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  182.   <section id="module-abc">

  183. <span id="abc-abstract-base-classes"></span><h1><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="Link to this heading">¶</a></h1>

  184. <p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.12/Lib/abc.py">Lib/abc.py</a></p>

  185. <hr class="docutils" />

  186. <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

  187. 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>;

  188. 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

  189. <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>

  190. <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

  191. ABCs; these can, of course, be further derived. In addition, the

  192. <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

  193. a class or instance provides a particular interface, for example, if it is

  194. <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>

  195. <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

  196. 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>

  197. <dl class="py class">

  198. <dt class="sig sig-object py" id="abc.ABC">

  199. <em class="property"><span class="pre">class</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>

  200. <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,

  201. 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>

  202. avoiding sometimes confusing metaclass usage, for example:</p>

  203. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">abc</span> <span class="kn">import</span> <span class="n">ABC</span>

  204. 

  205. <span class="k">class</span> <span class="nc">MyABC</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span>

  206.     <span class="k">pass</span>

  207. </pre></div>

  208. </div>

  209. <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

  210. inheriting from <code class="xref py py-class docutils literal notranslate"><span class="pre">ABC</span></code> requires the usual precautions regarding

  211. metaclass usage, as multiple inheritance may lead to metaclass conflicts.

  212. One may also define an abstract base class by passing the metaclass

  213. keyword and using <code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code> directly, for example:</p>

  214. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">abc</span> <span class="kn">import</span> <span class="n">ABCMeta</span>

  215. 

  216. <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>

  217.     <span class="k">pass</span>

  218. </pre></div>

  219. </div>

  220. <div class="versionadded">

  221. <p><span class="versionmodified added">New in version 3.4.</span></p>

  222. </div>

  223. </dd></dl>

  224. 

  225. <dl class="py class">

  226. <dt class="sig sig-object py" id="abc.ABCMeta">

  227. <em class="property"><span class="pre">class</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>

  228. <dd><p>Metaclass for defining Abstract Base Classes (ABCs).</p>

  229. <p>Use this metaclass to create an ABC.  An ABC can be subclassed directly, and

  230. then acts as a mix-in class.  You can also register unrelated concrete

  231. classes (even built-in classes) and unrelated ABCs as “virtual subclasses” –

  232. these and their descendants will be considered subclasses of the registering

  233. 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

  234. won’t show up in their MRO (Method Resolution Order) nor will method

  235. implementations defined by the registering ABC be callable (not even via

  236. <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>

  237. <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>

  238. <dl class="py method">

  239. <dt class="sig sig-object py" id="abc.ABCMeta.register">

  240. <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>

  241. <dd><p>Register <em>subclass</em> as a “virtual subclass” of this ABC. For

  242. example:</p>

  243. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">abc</span> <span class="kn">import</span> <span class="n">ABC</span>

  244. 

  245. <span class="k">class</span> <span class="nc">MyABC</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span>

  246.     <span class="k">pass</span>

  247. 

  248. <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>

  249. 

  250. <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>

  251. <span class="k">assert</span> <span class="nb">isinstance</span><span class="p">((),</span> <span class="n">MyABC</span><span class="p">)</span>

  252. </pre></div>

  253. </div>

  254. <div class="versionchanged">

  255. <p><span class="versionmodified changed">Changed in version 3.3: </span>Returns the registered subclass, to allow usage as a class decorator.</p>

  256. </div>

  257. <div class="versionchanged">

  258. <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

  259. <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>

  260. </div>

  261. </dd></dl>

  262. 

