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<div class="section" id="module-abc">
<span id="abc-abstract-base-classes"></span><h1>28.8. <a class="reference internal" href="#module-abc" title="abc: Abstract base classes according to PEP 3119."><code class="xref py py-mod docutils literal notranslate"><span class="pre">abc</span></code></a> — Abstract Base Classes<a class="headerlink" href="#module-abc" title="Permalink to this headline">¶</a></h1>
<div class="versionadded">
<p><span class="versionmodified added">New in version 2.6.</span></p>
</div>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/2.7/Lib/abc.py">Lib/abc.py</a></p>
<hr class="docutils" />
<p>This module provides the infrastructure for defining <a class="reference internal" href="../glossary.html#term-abstract-base-class"><span class="xref std std-term">abstract base
classes</span></a> (ABCs) in Python, as outlined in <span class="target" id="index-0"></span><a class="pep reference external" href="https://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-1"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3141"><strong>PEP 3141</strong></a> and the <a class="reference internal" href="numbers.html#module-numbers" title="numbers: Numeric abstract base classes (Complex, Real, Integral, etc.)."><code class="xref py py-mod docutils literal notranslate"><span class="pre">numbers</span></code></a> module
regarding a type hierarchy for numbers based on ABCs.)</p>
<p>The <a class="reference internal" href="collections.html#module-collections" title="collections: High-performance datatypes"><code class="xref py py-mod docutils literal notranslate"><span class="pre">collections</span></code></a> module has some concrete classes that derive from
ABCs; these can, of course, be further derived. In addition, the
<a class="reference internal" href="collections.html#module-collections" title="collections: High-performance datatypes"><code class="xref py py-mod docutils literal notranslate"><span class="pre">collections</span></code></a> module has some ABCs that can be used to test whether
a class or instance provides a particular interface, for example, if it is
hashable or if it is a mapping.</p>
<p>This module provides the following class:</p>
<dl class="class">
<dt id="abc.ABCMeta">
<em class="property">class </em><code class="descclassname">abc.</code><code class="descname">ABCMeta</code><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 class="reference internal" href="functions.html#issubclass" title="issubclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">issubclass()</span></code></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 class="reference internal" href="functions.html#super" title="super"><code class="xref py py-func docutils literal notranslate"><span class="pre">super()</span></code></a>). <a class="footnote-reference brackets" href="#id2" id="id1">1</a></p>
<p>Classes created with a metaclass of <a class="reference internal" href="#abc.ABCMeta" title="abc.ABCMeta"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code></a> have the following method:</p>
<dl class="method">
<dt id="abc.ABCMeta.register">
<code class="descname">register</code><span class="sig-paren">(</span><em>subclass</em><span class="sig-paren">)</span><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-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">abc</span> <span class="k">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__">
<code class="descname">__subclasshook__</code><span class="sig-paren">(</span><em>subclass</em><span class="sig-paren">)</span><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 <code class="docutils literal notranslate"><span class="pre">issubclass</span></code> further without the
need to call <a class="reference internal" href="#abc.ABCMeta.register" title="abc.ABCMeta.register"><code class="xref py py-meth docutils literal notranslate"><span class="pre">register()</span></code></a> on every class you want to consider a
subclass of the ABC. (This class method is called from the
<code class="xref py py-meth docutils literal notranslate"><span class="pre">__subclasscheck__()</span></code> method of the ABC.)</p>
<p>This method should return <code class="docutils literal notranslate"><span class="pre">True</span></code>, <code class="docutils literal notranslate"><span class="pre">False</span></code> or <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>. If
it returns <code class="docutils literal notranslate"><span class="pre">True</span></code>, the <em>subclass</em> is considered a subclass of this ABC.
