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Python Runtime Services</a> &#187;</li> <li class="right"> <div class="inline-search" style="display: none" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> <script type="text/javascript">$('.inline-search').show(0);</script> | </li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="module-abc"> <span id="abc-abstract-base-classes"></span><h1>29.7. <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> <p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.6/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: Container 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.abc.html#module-collections.abc" title="collections.abc: Abstract base classes for containers"><code class="xref py py-mod docutils literal notranslate"><span class="pre">collections.abc</span></code></a> submodule 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 metaclass <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> for defining ABCs and a helper class <a class="reference internal" href="#abc.ABC" title="abc.ABC"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABC</span></code></a> to alternatively define ABCs through inheritance:</p> <dl class="class"> <dt id="abc.ABC"> <em class="property">class </em><code class="descclassname">abc.</code><code class="descname">ABC</code><a class="headerlink" href="#abc.ABC" title="Permalink to this definition">¶</a></dt> <dd><p>A helper class that has <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> as its metaclass. With this class, an abstract base class can be created by simply deriving from <a class="reference internal" href="#abc.ABC" title="abc.ABC"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABC</span></code></a> avoiding sometimes confusing metaclass usage, for example:</p> <div class="highlight-python3 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">ABC</span> <span class="k">class</span> <span class="nc">MyABC</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span> <span class="k">pass</span> </pre></div> </div> <p>Note that the type of <a class="reference internal" href="#abc.ABC" title="abc.ABC"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABC</span></code></a> is still <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>, therefore inheriting from <a class="reference internal" href="#abc.ABC" title="abc.ABC"><code class="xref py py-class docutils literal notranslate"><span class="pre">ABC</span></code></a> requires the usual precautions regarding metaclass usage, as multiple inheritance may lead to metaclass conflicts. One may also define an abstract base class by passing the metaclass keyword and using <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> directly, for example:</p> <div class="highlight-python3 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="p">):</span> <span class="k">pass</span> </pre></div> </div> <div class="versionadded"> <p><span class="versionmodified">New in version 3.4.</span></p> </div> </dd></dl> <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" 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-python3 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">ABC</span> <span class="k">class</span> <span class="nc">MyABC</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span> <span class="k">pass</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> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.3: </span>Returns the registered subclass, to allow usage as a class decorator.</p> </div> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.4: </span>To detect calls to <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>, you can use the <a class="reference internal" href="#abc.get_cache_token" title="abc.get_cache_token"><code class="xref py py-func docutils literal notranslate"><span class="pre">get_cache_token()</span></code></a> function.</p> </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-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Foo</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">ABC</span><span class="p">):</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">&quot;__iter__&quot;</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>The <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> module also provides the following decorator:</p> <dl class="function"> <dt id="abc.abstractmethod"> <code class="descclassname">&#64;</code><code class="descclassname">abc.</code><code class="descname">abstractmethod</code><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. <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> may be used to declare abstract methods for properties and descriptors.</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>When <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> is applied in combination with other method descriptors, it should be applied as the innermost decorator, as shown in the following usage examples:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</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> <span class="nd">@classmethod</span> <span class="nd">@abstractmethod</span> <span class="k">def</span> <span class="nf">my_abstract_classmethod</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="o">...</span><span class="p">):</span> <span class="o">...</span> <span class="nd">@staticmethod</span> <span class="nd">@abstractmethod</span> <span class="k">def</span> <span class="nf">my_abstract_staticmethod</span><span class="p">(</span><span class="o">...</span><span class="p">):</span> <span class="o">...</span> <span class="nd">@property</span> <span class="nd">@abstractmethod</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> <span class="nd">@my_abstract_property</span><span class="o">.</span><span class="n">setter</span> <span class="nd">@abstractmethod</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="n">val</span><span class="p">):</span> <span class="o">...</span> <span class="nd">@abstractmethod</span> <span class="k">def</span> <span class="nf">_get_x</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="o">...</span> <span class="nd">@abstractmethod</span> <span class="k">def</span> <span class="nf">_set_x</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">val</span><span class="p">):</span> <span class="o">...</span> <span class="n">x</span> <span class="o">=</span> <span class="nb">property</span><span class="p">(</span><span class="n">_get_x</span><span class="p">,</span> <span class="n">_set_x</span><span class="p">)</span> </pre></div> </div> <p>In order to correctly interoperate with the abstract base class machinery, the descriptor must identify itself as abstract using <code class="xref py py-attr docutils literal notranslate"><span class="pre">__isabstractmethod__</span></code>. In general, this attribute should be <code class="docutils literal notranslate"><span class="pre">True</span></code> if any of the methods used to compose the descriptor are abstract. For example, Python’s built-in property does the equivalent of:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Descriptor</span><span class="p">:</span> <span class="o">...</span> <span class="nd">@property</span> <span class="k">def</span> <span class="nf">__isabstractmethod__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">return</span> <span class="nb">any</span><span class="p">(</span><span class="nb">getattr</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="s1">&#39;__isabstractmethod__&#39;</span><span class="p">,</span> <span class="kc">False</span><span class="p">)</span> <span class="k">for</span> <span class="n">f</span> <span class="ow">in</span> <span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">_fget</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">_fset</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">_fdel</span><span class="p">))</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 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> <p>The <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> module also supports the following legacy decorators:</p> <dl class="function"> <dt id="abc.abstractclassmethod"> <code class="descclassname">&#64;</code><code class="descclassname">abc.