes (ABCs).</p>
<p>Use this metaclass to create an ABC. An ABC can be subclassed directly, and
then acts as a mix-in class. You can also register unrelated concrete
classes (even built-in classes) and unrelated ABCs as “virtual subclasses” –
these and their descendants will be considered subclasses of the registering
ABC by the built-in <a title="issubclass" class="reference external" href="functions.html#issubclass"><tt class="xref docutils literal"><span class="pre">issubclass()</span></tt></a> function, but the registering ABC
won’t show up in their MRO (Method Resolution Order) nor will method
implementations defined by the registering ABC be callable (not even via
<a title="super" class="reference external" href="functions.html#super"><tt class="xref docutils literal"><span class="pre">super()</span></tt></a>). <a class="footnote-reference" href="#id2" id="id1">[1]</a></p>
<p>Classes created with a metaclass of <a title="abc.ABCMeta" class="reference internal" href="#abc.ABCMeta"><tt class="xref docutils literal"><span class="pre">ABCMeta</span></tt></a> have the following method:</p>
<dl class="method">
<dt id="abc.ABCMeta.register">
<tt class="descname">register</tt><big>(</big><em>subclass</em><big>)</big><a class="headerlink" href="#abc.ABCMeta.register" title="Permalink to this definition">¶</a></dt>
<dd><p>Register <em>subclass</em> as a “virtual subclass” of this ABC. For
example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">abc</span> <span class="kn">import</span> <span class="n">ABCMeta</span>
<span class="k">class</span> <span class="nc">MyABC</span><span class="p">:</span>
<span class="n">__metaclass__</span> <span class="o">=</span> <span class="n">ABCMeta</span>
<span class="n">MyABC</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="nb">tuple</span><span class="p">)</span>
<span class="k">assert</span> <span class="nb">issubclass</span><span class="p">(</span><span class="nb">tuple</span><span class="p">,</span> <span class="n">MyABC</span><span class="p">)</span>
<span class="k">assert</span> <span class="nb">isinstance</span><span class="p">((),</span> <span class="n">MyABC</span><span class="p">)</span>
</pre></div>
</div>
</dd></dl>
<p>You can also override this method in an abstract base class:</p>
<dl class="method">
<dt id="abc.ABCMeta.__subclasshook__">
<tt class="descname">__subclasshook__</tt><big>(</big><em>subclass</em><big>)</big><a class="headerlink" href="#abc.ABCMeta.__subclasshook__" title="Permalink to this definition">¶</a></dt>
<dd><p>(Must be defined as a class method.)</p>
<p>Check whether <em>subclass</em> is considered a subclass of this ABC. This means
that you can customize the behavior of <tt class="docutils literal"><span class="pre">issubclass</span></tt> further without the
need to call <a title="abc.ABCMeta.register" class="reference internal" href="#abc.ABCMeta.register"><tt class="xref docutils literal"><span class="pre">register()</span></tt></a> on every class you want to consider a
subclass of the ABC. (This class method is called from the
<tt class="xref docutils literal"><span class="pre">__subclasscheck__()</span></tt> method of the ABC.)</p>
<p>This method should return <tt class="xref docutils literal"><span class="pre">True</span></tt>, <tt class="xref docutils literal"><span class="pre">False</span></tt> or <tt class="docutils literal"><span class="pre">NotImplemented</span></tt>. If
it returns <tt class="xref docutils literal"><span class="pre">True</span></tt>, the <em>subclass</em> is considered a subclass of this ABC.
