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<div class="section" id="descriptor-howto-guide">
<h1><a class="toc-backref" href="#id1">Descriptor HowTo Guide</a><a class="headerlink" href="#descriptor-howto-guide" title="Permalink to this headline"></a></h1>
<dl class="field-list simple">
<dt class="field-odd">Author</dt>
<dd class="field-odd"><p>Raymond Hettinger</p>
</dd>
<dt class="field-even">Contact</dt>
<dd class="field-even"><p>&lt;python at rcn dot com&gt;</p>
</dd>
</dl>
<div class="contents topic" id="contents">
<p class="topic-title first">Contents</p>
<ul class="simple">
<li><p><a class="reference internal" href="#descriptor-howto-guide" id="id1">Descriptor HowTo Guide</a></p>
<ul>
<li><p><a class="reference internal" href="#abstract" id="id2">Abstract</a></p></li>
<li><p><a class="reference internal" href="#definition-and-introduction" id="id3">Definition and Introduction</a></p></li>
<li><p><a class="reference internal" href="#descriptor-protocol" id="id4">Descriptor Protocol</a></p></li>
<li><p><a class="reference internal" href="#invoking-descriptors" id="id5">Invoking Descriptors</a></p></li>
<li><p><a class="reference internal" href="#descriptor-example" id="id6">Descriptor Example</a></p></li>
<li><p><a class="reference internal" href="#properties" id="id7">Properties</a></p></li>
<li><p><a class="reference internal" href="#functions-and-methods" id="id8">Functions and Methods</a></p></li>
<li><p><a class="reference internal" href="#static-methods-and-class-methods" id="id9">Static Methods and Class Methods</a></p></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="abstract">
<h2><a class="toc-backref" href="#id2">Abstract</a><a class="headerlink" href="#abstract" title="Permalink to this headline"></a></h2>
<p>Defines descriptors, summarizes the protocol, and shows how descriptors are
called. Examines a custom descriptor and several built-in Python descriptors
including functions, properties, static methods, and class methods. Shows how
each works by giving a pure Python equivalent and a sample application.</p>
<p>Learning about descriptors not only provides access to a larger toolset, it
creates a deeper understanding of how Python works and an appreciation for the
elegance of its design.</p>
</div>
<div class="section" id="definition-and-introduction">
<h2><a class="toc-backref" href="#id3">Definition and Introduction</a><a class="headerlink" href="#definition-and-introduction" title="Permalink to this headline"></a></h2>
<p>In general, a descriptor is an object attribute with “binding behavior”, one
whose attribute access has been overridden by methods in the descriptor
protocol. Those methods are <a class="reference internal" href="../reference/datamodel.html#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a>, <a class="reference internal" href="../reference/datamodel.html#object.__set__" title="object.__set__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__set__()</span></code></a>, and
<a class="reference internal" href="../reference/datamodel.html#object.__delete__" title="object.__delete__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delete__()</span></code></a>. If any of those methods are defined for an object, it is
said to be a descriptor.</p>
<p>The default behavior for attribute access is to get, set, or delete the
attribute from an object’s dictionary. For instance, <code class="docutils literal notranslate"><span class="pre">a.x</span></code> has a lookup chain
starting with <code class="docutils literal notranslate"><span class="pre">a.__dict__['x']</span></code>, then <code class="docutils literal notranslate"><span class="pre">type(a).__dict__['x']</span></code>, and
continuing through the base classes of <code class="docutils literal notranslate"><span class="pre">type(a)</span></code> excluding metaclasses. If the
looked-up value is an object defining one of the descriptor methods, then Python
may override the default behavior and invoke the descriptor method instead.
Where this occurs in the precedence chain depends on which descriptor methods
were defined.</p>
<p>Descriptors are a powerful, general purpose protocol. They are the mechanism
behind properties, methods, static methods, class methods, and <a class="reference internal" href="../library/functions.html#super" title="super"><code class="xref py py-func docutils literal notranslate"><span class="pre">super()</span></code></a>.