</code><code class="descname">abstractclassmethod</code><a class="headerlink" href="#abc.abstractclassmethod" title="Permalink to this definition">¶</a></dt> <dd><div class="versionadded"> <p><span class="versionmodified">New in version 3.2.</span></p> </div> <div class="deprecated"> <p><span class="versionmodified">Deprecated since version 3.3: </span>It is now possible to use <a class="reference internal" href="functions.html#classmethod" title="classmethod"><code class="xref py py-class docutils literal notranslate"><span class="pre">classmethod</span></code></a> with <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>, making this decorator redundant.</p> </div> <p>A subclass of the built-in <a class="reference internal" href="functions.html#classmethod" title="classmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">classmethod()</span></code></a>, indicating an abstract classmethod. Otherwise it is similar to <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>.</p> <p>This special case is deprecated, as the <a class="reference internal" href="functions.html#classmethod" title="classmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">classmethod()</span></code></a> decorator is now correctly identified as abstract when applied to an abstract method:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span> <span class="nd">@classmethod</span> <span class="nd">@abstractmethod</span> <span class="k">def</span> <span class="nf">my_abstract_classmethod</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="o">...</span><span class="p">):</span> <span class="o">...</span> </pre></div> </div> </dd></dl> <dl class="function"> <dt id="abc.abstractstaticmethod"> <code class="descclassname">&#64;</code><code class="descclassname">abc.</code><code class="descname">abstractstaticmethod</code><a class="headerlink" href="#abc.abstractstaticmethod" title="Permalink to this definition">¶</a></dt> <dd><div class="versionadded"> <p><span class="versionmodified">New in version 3.2.</span></p> </div> <div class="deprecated"> <p><span class="versionmodified">Deprecated since version 3.3: </span>It is now possible to use <a class="reference internal" href="functions.html#staticmethod" title="staticmethod"><code class="xref py py-class docutils literal notranslate"><span class="pre">staticmethod</span></code></a> with <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>, making this decorator redundant.</p> </div> <p>A subclass of the built-in <a class="reference internal" href="functions.html#staticmethod" title="staticmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">staticmethod()</span></code></a>, indicating an abstract staticmethod. Otherwise it is similar to <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>.</p> <p>This special case is deprecated, as the <a class="reference internal" href="functions.html#staticmethod" title="staticmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">staticmethod()</span></code></a> decorator is now correctly identified as abstract when applied to an abstract method:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span> <span class="nd">@staticmethod</span> <span class="nd">@abstractmethod</span> <span class="k">def</span> <span class="nf">my_abstract_staticmethod</span><span class="p">(</span><span class="o">...</span><span class="p">):</span> <span class="o">...</span> </pre></div> </div> </dd></dl> <dl class="function"> <dt id="abc.abstractproperty"> <code class="descclassname">&#64;</code><code class="descclassname">abc.</code><code class="descname">abstractproperty</code><a class="headerlink" href="#abc.abstractproperty" title="Permalink to this definition">¶</a></dt> <dd><div class="deprecated"> <p><span class="versionmodified">Deprecated since version 3.3: </span>It is now possible to use <a class="reference internal" href="functions.html#property" title="property"><code class="xref py py-class docutils literal notranslate"><span class="pre">property</span></code></a>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">property.getter()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">property.setter()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">property.deleter()</span></code> with <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>, making this decorator redundant.</p> </div> <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>This special case is deprecated, as the <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> decorator is now correctly identified as abstract when applied to an abstract method:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span> <span class="nd">@property</span> <span class="nd">@abstractmethod</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>The above example defines a read-only property; you can also define a read-write abstract property by appropriately marking one or more of the underlying methods as abstract:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span> <span class="nd">@property</span> <span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="o">...</span> <span class="nd">@x</span><span class="o">.</span><span class="n">setter</span> <span class="nd">@abstractmethod</span> <span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">val</span><span class="p">):</span> <span class="o">...</span> </pre></div> </div> <p>If only some components are abstract, only those components need to be updated to create a concrete property in a subclass:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></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="nd">@C</span><span class="o">.</span><span class="n">x</span><span class="o">.</span><span class="n">setter</span> <span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">val</span><span class="p">):</span> <span class="o">...</span> </pre></div> </div> </dd></dl> <p>The <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> module also provides the following functions:</p> <dl class="function"> <dt id="abc.get_cache_token"> <code class="descclassname">abc.</code><code class="descname">get_cache_token</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#abc.get_cache_token" title="Permalink to this definition">¶</a></dt> <dd><p>Returns the current abstract base class cache token.</p> <p>The token is an opaque object (that supports equality testing) identifying the current version of the abstract base class cache for virtual subclasses. The token changes with every call to <a class="reference internal" href="#abc.ABCMeta.register" title="abc.ABCMeta.register"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ABCMeta.register()</span></code></a> on any ABC.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 3.4.</span></p> </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" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h4>Previous topic</h4> <p class="topless"><a href="contextlib.html" title="previous chapter">29.6. <code class="docutils literal notranslate"><span class="pre">contextlib</span></code> — Utilities for <code class="docutils literal notranslate"><span class="pre">with</span></code>-statement contexts</a></p> <h4>Next topic</h4> <p class="topless"><a href="atexit.html" title="next chapter">29.8. <code class="docutils literal notranslate"><span class="pre">atexit</span></code> — Exit handlers</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li> <a href="https://github.com/python/cpython/blob/3.6/Doc/library/abc.rst" rel="nofollow">Show Source </a> </li> </ul> </div> </div> </div> <div class="clearer"></div> </div> <div class="related" role="navigation" aria-label="related navigation"> <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="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="atexit.html" title="29.8. atexit — Exit handlers" >next</a> |</li> <li class="right" > <a href="contextlib.html" title="29.6. 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="https://www.python.org/">Python</a> &#187;</li> <li> <a href="../index.html">3.6.7 Documentation</a> &#187; </li> <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> &#187;</li> <li class="nav-item nav-item-2"><a href="python.html" >29. 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