If it returns <code class="docutils literal notranslate"><span class="pre">False</span></code>, the <em>subclass</em> is not considered a subclass of
this ABC, even if it would normally be one. If it returns
<code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>, 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-default notranslate"><div class="highlight"><pre><span></span><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="kc">False</span><span class="p">:</span>
<span class="k">yield</span> <span class="kc">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="fm">__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="bp">cls</span><span class="p">,</span> <span class="n">C</span><span class="p">):</span>
<span class="k">if</span> <span class="bp">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="s2">"__iter__"</span> <span class="ow">in</span> <span class="n">B</span><span class="o">.</span><span class="vm">__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="vm">__mro__</span><span class="p">):</span>
<span class="k">return</span> <span class="kc">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 <code class="docutils literal notranslate"><span class="pre">MyIterable</span></code> defines the standard iterable method,
<a class="reference internal" href="stdtypes.html#iterator.__iter__" title="iterator.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a>, as an abstract method. The implementation given
here can still be called from subclasses. The <code class="xref py py-meth docutils literal notranslate"><span class="pre">get_iterator()</span></code> method
is also part of the <code class="docutils literal notranslate"><span class="pre">MyIterable</span></code> abstract base class, but it does not have
to be overridden in non-abstract derived classes.</p>
<p>The <a class="reference internal" href="#abc.ABCMeta.__subclasshook__" title="abc.ABCMeta.__subclasshook__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__subclasshook__()</span></code></a> class method defined here says that any class
that has an <a class="reference internal" href="stdtypes.html#iterator.__iter__" title="iterator.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a> method in its
<a class="reference internal" href="stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a> (or in that of one of its base classes, accessed
via the <a class="reference internal" href="stdtypes.html#class.__mro__" title="class.__mro__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__mro__</span></code></a> list) is considered a <code class="docutils literal notranslate"><span class="pre">MyIterable</span></code> too.</p>
<p>Finally, the last line makes <code class="docutils literal notranslate"><span class="pre">Foo</span></code> a virtual subclass of <code class="docutils literal notranslate"><span class="pre">MyIterable</span></code>,
even though it does not define an <a class="reference internal" href="stdtypes.html#iterator.__iter__" title="iterator.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a> method (it uses
the old-style iterable protocol, defined in terms of <a class="reference internal" href="../reference/datamodel.html#object.__len__" title="object.__len__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__len__()</span></code></a> and
<a class="reference internal" href="../reference/datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>). Note that this will not make <code class="docutils literal notranslate"><span class="pre">get_iterator</span></code>
available as a method of <code class="docutils literal notranslate"><span class="pre">Foo</span></code>, so it is provided separately.</p>
</dd></dl>
<p>It also provides the following decorators:</p>
<dl class="function">
<dt id="abc.abstractmethod">
<code class="descclassname">abc.</code><code class="descname">abstractmethod</code><span class="sig-paren">(</span><em>function</em><span class="sig-paren">)</span><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 class="reference internal" href="#abc.ABCMeta" title="abc.ABCMeta"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code></a> or
is derived from it.
A class that has a metaclass derived from <a class="reference internal" href="#abc.ABCMeta" title="abc.ABCMeta"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code></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 class="reference internal" href="#abc.abstractmethod" title="abc.abstractmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">abstractmethod()</span></code></a> only affects subclasses derived using
regular inheritance; “virtual subclasses” registered with the ABC’s
<code class="xref py py-meth docutils literal notranslate"><span class="pre">register()</span></code> method are not affected.</p>
<p>Usage:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><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="admonition-title">Note</p>
<p>Unlike Java abstract methods, these abstract
methods may have an implementation. This implementation can be
called via the <a class="reference internal" href="functions.html#super" title="super"><code class="xref py py-func docutils literal notranslate"><span class="pre">super()</span></code></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">
<code class="descclassname">abc.</code><code class="descname">abstractproperty</code><span class="sig-paren">(</span><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><span class="sig-paren">)</span><a class="headerlink" href="#abc.abstractproperty" title="Permalink to this definition">¶</a></dt>
<dd><p>A subclass of the built-in <a class="reference internal" href="functions.html#property" title="property"><code class="xref py py-func docutils literal notranslate"><span class="pre">property()</span></code></a>, indicating an abstract property.</p>
<p>Using this function requires that the class’s metaclass is <a class="reference internal" href="#abc.ABCMeta" title="abc.ABCMeta"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code></a> or
is derived from it.
A class that has a metaclass derived from <a class="reference internal" href="#abc.ABCMeta" title="abc.ABCMeta"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code></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-default notranslate"><div class="highlight"><pre><span></span><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-default notranslate"><div class="highlight"><pre><span></span><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>
<dl class="footnote brackets">
<dt class="label" id="id2"><span class="brackets"><a class="fn-backref" href="#id1">1</a></span></dt>
<dd><p>C++ programmers should note that Python’s virtual base class
concept is not the same as C++’s.</p>
</dd>
</dl>
</div>
</div>
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