They are used throughout Python itself to implement the new style classes
introduced in version 2.2. Descriptors simplify the underlying C-code and offer
a flexible set of new tools for everyday Python programs.</p>
</div>
<div class="section" id="descriptor-protocol">
<h2><a class="toc-backref" href="#id4">Descriptor Protocol</a><a class="headerlink" href="#descriptor-protocol" title="Permalink to this headline"></a></h2>
<p><code class="docutils literal notranslate"><span class="pre">descr.__get__(self,</span> <span class="pre">obj,</span> <span class="pre">type=None)</span> <span class="pre">-&gt;</span> <span class="pre">value</span></code></p>
<p><code class="docutils literal notranslate"><span class="pre">descr.__set__(self,</span> <span class="pre">obj,</span> <span class="pre">value)</span> <span class="pre">-&gt;</span> <span class="pre">None</span></code></p>
<p><code class="docutils literal notranslate"><span class="pre">descr.__delete__(self,</span> <span class="pre">obj)</span> <span class="pre">-&gt;</span> <span class="pre">None</span></code></p>
<p>That is all there is to it. Define any of these methods and an object is
considered a descriptor and can override default behavior upon being looked up
as an attribute.</p>
<p>If an object defines both <a class="reference internal" href="../reference/datamodel.html#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a> and <a class="reference internal" href="../reference/datamodel.html#object.__set__" title="object.__set__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__set__()</span></code></a>, it is considered
a data descriptor. Descriptors that only define <a class="reference internal" href="../reference/datamodel.html#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a> are called
non-data descriptors (they are typically used for methods but other uses are
possible).</p>
<p>Data and non-data descriptors differ in how overrides are calculated with
respect to entries in an instance’s dictionary. If an instance’s dictionary
has an entry with the same name as a data descriptor, the data descriptor
takes precedence. If an instance’s dictionary has an entry with the same
name as a non-data descriptor, the dictionary entry takes precedence.</p>
<p>To make a read-only data descriptor, define both <a class="reference internal" href="../reference/datamodel.html#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a> and
<a class="reference internal" href="../reference/datamodel.html#object.__set__" title="object.__set__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__set__()</span></code></a> with the <a class="reference internal" href="../reference/datamodel.html#object.__set__" title="object.__set__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__set__()</span></code></a> raising an <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> when
called. Defining the <a class="reference internal" href="../reference/datamodel.html#object.__set__" title="object.__set__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__set__()</span></code></a> method with an exception raising
placeholder is enough to make it a data descriptor.</p>
</div>
<div class="section" id="invoking-descriptors">
<h2><a class="toc-backref" href="#id5">Invoking Descriptors</a><a class="headerlink" href="#invoking-descriptors" title="Permalink to this headline"></a></h2>
<p>A descriptor can be called directly by its method name. For example,
<code class="docutils literal notranslate"><span class="pre">d.__get__(obj)</span></code>.</p>
<p>Alternatively, it is more common for a descriptor to be invoked automatically
upon attribute access. For example, <code class="docutils literal notranslate"><span class="pre">obj.d</span></code> looks up <code class="docutils literal notranslate"><span class="pre">d</span></code> in the dictionary
of <code class="docutils literal notranslate"><span class="pre">obj</span></code>. If <code class="docutils literal notranslate"><span class="pre">d</span></code> defines the method <a class="reference internal" href="../reference/datamodel.html#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a>, then <code class="docutils literal notranslate"><span class="pre">d.__get__(obj)</span></code>
is invoked according to the precedence rules listed below.</p>
<p>The details of invocation depend on whether <code class="docutils literal notranslate"><span class="pre">obj</span></code> is an object or a class.</p>
<p>For objects, the machinery is in <a class="reference internal" href="../reference/datamodel.html#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__getattribute__()</span></code></a> which
transforms <code class="docutils literal notranslate"><span class="pre">b.x</span></code> into <code class="docutils literal notranslate"><span class="pre">type(b).__dict__['x'].__get__(b,</span> <span class="pre">type(b))</span></code>. The
implementation works through a precedence chain that gives data descriptors
priority over instance variables, instance variables priority over non-data
descriptors, and assigns lowest priority to <a class="reference internal" href="../reference/datamodel.html#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattr__()</span></code></a> if provided.
