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Python Runtime Services</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="module-abc"> <h1>27.7. <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-211"></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-212"></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> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <h4>Previous topic</h4> <p class="topless"><a href="contextlib.html" title="previous chapter">27.6. <tt class="docutils literal"><span class="pre">contextlib</span></tt> — Utilities for <tt class="docutils literal"><span class="pre">with</span></tt>-statement contexts</a></p> <h4>Next topic</h4> <p class="topless"><a href="atexit.html" title="next chapter">27.8. <tt class="docutils literal"><span class="pre">atexit</span></tt> — Exit handlers</a></p> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li><a href="../_sources/library/abc.txt" rel="nofollow">Show Source</a></li> </ul> <div id="searchbox" style="display: none"> <h3>Quick search</h3> <form class="search" action="../search.html" method="get"> <input type="text" name="q" size="18" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> <p class="searchtip" style="font-size: 90%"> Enter search terms or a module, class or function name. </p> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../modindex.html" title="Global Module Index" >modules</a> |</li> <li class="right" > <a href="atexit.html" title="27.8. atexit — Exit handlers" >next</a> |</li> <li class="right" > <a href="contextlib.html" title="27.6. contextlib — Utilities for with-statement contexts" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="../index.html">Python v2.6.6 documentation</a> »</li> <li><a href="index.html" >The Python Standard Library</a> »</li> <li><a href="python.html" >27. 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