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<section id="module-abc">
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<span id="abc-abstract-base-classes"></span><h1><code class="xref py py-mod docutils literal notranslate"><span class="pre">abc</span></code> — Abstract Base Classes<a class="headerlink" href="#module-abc" title="Link to this heading">¶</a></h1>
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<p><strong>Source code:</strong> <a class="extlink-source reference external" href="https://github.com/python/cpython/tree/3.13/Lib/abc.py">Lib/abc.py</a></p>
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<hr class="docutils" />
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<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
|
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classes</span></a> (ABCs) in Python, as outlined in <span class="target" id="index-0"></span><a class="pep reference external" href="https://peps.python.org/pep-3119/"><strong>PEP 3119</strong></a>;
|
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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://peps.python.org/pep-3141/"><strong>PEP 3141</strong></a> and the
|
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<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>
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<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
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<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, if it is
|
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<a class="reference internal" href="../glossary.html#term-hashable"><span class="xref std std-term">hashable</span></a> or if it is a <a class="reference internal" href="../glossary.html#term-mapping"><span class="xref std std-term">mapping</span></a>.</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="py class">
|
||
<dt class="sig sig-object py" id="abc.ABC">
|
||
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">abc.</span></span><span class="sig-name descname"><span class="pre">ABC</span></span><a class="headerlink" href="#abc.ABC" title="Link 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 <code class="xref py py-class docutils literal notranslate"><span class="pre">ABC</span></code>
|
||
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="w"> </span><span class="nn">abc</span><span class="w"> </span><span class="kn">import</span> <span class="n">ABC</span>
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<span class="k">class</span><span class="w"> </span><span class="nc">MyABC</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span>
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<span class="k">pass</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>Note that the type of <code class="xref py py-class docutils literal notranslate"><span class="pre">ABC</span></code> 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 <code class="xref py py-class docutils literal notranslate"><span class="pre">ABC</span></code> 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 <code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code> directly, for example:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span><span class="w"> </span><span class="nn">abc</span><span class="w"> </span><span class="kn">import</span> <span class="n">ABCMeta</span>
|
||
|
||
<span class="k">class</span><span class="w"> </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>
|
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<span class="k">pass</span>
|
||
</pre></div>
|
||
</div>
|
||
<div class="versionadded">
|
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<p><span class="versionmodified added">Added in version 3.4.</span></p>
|
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</div>
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</dd></dl>
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||
|
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<dl class="py class">
|
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<dt class="sig sig-object py" id="abc.ABCMeta">
|
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<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">abc.</span></span><span class="sig-name descname"><span class="pre">ABCMeta</span></span><a class="headerlink" href="#abc.ABCMeta" title="Link 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" role="doc-noteref"><span class="fn-bracket">[</span>1<span class="fn-bracket">]</span></a></p>
|
||
<p>Classes created with a metaclass of <code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code> have the following method:</p>
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py" id="abc.ABCMeta.register">
|
||
<span class="sig-name descname"><span class="pre">register</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">subclass</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#abc.ABCMeta.register" title="Link 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="w"> </span><span class="nn">abc</span><span class="w"> </span><span class="kn">import</span> <span class="n">ABC</span>
|
||
|
||
<span class="k">class</span><span class="w"> </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">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">Changed in version 3.4: </span>To detect calls to <code class="xref py py-meth docutils literal notranslate"><span class="pre">register()</span></code>, 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="py method">
|
||
<dt class="sig sig-object py" id="abc.ABCMeta.__subclasshook__">
|
||
<span class="sig-name descname"><span class="pre">__subclasshook__</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">subclass</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#abc.ABCMeta.__subclasshook__" title="Link 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 <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> 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
|
||
<a class="reference internal" href="../reference/datamodel.html#type.__subclasscheck__" title="type.__subclasscheck__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__subclasscheck__()</span></code></a> 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 <a class="reference internal" href="constants.html#NotImplemented" title="NotImplemented"><code class="xref py py-data docutils literal notranslate"><span class="pre">NotImplemented</span></code></a>. 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="xref py py-data 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="w"> </span><span class="nc">Foo</span><span class="p">:</span>
|
||
<span class="k">def</span><span class="w"> </span><span class="fm">__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="w"> </span><span class="fm">__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="w"> </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="w"> </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="w"> </span><span class="fm">__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="w"> </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="w"> </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="../reference/datamodel.html#object.__iter__" title="object.__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="../reference/datamodel.html#object.__iter__" title="object.__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="../reference/datamodel.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="../reference/datamodel.html#type.__mro__" title="type.__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="../reference/datamodel.html#object.__iter__" title="object.__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 <code class="xref py py-mod docutils literal notranslate"><span class="pre">abc</span></code> module also provides the following decorator:</p>
|
||
<dl class="py function">
|
||
<dt class="sig sig-object py" id="abc.abstractmethod">
|
||
<span class="sig-prename descclassname"><span class="pre">@</span></span><span class="sig-prename descclassname"><span class="pre">abc.</span></span><span class="sig-name descname"><span class="pre">abstractmethod</span></span><a class="headerlink" href="#abc.abstractmethod" title="Link 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
|
||