The full C implementation can be found in <a class="reference internal" href="../c-api/object.html#c.PyObject_GenericGetAttr" title="PyObject_GenericGetAttr"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_GenericGetAttr()</span></code></a> in
<a class="reference external" href="https://github.com/python/cpython/tree/3.7/Objects/object.c">Objects/object.c</a>.</p>
<p>For classes, the machinery is in <code class="xref py py-meth docutils literal notranslate"><span class="pre">type.__getattribute__()</span></code> which transforms
<code class="docutils literal notranslate"><span class="pre">B.x</span></code> into <code class="docutils literal notranslate"><span class="pre">B.__dict__['x'].__get__(None,</span> <span class="pre">B)</span></code>. In pure Python, it looks
like:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">__getattribute__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">key</span><span class="p">):</span>
<span class="s2">&quot;Emulate type_getattro() in Objects/typeobject.c&quot;</span>
<span class="n">v</span> <span class="o">=</span> <span class="nb">object</span><span class="o">.</span><span class="fm">__getattribute__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">key</span><span class="p">)</span>
<span class="k">if</span> <span class="nb">hasattr</span><span class="p">(</span><span class="n">v</span><span class="p">,</span> <span class="s1">&#39;__get__&#39;</span><span class="p">):</span>
<span class="k">return</span> <span class="n">v</span><span class="o">.</span><span class="fm">__get__</span><span class="p">(</span><span class="kc">None</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span>
<span class="k">return</span> <span class="n">v</span>
</pre></div>
</div>
<p>The important points to remember are:</p>
<ul class="simple">
<li><p>descriptors are invoked by the <a class="reference internal" href="../reference/datamodel.html#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a> method</p></li>
<li><p>overriding <a class="reference internal" href="../reference/datamodel.html#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a> prevents automatic descriptor calls</p></li>
<li><p><a class="reference internal" href="../reference/datamodel.html#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__getattribute__()</span></code></a> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">type.__getattribute__()</span></code> make
different calls to <a class="reference internal" href="../reference/datamodel.html#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a>.</p></li>
<li><p>data descriptors always override instance dictionaries.</p></li>
<li><p>non-data descriptors may be overridden by instance dictionaries.</p></li>
</ul>
<p>The object returned by <code class="docutils literal notranslate"><span class="pre">super()</span></code> also has a custom <a class="reference internal" href="../reference/datamodel.html#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a>
method for invoking descriptors. The call <code class="docutils literal notranslate"><span class="pre">super(B,</span> <span class="pre">obj).m()</span></code> searches
<code class="docutils literal notranslate"><span class="pre">obj.__class__.__mro__</span></code> for the base class <code class="docutils literal notranslate"><span class="pre">A</span></code> immediately following <code class="docutils literal notranslate"><span class="pre">B</span></code>
and then returns <code class="docutils literal notranslate"><span class="pre">A.__dict__['m'].__get__(obj,</span> <span class="pre">B)</span></code>. If not a descriptor,
<code class="docutils literal notranslate"><span class="pre">m</span></code> is returned unchanged. If not in the dictionary, <code class="docutils literal notranslate"><span class="pre">m</span></code> reverts to a
search using <a class="reference internal" href="../reference/datamodel.html#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__getattribute__()</span></code></a>.</p>
<p>The implementation details are in <code class="xref c c-func docutils literal notranslate"><span class="pre">super_getattro()</span></code> in
<a class="reference external" href="https://github.com/python/cpython/tree/3.7/Objects/typeobject.c">Objects/typeobject.c</a>. and a pure Python equivalent can be found in
<a class="reference external" href="https://www.python.org/download/releases/2.2.3/descrintro/#cooperation">Guido’s Tutorial</a>.</p>
<p>The details above show that the mechanism for descriptors is embedded in the
<a class="reference internal" href="../reference/datamodel.html#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a> methods for <a class="reference internal" href="../library/functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate"><span class="pre">object</span></code></a>, <a class="reference internal" href="../library/functions.html#type" title="type"><code class="xref py py-class docutils literal notranslate"><span class="pre">type</span></code></a>, and
<a class="reference internal" href="../library/functions.html#super" title="super"><code class="xref py py-func docutils literal notranslate"><span class="pre">super()</span></code></a>. Classes inherit this machinery when they derive from
<a class="reference internal" href="../library/functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate"><span class="pre">object</span></code></a> or if they have a meta-class providing similar functionality.