If it returns <tt class="xref docutils literal"><span class="pre">False</span></tt>, the <em>subclass</em> is not considered a subclass of
this ABC, even if it would normally be one. If it returns
<tt class="docutils literal"><span class="pre">NotImplemented</span></tt>, the subclass check is continued with the usual
mechanism.</p>
</dd></dl>
<p>For a demonstration of these concepts, look at this example ABC definition:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Foo</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__getitem__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">index</span><span class="p">):</span>
<span class="o">...</span>
<span class="k">def</span> <span class="nf">__len__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="o">...</span>
<span class="k">def</span> <span class="nf">get_iterator</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="nb">iter</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">MyIterable</span><span class="p">:</span>
<span class="n">__metaclass__</span> <span class="o">=</span> <span class="n">ABCMeta</span>
<span class="nd">@abstractmethod</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">while</span> <span class="bp">False</span><span class="p">:</span>
<span class="k">yield</span> <span class="bp">None</span>
<span class="k">def</span> <span class="nf">get_iterator</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="o">.</span><span class="n">__iter__</span><span class="p">()</span>
<span class="nd">@classmethod</span>
<span class="k">def</span> <span class="nf">__subclasshook__</span><span class="p">(</span><span class="n">cls</span><span class="p">,</span> <span class="n">C</span><span class="p">):</span>
<span class="k">if</span> <span class="n">cls</span> <span class="ow">is</span> <span class="n">MyIterable</span><span class="p">:</span>
<span class="k">if</span> <span class="nb">any</span><span class="p">(</span><span class="s">"__iter__"</span> <span class="ow">in</span> <span class="n">B</span><span class="o">.</span><span class="n">__dict__</span> <span class="k">for</span> <span class="n">B</span> <span class="ow">in</span> <span class="n">C</span><span class="o">.</span><span class="n">__mro__</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">True</span>
<span class="k">return</span> <span class="bp">NotImplemented</span>
<span class="n">MyIterable</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="n">Foo</span><span class="p">)</span>
</pre></div>
</div>
<p>The ABC <tt class="docutils literal"><span class="pre">MyIterable</span></tt> defines the standard iterable method,
<a title="object.__iter__" class="reference external" href="../reference/datamodel.html#object.__iter__"><tt class="xref docutils literal"><span class="pre">__iter__()</span></tt></a>, as an abstract method. The implementation given here can
still be called from subclasses. The <tt class="xref docutils literal"><span class="pre">get_iterator()</span></tt> method is also
part of the <tt class="docutils literal"><span class="pre">MyIterable</span></tt> abstract base class, but it does not have to be
overridden in non-abstract derived classes.</p>
<p>The <a title="abc.ABCMeta.__subclasshook__" class="reference internal" href="#abc.ABCMeta.__subclasshook__"><tt class="xref docutils literal"><span class="pre">__subclasshook__()</span></tt></a> class method defined here says that any class
that has an <a title="object.__iter__" class="reference external" href="../reference/datamodel.html#object.__iter__"><tt class="xref docutils literal"><span class="pre">__iter__()</span></tt></a> method in its <tt class="xref docutils literal"><span class="pre">__dict__</span></tt> (or in that of
one of its base classes, accessed via the <tt class="xref docutils literal"><span class="pre">__mro__</span></tt> list) is
considered a <tt class="docutils literal"><span class="pre">MyIterable</span></tt> too.</p>
<p>Finally, the last line makes <tt class="docutils literal"><span class="pre">Foo</span></tt> a virtual subclass of <tt class="docutils literal"><span class="pre">MyIterable</span></tt>,
even though it does not define an <a title="object.__iter__" class="reference external" href="../reference/datamodel.html#object.__iter__"><tt class="xref docutils literal"><span class="pre">__iter__()</span></tt></a> method (it uses the
old-style iterable protocol, defined in terms of <a title="object.__len__" class="reference external" href="../reference/datamodel.html#object.__len__"><tt class="xref docutils literal"><span class="pre">__len__()</span></tt></a> and
<a title="object.__getitem__" class="reference external" href="../reference/datamodel.html#object.__getitem__"><tt class="xref docutils literal"><span class="pre">__getitem__()</span></tt></a>). Note that this will not make <tt class="docutils literal"><span class="pre">get_iterator</span></tt>
available as a method of <tt class="docutils literal"><span class="pre">Foo</span></tt>, so it is provided separately.</p>
</dd></dl>
<p>It also provides the following decorators:</p>
<dl class="function">
<dt id="abc.abstractmethod">
<tt class="descclassname">abc.</tt><tt class="descname">abstractmethod</tt><big>(</big><em>function</em><big>)</big><a class="headerlink" href="#abc.abstractmethod" title="Permalink to this definition">¶</a></dt>
<dd><p>A decorator indicating abstract methods.</p>
<p>Using this decorator requires that the class’s metaclass is <a title="abc.ABCMeta" class="reference internal" href="#abc.ABCMeta"><tt class="xref docutils literal"><span class="pre">ABCMeta</span></tt></a> or
is derived from it.