  263. <p>You can also override this method in an abstract base class:</p>

  264. <dl class="py method">

  265. <dt class="sig sig-object py" id="abc.ABCMeta.__subclasshook__">

  266. <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>

  267. <dd><p>(Must be defined as a class method.)</p>

  268. <p>Check whether <em>subclass</em> is considered a subclass of this ABC.  This means

  269. 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

  270. 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

  271. subclass of the ABC.  (This class method is called from the

  272. <a class="reference internal" href="../reference/datamodel.html#class.__subclasscheck__" title="class.__subclasscheck__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__subclasscheck__()</span></code></a> method of the ABC.)</p>

  273. <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

  274. it returns <code class="docutils literal notranslate"><span class="pre">True</span></code>, the <em>subclass</em> is considered a subclass of this ABC.

  275. If it returns <code class="docutils literal notranslate"><span class="pre">False</span></code>, the <em>subclass</em> is not considered a subclass of

  276. this ABC, even if it would normally be one.  If it returns

  277. <code class="xref py py-data docutils literal notranslate"><span class="pre">NotImplemented</span></code>, the subclass check is continued with the usual

  278. mechanism.</p>

  279. </dd></dl>

  280. 

  281. <p>For a demonstration of these concepts, look at this example ABC definition:</p>

  282. <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>

  283.     <span class="k">def</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>

  284.         <span class="o">...</span>

  285.     <span class="k">def</span> <span class="fm">__len__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  286.         <span class="o">...</span>

  287.     <span class="k">def</span> <span class="nf">get_iterator</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  288.         <span class="k">return</span> <span class="nb">iter</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span>

  289. 

  290. <span class="k">class</span> <span class="nc">MyIterable</span><span class="p">(</span><span class="n">ABC</span><span class="p">):</span>

  291. 

  292.     <span class="nd">@abstractmethod</span>

  293.     <span class="k">def</span> <span class="fm">__iter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  294.         <span class="k">while</span> <span class="kc">False</span><span class="p">:</span>

  295.             <span class="k">yield</span> <span class="kc">None</span>

  296. 

  297.     <span class="k">def</span> <span class="nf">get_iterator</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  298.         <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="fm">__iter__</span><span class="p">()</span>

  299. 

  300.     <span class="nd">@classmethod</span>

  301.     <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>

  302.         <span class="k">if</span> <span class="bp">cls</span> <span class="ow">is</span> <span class="n">MyIterable</span><span class="p">:</span>

  303.             <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>

  304.                 <span class="k">return</span> <span class="kc">True</span>

  305.         <span class="k">return</span> <span class="bp">NotImplemented</span>

  306. 

  307. <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>

  308. </pre></div>

  309. </div>

  310. <p>The ABC <code class="docutils literal notranslate"><span class="pre">MyIterable</span></code> defines the standard iterable method,

  311. <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

  312. 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

  313. is also part of the <code class="docutils literal notranslate"><span class="pre">MyIterable</span></code> abstract base class, but it does not have

  314. to be overridden in non-abstract derived classes.</p>

  315. <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

  316. 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

  317. <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

  318. 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>

  319. <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>,

  320. 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

  321. 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

  322. <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>

  323. available as a method of <code class="docutils literal notranslate"><span class="pre">Foo</span></code>, so it is provided separately.</p>

  324. </dd></dl>

  325. 

  326. <p>The <code class="xref py py-mod docutils literal notranslate"><span class="pre">abc</span></code> module also provides the following decorator:</p>

  327. <dl class="py function">

  328. <dt class="sig sig-object py" id="abc.abstractmethod">

  329. <span class="sig-prename descclassname"><span class="pre">&#64;</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>

  330. <dd><p>A decorator indicating abstract methods.</p>

  331. <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>

  332. or is derived from it.  A class that has a metaclass derived from

  333. <code class="xref py py-class docutils literal notranslate"><span class="pre">ABCMeta</span></code> cannot be instantiated unless all of its abstract methods

  334. and properties are overridden.  The abstract methods can be called using any

  335. of the normal ‘super’ call mechanisms.  <code class="xref py py-func docutils literal notranslate"><span class="pre">abstractmethod()</span></code> may be used

  336. to declare abstract methods for properties and descriptors.</p>

  337. <p>Dynamically adding abstract methods to a class, or attempting to modify the