Likewise, classes can turn-off descriptor invocation by overriding
<a class="reference internal" href="../reference/datamodel.html#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a>.</p>
</div>
<div class="section" id="descriptor-example">
<h2><a class="toc-backref" href="#id6">Descriptor Example</a><a class="headerlink" href="#descriptor-example" title="Permalink to this headline"></a></h2>
<p>The following code creates a class whose objects are data descriptors which
print a message for each get or set. Overriding <a class="reference internal" href="../reference/datamodel.html#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a> is
alternate approach that could do this for every attribute. However, this
descriptor is useful for monitoring just a few chosen attributes:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">RevealAccess</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="sd">&quot;&quot;&quot;A data descriptor that sets and returns values</span>
<span class="sd"> normally and prints a message logging their access.</span>
<span class="sd"> &quot;&quot;&quot;</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">initval</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="s1">&#39;var&#39;</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">val</span> <span class="o">=</span> <span class="n">initval</span>
<span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
<span class="k">def</span> <span class="nf">__get__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">obj</span><span class="p">,</span> <span class="n">objtype</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Retrieving&#39;</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">val</span>
<span class="k">def</span> <span class="nf">__set__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">obj</span><span class="p">,</span> <span class="n">val</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Updating&#39;</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">val</span> <span class="o">=</span> <span class="n">val</span>
<span class="o">&gt;&gt;&gt;</span> <span class="k">class</span> <span class="nc">MyClass</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="o">...</span> <span class="n">x</span> <span class="o">=</span> <span class="n">RevealAccess</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="s1">&#39;var &quot;x&quot;&#39;</span><span class="p">)</span>
<span class="o">...</span> <span class="n">y</span> <span class="o">=</span> <span class="mi">5</span>
<span class="o">...</span>
<span class="o">&gt;&gt;&gt;</span> <span class="n">m</span> <span class="o">=</span> <span class="n">MyClass</span><span class="p">()</span>
<span class="o">&gt;&gt;&gt;</span> <span class="n">m</span><span class="o">.</span><span class="n">x</span>
<span class="n">Retrieving</span> <span class="n">var</span> <span class="s2">&quot;x&quot;</span>
<span class="mi">10</span>
<span class="o">&gt;&gt;&gt;</span> <span class="n">m</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="mi">20</span>
<span class="n">Updating</span> <span class="n">var</span> <span class="s2">&quot;x&quot;</span>
<span class="o">&gt;&gt;&gt;</span> <span class="n">m</span><span class="o">.</span><span class="n">x</span>
<span class="n">Retrieving</span> <span class="n">var</span> <span class="s2">&quot;x&quot;</span>
<span class="mi">20</span>
<span class="o">&gt;&gt;&gt;</span> <span class="n">m</span><span class="o">.</span><span class="n">y</span>
<span class="mi">5</span>
</pre></div>
</div>
<p>The protocol is simple and offers exciting possibilities. Several use cases are
so common that they have been packaged into individual function calls.
