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<div class="section" id="static-and-class-methods">
<span id="id1"></span><h1>Static and Class Methods<a class="headerlink" href="#static-and-class-methods" title="Permalink to this headline"></a></h1>
<p>You’ve seen how methods of a class are <em>bound</em> to an instance when it is
created.</p>
<p>And you’ve seen how the argument <code class="docutils literal notranslate"><span class="pre">self</span></code> is then automatically passed to the method when it is called.</p>
<p>And you’ve seen how you can call <em>unbound</em> methods on a class object so
long as you pass an instance of that class as the first argument.</p>
<p><strong>But what if you don’t want or need an instance?</strong></p>
<div class="section" id="static-methods">
<h2>Static Methods<a class="headerlink" href="#static-methods" title="Permalink to this headline"></a></h2>
<p>A <em>static method</em> is a method that doesn’t get self:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [36]: </span><span class="k">class</span> <span class="nc">StaticAdder</span><span class="p">:</span>
<span class="gp"> ....: </span> <span class="nd">@staticmethod</span>
<span class="gp"> ....: </span> <span class="k">def</span> <span class="nf">add</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="k">return</span> <span class="n">a</span> <span class="o">+</span> <span class="n">b</span>
<span class="gp"> ....:</span>
<span class="gp">In [37]: </span><span class="n">StaticAdder</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">6</span><span class="p">)</span>
<span class="gh">Out[37]: </span><span class="go">9</span>
</pre></div>
</div>
<p>Where are static methods useful?</p>
<p>Usually they aren’t. It is often better just to write a module-level function.</p>
<p>An example from the Standard Library (tarfile.py):</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">TarInfo</span><span class="p">:</span>
<span class="c1"># ...</span>
<span class="nd">@staticmethod</span>
<span class="k">def</span> <span class="nf">_create_payload</span><span class="p">(</span><span class="n">payload</span><span class="p">):</span>
<span class="sd">"""Return the string payload filled with zero bytes</span>
<span class="sd"> up to the next 512 byte border.</span>
<span class="sd"> """</span>
<span class="n">blocks</span><span class="p">,</span> <span class="n">remainder</span> <span class="o">=</span> <span class="nb">divmod</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">payload</span><span class="p">),</span> <span class="n">BLOCKSIZE</span><span class="p">)</span>
<span class="k">if</span> <span class="n">remainder</span> <span class="o">></span> <span class="mi">0</span><span class="p">:</span>
<span class="n">payload</span> <span class="o">+=</span> <span class="p">(</span><span class="n">BLOCKSIZE</span> <span class="o">-</span> <span class="n">remainder</span><span class="p">)</span> <span class="o">*</span> <span class="n">NUL</span>
<span class="k">return</span> <span class="n">payload</span>
</pre></div>
</div>
</div>
<div class="section" id="class-methods">
<h2>Class Methods<a class="headerlink" href="#class-methods" title="Permalink to this headline"></a></h2>
<p>A class method gets the class object, rather than an instance, as the first
argument</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [41]: </span><span class="k">class</span> <span class="nc">Classy</span><span class="p">:</span>
<span class="gp"> ....: </span> <span class="n">x</span> <span class="o">=</span> <span class="mi">2</span>
<span class="gp"> ....: </span> <span class="nd">@classmethod</span>
<span class="gp"> ....: </span> <span class="k">def</span> <span class="nf">a_class_method</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">y</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"in a class method: "</span><span class="p">,</span> <span class="bp">cls</span><span class="p">)</span>
<span class="gp"> ....: </span> <span class="k">return</span> <span class="n">y</span> <span class="o">**</span> <span class="bp">cls</span><span class="o">.</span><span class="n">x</span>
<span class="gp"> ....:</span>
<span class="gp">In [42]: </span><span class="n">Classy</span><span class="o">.</span><span class="n">a_class_method</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span>
<span class="go">in a class method: <class '__main__.Classy'></span>
<span class="gh">Out[42]: </span><span class="go">16</span>
</pre></div>
</div>
</div>
<div class="section" id="why">
<h2>Why?<a class="headerlink" href="#why" title="Permalink to this headline"></a></h2>
<p>Unlike static methods, class methods are quite common.</p>
<p>They have the advantage of being friendly to subclassing.</p>
<p>Consider this:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [44]: </span><span class="k">class</span> <span class="nc">SubClassy</span><span class="p">(</span><span class="n">Classy</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="n">x</span> <span class="o">=</span> <span class="mi">3</span>
<span class="gp"> ....:</span>
<span class="gp">In [45]: </span><span class="n">SubClassy</span><span class="o">.</span><span class="n">a_class_method</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span>
<span class="go">in a class method: <class '__main__.SubClassy'></span>
<span class="gh">Out[45]: </span><span class="go">64</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">a_class_method</span></code> is defined in the <code class="docutils literal notranslate"><span class="pre">Classy</span></code> class (see above).