  338. abstraction status of a method or class once it is created, are only

  339. 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

  340. <code class="xref py py-func docutils literal notranslate"><span class="pre">abstractmethod()</span></code> only affects subclasses derived using regular

  341. inheritance; “virtual subclasses” registered with the ABC’s

  342. <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>

  343. <p>When <code class="xref py py-func docutils literal notranslate"><span class="pre">abstractmethod()</span></code> is applied in combination with other method

  344. descriptors, it should be applied as the innermost decorator, as shown in

  345. the following usage examples:</p>

  346. <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>

  347.     <span class="nd">@abstractmethod</span>

  348.     <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="n">arg1</span><span class="p">):</span>

  349.         <span class="o">...</span>

  350.     <span class="nd">@classmethod</span>

  351.     <span class="nd">@abstractmethod</span>

  352.     <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="n">arg2</span><span class="p">):</span>

  353.         <span class="o">...</span>

  354.     <span class="nd">@staticmethod</span>

  355.     <span class="nd">@abstractmethod</span>

  356.     <span class="k">def</span> <span class="nf">my_abstract_staticmethod</span><span class="p">(</span><span class="n">arg3</span><span class="p">):</span>

  357.         <span class="o">...</span>

  358. 

  359.     <span class="nd">@property</span>

  360.     <span class="nd">@abstractmethod</span>

  361.     <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>

  362.         <span class="o">...</span>

  363.     <span class="nd">@my_abstract_property</span><span class="o">.</span><span class="n">setter</span>

  364.     <span class="nd">@abstractmethod</span>

  365.     <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>

  366.         <span class="o">...</span>

  367. 

  368.     <span class="nd">@abstractmethod</span>

  369.     <span class="k">def</span> <span class="nf">_get_x</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  370.         <span class="o">...</span>

  371.     <span class="nd">@abstractmethod</span>

  372.     <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>

  373.         <span class="o">...</span>

  374.     <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>

  375. </pre></div>

  376. </div>

  377. <p>In order to correctly interoperate with the abstract base class machinery,

  378. the descriptor must identify itself as abstract using

  379. <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>

  380. if any of the methods used to compose the descriptor are abstract. For

  381. 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>

  382. <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>

  383.     <span class="o">...</span>

  384.     <span class="nd">@property</span>

  385.     <span class="k">def</span> <span class="nf">__isabstractmethod__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  386.         <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>

  387.                    <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>

  388. </pre></div>

  389. </div>

  390. <div class="admonition note">

  391. <p class="admonition-title">Note</p>

  392. <p>Unlike Java abstract methods, these abstract

  393. methods may have an implementation. This implementation can be

  394. 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

  395. overrides it.  This could be useful as an end-point for a

  396. super-call in a framework that uses cooperative

  397. multiple-inheritance.</p>

  398. </div>

  399. </dd></dl>

  400. 

  401. <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>

  402. <dl class="py function">

  403. <dt class="sig sig-object py" id="abc.abstractclassmethod">

  404. <span class="sig-prename descclassname"><span class="pre">&#64;</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>

  405. <dd><div class="versionadded">

  406. <p><span class="versionmodified added">New in version 3.2.</span></p>

  407. </div>

  408. <div class="deprecated">

  409. <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

  410. <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>

  411. </div>

  412. <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

  413. 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>

  414. <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

  415. is now correctly identified as abstract when applied to an abstract

  416. method:</p>

  417. <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>

  418.     <span class="nd">@classmethod</span>

  419.     <span class="nd">@abstractmethod</span>

  420.     <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="n">arg</span><span class="p">):</span>

  421.         <span class="o">...</span>

  422. </pre></div>

  423. </div>

  424. </dd></dl>

  425. 