Properties, bound methods, static methods, and class methods are all
based on the descriptor protocol.</p>
</div>
<div class="section" id="properties">
<h2><a class="toc-backref" href="#id7">Properties</a><a class="headerlink" href="#properties" title="Permalink to this headline"></a></h2>
<p>Calling <a class="reference internal" href="../library/functions.html#property" title="property"><code class="xref py py-func docutils literal notranslate"><span class="pre">property()</span></code></a> is a succinct way of building a data descriptor that
triggers function calls upon access to an attribute. Its signature is:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nb">property</span><span class="p">(</span><span class="n">fget</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">fset</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">fdel</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">doc</span><span class="o">=</span><span class="kc">None</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">property</span> <span class="n">attribute</span>
</pre></div>
</div>
<p>The documentation shows a typical use to define a managed attribute <code class="docutils literal notranslate"><span class="pre">x</span></code>:</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="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">getx</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">__x</span>
<span class="k">def</span> <span class="nf">setx</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">__x</span> <span class="o">=</span> <span class="n">value</span>
<span class="k">def</span> <span class="nf">delx</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">del</span> <span class="bp">self</span><span class="o">.</span><span class="n">__x</span>
<span class="n">x</span> <span class="o">=</span> <span class="nb">property</span><span class="p">(</span><span class="n">getx</span><span class="p">,</span> <span class="n">setx</span><span class="p">,</span> <span class="n">delx</span><span class="p">,</span> <span class="s2">&quot;I&#39;m the &#39;x&#39; property.&quot;</span><span class="p">)</span>
</pre></div>
</div>
<p>To see how <a class="reference internal" href="../library/functions.html#property" title="property"><code class="xref py py-func docutils literal notranslate"><span class="pre">property()</span></code></a> is implemented in terms of the descriptor protocol,
here is a pure Python equivalent:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Property</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="s2">&quot;Emulate PyProperty_Type() in Objects/descrobject.c&quot;</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">fget</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">fset</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">fdel</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">doc</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">fget</span> <span class="o">=</span> <span class="n">fget</span>
<span class="bp">self</span><span class="o">.</span><span class="n">fset</span> <span class="o">=</span> <span class="n">fset</span>
<span class="bp">self</span><span class="o">.</span><span class="n">fdel</span> <span class="o">=</span> <span class="n">fdel</span>
<span class="k">if</span> <span class="n">doc</span> <span class="ow">is</span> <span class="kc">None</span> <span class="ow">and</span> <span class="n">fget</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<span class="n">doc</span> <span class="o">=</span> <span class="n">fget</span><span class="o">.</span><span class="vm">__doc__</span>
<span class="bp">self</span><span class="o">.</span><span class="vm">__doc__</span> <span class="o">=</span> <span class="n">doc</span>
<span class="k">def</span> <span class="nf">__get__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">obj</span><span class="p">,</span> <span class="n">objtype</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="k">if</span> <span class="n">obj</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="k">return</span> <span class="bp">self</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">fget</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">AttributeError</span><span class="p">(</span><span class="s2">&quot;unreadable attribute&quot;</span><span class="p">)</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">fget</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">__set__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">obj</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">fset</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">AttributeError</span><span class="p">(</span><span class="s2">&quot;can&#39;t set attribute&quot;</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="n">obj</span><span class="p">,</span> <span class="n">value</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">__delete__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">obj</span><span class="p">):</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">fdel</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">AttributeError</span><span class="p">(</span><span class="s2">&quot;can&#39;t delete attribute&quot;</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">fdel</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">getter</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">fget</span><span class="p">):</span>
<span class="k">return</span> <span class="nb">type</span><span class="p">(</span><span class="bp">self</span><span class="p">)(</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> <span class="bp">self</span><span class="o">.</span><span class="vm">__doc__</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">setter</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">fset</span><span class="p">):</span>
<span class="k">return</span> <span class="nb">type</span><span class="p">(</span><span class="bp">self</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="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> <span class="bp">self</span><span class="o">.</span><span class="vm">__doc__</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">deleter</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">fdel</span><span class="p">):</span>
<span class="k">return</span> <span class="nb">type</span><span class="p">(</span><span class="bp">self</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="n">fdel</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="vm">__doc__</span><span class="p">)</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="../library/functions.html#property" title="property"><code class="xref py py-func docutils literal notranslate"><span class="pre">property()</span></code></a> builtin helps whenever a user interface has granted
attribute access and then subsequent changes require the intervention of a
method.</p>
<p>For instance, a spreadsheet class may grant access to a cell value through
<code class="docutils literal notranslate"><span class="pre">Cell('b10').value</span></code>. Subsequent improvements to the program require the cell
to be recalculated on every access; however, the programmer does not want to
affect existing client code accessing the attribute directly. The solution is
to wrap access to the value attribute in a property data descriptor:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Cell</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="o">.</span> <span class="o">.</span> <span class="o">.</span>
<span class="k">def</span> <span class="nf">getvalue</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="s2">&quot;Recalculate the cell before returning value&quot;</span>
<span class="bp">self</span><span class="o">.</span><span class="n">recalc</span><span class="p">()</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">_value</span>
<span class="n">value</span> <span class="o">=</span> <span class="nb">property</span><span class="p">(</span><span class="n">getvalue</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="functions-and-methods">
<h2><a class="toc-backref" href="#id8">Functions and Methods</a><a class="headerlink" href="#functions-and-methods" title="Permalink to this headline"></a></h2>
<p>Python’s object oriented features are built upon a function based environment.