A class that has a metaclass derived from <a title="abc.ABCMeta" class="reference internal" href="#abc.ABCMeta"><tt class="xref docutils literal"><span class="pre">ABCMeta</span></tt></a>
cannot be instantiated unless all of its abstract methods and
properties are overridden.
The abstract methods can be called using any of the normal ‘super’ call
mechanisms.</p>
<p>Dynamically adding abstract methods to a class, or attempting to modify the
abstraction status of a method or class once it is created, are not
supported. The <a title="abc.abstractmethod" class="reference internal" href="#abc.abstractmethod"><tt class="xref docutils literal"><span class="pre">abstractmethod()</span></tt></a> only affects subclasses derived using
regular inheritance; “virtual subclasses” registered with the ABC’s
<tt class="xref docutils literal"><span class="pre">register()</span></tt> method are not affected.</p>
<p>Usage:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
<span class="n">__metaclass__</span> <span class="o">=</span> <span class="n">ABCMeta</span>
<span class="nd">@abstractmethod</span>
<span class="k">def</span> <span class="nf">my_abstract_method</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">...</span><span class="p">):</span>
<span class="o">...</span>
</pre></div>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Unlike Java abstract methods, these abstract
methods may have an implementation. This implementation can be
called via the <a title="super" class="reference external" href="functions.html#super"><tt class="xref docutils literal"><span class="pre">super()</span></tt></a> mechanism from the class that
overrides it. This could be useful as an end-point for a
super-call in a framework that uses cooperative
multiple-inheritance.</p>
</div>
</dd></dl>
<dl class="function">
<dt id="abc.abstractproperty">
<tt class="descclassname">abc.</tt><tt class="descname">abstractproperty</tt><big>(</big><span class="optional">[</span><em>fget</em><span class="optional">[</span>, <em>fset</em><span class="optional">[</span>, <em>fdel</em><span class="optional">[</span>, <em>doc</em><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><big>)</big><a class="headerlink" href="#abc.abstractproperty" title="Permalink to this definition">¶</a></dt>
<dd><p>A subclass of the built-in <a title="property" class="reference external" href="functions.html#property"><tt class="xref docutils literal"><span class="pre">property()</span></tt></a>, indicating an abstract property.</p>
<p>Using this function requires that the class’s metaclass is <a title="abc.ABCMeta" class="reference internal" href="#abc.ABCMeta"><tt class="xref docutils literal"><span class="pre">ABCMeta</span></tt></a> or
is derived from it.
A class that has a metaclass derived from <a title="abc.ABCMeta" class="reference internal" href="#abc.ABCMeta"><tt class="xref docutils literal"><span class="pre">ABCMeta</span></tt></a> cannot be
instantiated unless all of its abstract methods and properties are overridden.
The abstract properties can be called using any of the normal
‘super’ call mechanisms.</p>
<p>Usage:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
<span class="n">__metaclass__</span> <span class="o">=</span> <span class="n">ABCMeta</span>
<span class="nd">@abstractproperty</span>
<span class="k">def</span> <span class="nf">my_abstract_property</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="o">...</span>
</pre></div>
</div>
<p>This defines a read-only property; you can also define a read-write abstract
property using the ‘long’ form of property declaration:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
<span class="n">__metaclass__</span> <span class="o">=</span> <span class="n">ABCMeta</span>
<span class="k">def</span> <span class="nf">getx</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="o">...</span>
<span class="k">def</span> <span class="nf">setx</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span> <span class="o">...</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">abstractproperty</span><span class="p">(</span><span class="n">getx</span><span class="p">,</span> <span class="n">setx</span><span class="p">)</span>
</pre></div>
</div>
</dd></dl>
<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>C++ programmers should note that Python’s virtual base class
concept is not the same as C++’s.</td></tr>
</tbody>
</table>
</div>
</div>
</div>
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