<code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code> 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. <code class="xref py py-func docutils literal notranslate"><span class="pre">abstractmethod()</span></code> 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 only
|
||
supported using the <a class="reference internal" href="#abc.update_abstractmethods" title="abc.update_abstractmethods"><code class="xref py py-func docutils literal notranslate"><span class="pre">update_abstractmethods()</span></code></a> function. The
|
||
<code class="xref py py-func docutils literal notranslate"><span class="pre">abstractmethod()</span></code> only affects subclasses derived using regular
|
||
inheritance; “virtual subclasses” registered with the ABC’s
|
||
<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> method are not affected.</p>
|
||
<p>When <code class="xref py py-func docutils literal notranslate"><span class="pre">abstractmethod()</span></code> 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="w"> </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="w"> </span><span class="nf">my_abstract_method</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">arg1</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="w"> </span><span class="nf">my_abstract_classmethod</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">arg2</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="w"> </span><span class="nf">my_abstract_staticmethod</span><span class="p">(</span><span class="n">arg3</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="w"> </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="w"> </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="w"> </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="w"> </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 <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> does the equivalent of:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span><span class="w"> </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="w"> </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">'__isabstractmethod__'</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="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>
|
||
|
||
<p>The <code class="xref py py-mod docutils literal notranslate"><span class="pre">abc</span></code> module also supports the following legacy decorators:</p>
|
||
<dl class="py function">
|
||
<dt class="sig sig-object py" id="abc.abstractclassmethod">
|
||
<span class="sig-prename descclassname"><span class="pre">@</span></span><span class="sig-prename descclassname"><span class="pre">abc.</span></span><span class="sig-name descname"><span class="pre">abstractclassmethod</span></span><a class="headerlink" href="#abc.abstractclassmethod" title="Link to this definition">¶</a></dt>
|
||
<dd><div class="versionadded">
|
||
<p><span class="versionmodified added">Added in version 3.2.</span></p>
|
||
</div>
|
||
<div class="deprecated">
|
||
<p><span class="versionmodified deprecated">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="w"> </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="w"> </span><span class="nf">my_abstract_classmethod</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">arg</span><span class="p">):</span>
|
||
<span class="o">...</span>
|
||
</pre></div>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="py function">
|
||
<dt class="sig sig-object py" id="abc.abstractstaticmethod">
|
||
<span class="sig-prename descclassname"><span class="pre">@</span></span><span class="sig-prename descclassname"><span class="pre">abc.</span></span><span class="sig-name descname"><span class="pre">abstractstaticmethod</span></span><a class="headerlink" href="#abc.abstractstaticmethod" title="Link to this definition">¶</a></dt>
|
||
<dd><div class="versionadded">
|
||
<p><span class="versionmodified added">Added in version 3.2.</span></p>
|
||
</div>
|
||
<div class="deprecated">
|
||
<p><span class="versionmodified deprecated">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="w"> </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="w"> </span><span class="nf">my_abstract_staticmethod</span><span class="p">(</span><span class="n">arg</span><span class="p">):</span>
|
||
<span class="o">...</span>
|
||
</pre></div>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="py function">
|
||
<dt class="sig sig-object py" id="abc.abstractproperty">
|
||
<span class="sig-prename descclassname"><span class="pre">@</span></span><span class="sig-prename descclassname"><span class="pre">abc.</span></span><span class="sig-name descname"><span class="pre">abstractproperty</span></span><a class="headerlink" href="#abc.abstractproperty" title="Link to this definition">¶</a></dt>
|
||
<dd><div class="deprecated">
|
||
<p><span class="versionmodified deprecated">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>, <a class="reference internal" href="functions.html#property.getter" title="property.getter"><code class="xref py py-meth docutils literal notranslate"><span class="pre">property.getter()</span></code></a>,
|
||
<a class="reference internal" href="functions.html#property.setter" title="property.setter"><code class="xref py py-meth docutils literal notranslate"><span class="pre">property.setter()</span></code></a> and <a class="reference internal" href="functions.html#property.deleter" title="property.deleter"><code class="xref py py-meth docutils literal notranslate"><span class="pre">property.deleter()</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#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="w"> </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="w"> </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="w"> </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="w"> </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="w"> </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="w"> </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="w"> </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 <code class="xref py py-mod docutils literal notranslate"><span class="pre">abc</span></code> module also provides the following functions:</p>
|
||
<dl class="py function">
|
||
<dt class="sig sig-object py" id="abc.get_cache_token">
|
||
<span class="sig-prename descclassname"><span class="pre">abc.</span></span><span class="sig-name descname"><span class="pre">get_cache_token</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#abc.get_cache_token" title="Link 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 added">Added in version 3.4.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="py function">
|
||
<dt class="sig sig-object py" id="abc.update_abstractmethods">
|
||
<span class="sig-prename descclassname"><span class="pre">abc.</span></span><span class="sig-name descname"><span class="pre">update_abstractmethods</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">cls</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#abc.update_abstractmethods" title="Link to this definition">¶</a></dt>
|
||
<dd><p>A function to recalculate an abstract class’s abstraction status. This
|
||
function should be called if a class’s abstract methods have been
|
||
implemented or changed after it was created. Usually, this function should
|
||
be called from within a class decorator.</p>
|
||
<p>Returns <em>cls</em>, to allow usage as a class decorator.</p>
|
||
<p>If <em>cls</em> is not an instance 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>, does nothing.</p>
|
||
<div class="admonition note">
|
||
<p class="admonition-title">Note</p>
|
||
<p>This function assumes that <em>cls</em>’s superclasses are already updated.
|
||
It does not update any subclasses.</p>
|
||
</div>
|
||
<div class="versionadded">
|
||
<p><span class="versionmodified added">Added in version 3.10.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<p class="rubric">Footnotes</p>
|
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|
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<span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id1">1</a><span class="fn-bracket">]</span></span>
|
||
<p>C++ programmers should note that Python’s virtual base class
|
||
concept is not the same as C++’s.</p>
|
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|
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