Using non-data descriptors, the two are merged seamlessly.</p>
<p>Class dictionaries store methods as functions. In a class definition, methods
are written using <a class="reference internal" href="../reference/compound_stmts.html#def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">def</span></code></a> or <a class="reference internal" href="../reference/expressions.html#lambda"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">lambda</span></code></a>, the usual tools for
creating functions. Methods only differ from regular functions in that the
first argument is reserved for the object instance. By Python convention, the
instance reference is called <em>self</em> but may be called <em>this</em> or any other
variable name.</p>
<p>To support method calls, functions include the <a class="reference internal" href="../reference/datamodel.html#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a> method for
binding methods during attribute access. This means that all functions are
non-data descriptors which return bound methods when they are invoked from an
object. In pure Python, it works like this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Function</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="o">.</span> <span class="o">.</span> <span class="o">.</span>
<span class="k">def</span> <span class="nf">__get__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">obj</span><span class="p">,</span> <span class="n">objtype</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="s2">&quot;Simulate func_descr_get() in Objects/funcobject.c&quot;</span>
<span class="k">if</span> <span class="n">obj</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="k">return</span> <span class="bp">self</span>
<span class="k">return</span> <span class="n">types</span><span class="o">.</span><span class="n">MethodType</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">obj</span><span class="p">)</span>
</pre></div>
</div>
<p>Running the interpreter shows how the function descriptor works in practice:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">D</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">return</span> <span class="n">x</span>
<span class="gp">...</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">d</span> <span class="o">=</span> <span class="n">D</span><span class="p">()</span>
<span class="go"># Access through the class dictionary does not invoke __get__.</span>
<span class="go"># It just returns the underlying function object.</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">D</span><span class="o">.</span><span class="vm">__dict__</span><span class="p">[</span><span class="s1">&#39;f&#39;</span><span class="p">]</span>
<span class="go">&lt;function D.f at 0x00C45070&gt;</span>
<span class="go"># Dotted access from a class calls __get__() which just returns</span>
<span class="go"># the underlying function unchanged.</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">D</span><span class="o">.</span><span class="n">f</span>
<span class="go">&lt;function D.f at 0x00C45070&gt;</span>
<span class="go"># The function has a __qualname__ attribute to support introspection</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">D</span><span class="o">.</span><span class="n">f</span><span class="o">.</span><span class="vm">__qualname__</span>
<span class="go">&#39;D.f&#39;</span>
<span class="go"># Dotted access from an instance calls __get__() which returns the</span>
<span class="go"># function wrapped in a bound method object</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">d</span><span class="o">.</span><span class="n">f</span>
<span class="go">&lt;bound method D.f of &lt;__main__.D object at 0x00B18C90&gt;&gt;</span>
<span class="go"># Internally, the bound method stores the underlying function,</span>