  426. <dl class="py function">

  427. <dt class="sig sig-object py" id="abc.abstractstaticmethod">

  428. <span class="sig-prename descclassname"><span class="pre">&#64;</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>

  429. <dd><div class="versionadded">

  430. <p><span class="versionmodified added">New in version 3.2.</span></p>

  431. </div>

  432. <div class="deprecated">

  433. <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

  434. <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>

  435. </div>

  436. <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

  437. 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>

  438. <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

  439. is now correctly identified as abstract when applied to an abstract

  440. method:</p>

  441. <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>

  442.     <span class="nd">@staticmethod</span>

  443.     <span class="nd">@abstractmethod</span>

  444.     <span class="k">def</span> <span class="nf">my_abstract_staticmethod</span><span class="p">(</span><span class="n">arg</span><span class="p">):</span>

  445.         <span class="o">...</span>

  446. </pre></div>

  447. </div>

  448. </dd></dl>

  449. 

  450. <dl class="py function">

  451. <dt class="sig sig-object py" id="abc.abstractproperty">

  452. <span class="sig-prename descclassname"><span class="pre">&#64;</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>

  453. <dd><div class="deprecated">

  454. <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>,

  455. <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

  456. <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>

  457. </div>

  458. <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

  459. property.</p>

  460. <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

  461. is now correctly identified as abstract when applied to an abstract

  462. method:</p>

  463. <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>

  464.     <span class="nd">@property</span>

  465.     <span class="nd">@abstractmethod</span>

  466.     <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>

  467.         <span class="o">...</span>

  468. </pre></div>

  469. </div>

  470. <p>The above example defines a read-only property; you can also define a

  471. read-write abstract property by appropriately marking one or more of the

  472. underlying methods as abstract:</p>

  473. <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>

  474.     <span class="nd">@property</span>

  475.     <span class="k">def</span> <span class="nf">x</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  476.         <span class="o">...</span>

  477. 

  478.     <span class="nd">@x</span><span class="o">.</span><span class="n">setter</span>

  479.     <span class="nd">@abstractmethod</span>

  480.     <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>

  481.         <span class="o">...</span>

  482. </pre></div>

  483. </div>

  484. <p>If only some components are abstract, only those components need to be

  485. updated to create a concrete property in a subclass:</p>

  486. <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>

  487.     <span class="nd">@C</span><span class="o">.</span><span class="n">x</span><span class="o">.</span><span class="n">setter</span>

  488.     <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>

  489.         <span class="o">...</span>

  490. </pre></div>

  491. </div>

  492. </dd></dl>

  493. 

  494. <p>The <code class="xref py py-mod docutils literal notranslate"><span class="pre">abc</span></code> module also provides the following functions:</p>

  495. <dl class="py function">

  496. <dt class="sig sig-object py" id="abc.get_cache_token">

  497. <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>

  498. <dd><p>Returns the current abstract base class cache token.</p>

  499. <p>The token is an opaque object (that supports equality testing) identifying

  500. the current version of the abstract base class cache for virtual subclasses.

  501. 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>

  502. <div class="versionadded">

  503. <p><span class="versionmodified added">New in version 3.4.</span></p>

  504. </div>

  505. </dd></dl>

  506. 

  507. <dl class="py function">

  508. <dt class="sig sig-object py" id="abc.update_abstractmethods">

  509. <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>

  510. <dd><p>A function to recalculate an abstract class’s abstraction status. This

  511. function should be called if a class’s abstract methods have been

  512. implemented or changed after it was created. Usually, this function should

  513. be called from within a class decorator.</p>

  514. <p>Returns <em>cls</em>, to allow usage as a class decorator.</p>

  515. <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>

  516. <div class="admonition note">

  517. <p class="admonition-title">Note</p>

  518. <p>This function assumes that <em>cls</em>’s superclasses are already updated.

  519. It does not update any subclasses.</p>

  520. </div>

  521. <div class="versionadded">

  522. <p><span class="versionmodified added">New in version 3.10.</span></p>

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