And it prints the class that it is called on. But despite being defined in <code class="docutils literal notranslate"><span class="pre">Classy</span></code>, it gets the <code class="docutils literal notranslate"><span class="pre">SubClassy</span></code> class object as the first parameter (<code class="docutils literal notranslate"><span class="pre">cls</span></code>).
So a classmethod will “do the right thing” when used in a subclass.</p>
</div>
<div class="section" id="alternate-constructors">
<h2>Alternate Constructors<a class="headerlink" href="#alternate-constructors" title="Permalink to this headline"></a></h2>
<p>Because of this friendliness to subclassing, class methods are often used to
build alternate constructors.</p>
<p>Consider the case of wanting to build a dictionary with a given iterable of
keys:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [57]: </span><span class="n">d</span> <span class="o">=</span> <span class="nb">dict</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">])</span>
<span class="gt">---------------------------------------------------------------------------</span>
<span class="ne">TypeError</span><span class="g g-Whitespace"> </span>Traceback (most recent call last)
<span class="nn"><ipython-input-57-50c56a77d95f></span> in <span class="ni"><module></span><span class="nt">()</span>
<span class="ne">----> </span><span class="mi">1</span> <span class="n">d</span> <span class="o">=</span> <span class="nb">dict</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">])</span>
<span class="ne">TypeError</span>: cannot convert dictionary update sequence element #0 to a sequence
</pre></div>
</div>
<p>The stock constructor for a dictionary won’t work this way. So the dict object
implements an alternate constructor that <em>can</em>.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="nd">@classmethod</span>
<span class="k">def</span> <span class="nf">fromkeys</span><span class="p">(</span><span class="bp">cls</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="sd">'''OD.fromkeys(S[, v]) -> New ordered dictionary with keys from S.</span>
<span class="sd"> If not specified, the value defaults to None.</span>
<span class="sd"> '''</span>
<span class="bp">self</span> <span class="o">=</span> <span class="bp">cls</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="bp">self</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="bp">self</span>
</pre></div>
</div>
<p>(This is actually from the <code class="docutils literal notranslate"><span class="pre">OrderedDict</span></code> implementation in <code class="docutils literal notranslate"><span class="pre">collections.py</span></code>).</p>
<p>See also <code class="docutils literal notranslate"><span class="pre">datetime.datetime.now()</span></code>, etc….</p>
<p>Properties, Static Methods and Class Methods are powerful features of Python’s OO model.</p>
<p>They are implemented using an underlying structure called <em>descriptors</em></p>
<p><a class="reference external" href="https://docs.python.org/2/howto/descriptor.html">Here is a low level look</a> at how the descriptor protocol works.</p>
<p>The cool part is that this mechanism is available to you, the programmer, as
well.</p>
<p>For the Circle Excercise: use a class method to make an alternate constructor that takes the diameter instead.</p>
<p>Ultimately, make a subclass of <code class="docutils literal notranslate"><span class="pre">Circle</span></code>, called <code class="docutils literal notranslate"><span class="pre">Sphere</span></code>. Check and see if the <code class="docutils literal notranslate"><span class="pre">.from_diameter</span></code> alternate consructor still works!</p>
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