<span class="go"># the bound instance, and the class of the bound instance.</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">d</span><span class="o">.</span><span class="n">f</span><span class="o">.</span><span class="vm">__func__</span>
<span class="go">&lt;function D.f at 0x1012e5ae8&gt;</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">d</span><span class="o">.</span><span class="n">f</span><span class="o">.</span><span class="vm">__self__</span>
<span class="go">&lt;__main__.D object at 0x1012e1f98&gt;</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">d</span><span class="o">.</span><span class="n">f</span><span class="o">.</span><span class="vm">__class__</span>
<span class="go">&lt;class &#39;method&#39;&gt;</span>
</pre></div>
</div>
</div>
<div class="section" id="static-methods-and-class-methods">
<h2><a class="toc-backref" href="#id9">Static Methods and Class Methods</a><a class="headerlink" href="#static-methods-and-class-methods" title="Permalink to this headline"></a></h2>
<p>Non-data descriptors provide a simple mechanism for variations on the usual
patterns of binding functions into methods.</p>
<p>To recap, functions have a <a class="reference internal" href="../reference/datamodel.html#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a> method so that they can be converted
to a method when accessed as attributes. The non-data descriptor transforms an
<code class="docutils literal notranslate"><span class="pre">obj.f(*args)</span></code> call into <code class="docutils literal notranslate"><span class="pre">f(obj,</span> <span class="pre">*args)</span></code>. Calling <code class="docutils literal notranslate"><span class="pre">klass.f(*args)</span></code>
becomes <code class="docutils literal notranslate"><span class="pre">f(*args)</span></code>.</p>
<p>This chart summarizes the binding and its two most useful variants:</p>
<blockquote>
<div><table class="docutils align-center">
<colgroup>
<col style="width: 30%" />
<col style="width: 39%" />
<col style="width: 32%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Transformation</p></th>
<th class="head"><p>Called from an
Object</p></th>
<th class="head"><p>Called from a
Class</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p>function</p></td>
<td><p>f(obj, *args)</p></td>
<td><p>f(*args)</p></td>
</tr>
<tr class="row-odd"><td><p>staticmethod</p></td>
<td><p>f(*args)</p></td>
<td><p>f(*args)</p></td>
</tr>
<tr class="row-even"><td><p>classmethod</p></td>
<td><p>f(type(obj), *args)</p></td>
<td><p>f(klass, *args)</p></td>
</tr>
</tbody>
</table>
</div></blockquote>
<p>Static methods return the underlying function without changes. Calling either
<code class="docutils literal notranslate"><span class="pre">c.f</span></code> or <code class="docutils literal notranslate"><span class="pre">C.f</span></code> is the equivalent of a direct lookup into
<code class="docutils literal notranslate"><span class="pre">object.__getattribute__(c,</span> <span class="pre">&quot;f&quot;)</span></code> or <code class="docutils literal notranslate"><span class="pre">object.__getattribute__(C,</span> <span class="pre">&quot;f&quot;)</span></code>. As a
result, the function becomes identically accessible from either an object or a
class.</p>
<p>Good candidates for static methods are methods that do not reference the
<code class="docutils literal notranslate"><span class="pre">self</span></code> variable.</p>
<p>For instance, a statistics package may include a container class for
experimental data. The class provides normal methods for computing the average,
mean, median, and other descriptive statistics that depend on the data. However,
there may be useful functions which are conceptually related but do not depend
on the data. For instance, <code class="docutils literal notranslate"><span class="pre">erf(x)</span></code> is handy conversion routine that comes up
in statistical work but does not directly depend on a particular dataset.
It can be called either from an object or the class: <code class="docutils literal notranslate"><span class="pre">s.erf(1.5)</span> <span class="pre">--&gt;</span> <span class="pre">.9332</span></code> or
<code class="docutils literal notranslate"><span class="pre">Sample.erf(1.5)</span> <span class="pre">--&gt;</span> <span class="pre">.9332</span></code>.</p>
<p>Since staticmethods return the underlying function with no changes, the example
calls are unexciting:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">E</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="gp">... </span> <span class="n">f</span> <span class="o">=</span> <span class="nb">staticmethod</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">E</span><span class="o">.</span><span class="n">f</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span>
<span class="go">3</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">E</span><span class="p">()</span><span class="o">.</span><span class="n">f</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span>
<span class="go">3</span>
</pre></div>
</div>
<p>Using the non-data descriptor protocol, a pure Python version of
<a class="reference internal" href="../library/functions.html#staticmethod" title="staticmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">staticmethod()</span></code></a> would look like this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">StaticMethod</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="s2">&quot;Emulate PyStaticMethod_Type() in Objects/funcobject.c&quot;</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">f</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">f</span> <span class="o">=</span> <span class="n">f</span>
<span class="k">def</span> <span class="nf">__get__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">obj</span><span class="p">,</span> <span class="n">objtype</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">f</span>
</pre></div>
</div>
<p>Unlike static methods, class methods prepend the class reference to the
argument list before calling the function. This format is the same
for whether the caller is an object or a class:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">E</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">klass</span><span class="p">,</span> <span class="n">x</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">return</span> <span class="n">klass</span><span class="o">.</span><span class="vm">__name__</span><span class="p">,</span> <span class="n">x</span>
<span class="gp">... </span> <span class="n">f</span> <span class="o">=</span> <span class="nb">classmethod</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="n">E</span><span class="o">.</span><span class="n">f</span><span class="p">(</span><span class="mi">3</span><span class="p">))</span>
<span class="go">(&#39;E&#39;, 3)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="n">E</span><span class="p">()</span><span class="o">.</span><span class="n">f</span><span class="p">(</span><span class="mi">3</span><span class="p">))</span>
<span class="go">(&#39;E&#39;, 3)</span>
</pre></div>
</div>
<p>This behavior is useful whenever the function only needs to have a class
reference and does not care about any underlying data. One use for classmethods
is to create alternate class constructors. In Python 2.3, the classmethod
<a class="reference internal" href="../library/stdtypes.html#dict.fromkeys" title="dict.fromkeys"><code class="xref py py-func docutils literal notranslate"><span class="pre">dict.fromkeys()</span></code></a> creates a new dictionary from a list of keys. The pure
Python equivalent is:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Dict</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="o">.</span> <span class="o">.</span> <span class="o">.</span>
<span class="k">def</span> <span class="nf">fromkeys</span><span class="p">(</span><span class="n">klass</span><span class="p">,</span> <span class="n">iterable</span><span class="p">,</span> <span class="n">value</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="s2">&quot;Emulate dict_fromkeys() in Objects/dictobject.c&quot;</span>
<span class="n">d</span> <span class="o">=</span> <span class="n">klass</span><span class="p">()</span>
<span class="k">for</span> <span class="n">key</span> <span class="ow">in</span> <span class="n">iterable</span><span class="p">:</span>
<span class="n">d</span><span class="p">[</span><span class="n">key</span><span class="p">]</span> <span class="o">=</span> <span class="n">value</span>
<span class="k">return</span> <span class="n">d</span>
<span class="n">fromkeys</span> <span class="o">=</span> <span class="nb">classmethod</span><span class="p">(</span><span class="n">fromkeys</span><span class="p">)</span>
</pre></div>
</div>
<p>Now a new dictionary of unique keys can be constructed like this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">Dict</span><span class="o">.</span><span class="n">fromkeys</span><span class="p">(</span><span class="s1">&#39;abracadabra&#39;</span><span class="p">)</span>
<span class="go">{&#39;a&#39;: None, &#39;r&#39;: None, &#39;b&#39;: None, &#39;c&#39;: None, &#39;d&#39;: None}</span>
</pre></div>
</div>
<p>Using the non-data descriptor protocol, a pure Python version of
<a class="reference internal" href="../library/functions.html#classmethod" title="classmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">classmethod()</span></code></a> would look like this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">ClassMethod</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="s2">&quot;Emulate PyClassMethod_Type() in Objects/funcobject.c&quot;</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">f</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">f</span> <span class="o">=</span> <span class="n">f</span>
<span class="k">def</span> <span class="nf">__get__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">obj</span><span class="p">,</span> <span class="n">klass</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="k">if</span> <span class="n">klass</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="n">klass</span> <span class="o">=</span> <span class="nb">type</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">newfunc</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">f</span><span class="p">(</span><span class="n">klass</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">)</span>
<span class="k">return</span> <span class="n">newfunc</span>
</pre></div>
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