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<!DOCTYPE html>

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<title>Static and Class Methods — Programming in Python 7.0 documentation</title>

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<li class="toctree-l1"><a class="reference internal" href="../topics/01-setting_up/index.html">1. Setting up your Environment</a></li>

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<li class="toctree-l1"><a class="reference internal" href="../topics/04-sequences_iteration/index.html">4. Sequences and Iteration</a></li>

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<li class="toctree-l1 current"><a class="reference internal" href="../topics/14-magic_methods/index.html">14. Properties and Magic Methods</a><ul class="current">

<li class="toctree-l2"><a class="reference internal" href="Properties.html">Properties</a></li>

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<li class="toctree-l2"><a class="reference internal" href="../exercises/circle/circle_class.html">Circle Class Exercise</a></li>

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<li><a href="../topics/14-magic_methods/index.html">14. Properties and Magic Methods</a> »</li>

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<h1>Static and Class Methods<a class="headerlink" href="#static-and-class-methods" title="Permalink to this headline"></a></h1>

You’ve seen how methods of a class are bound to an instance when it is

created.

And you’ve seen how the argument <code class="docutils literal notranslate">self</code> is then automatically passed to the method when it is called.

And you’ve seen how you can call unbound methods on a class object so

long as you pass an instance of that class as the first argument.

But what if you don’t want or need an instance?

<h2>Static Methods<a class="headerlink" href="#static-methods" title="Permalink to this headline"></a></h2>

A static method is a method that doesn’t get self:

<div class="highlight-ipython notranslate"><div class="highlight"><pre>In [36]: class StaticAdder:

....: @staticmethod

....: def add(a, b):

....: return a + b

....:

In [37]: StaticAdder.add(3, 6)

Out[37]: 9

</pre></div>

</div>

Where are static methods useful?

Usually they aren’t. It is often better just to write a module-level function.

An example from the Standard Library (tarfile.py):

<div class="highlight-python notranslate"><div class="highlight"><pre>class TarInfo:

# ...

@staticmethod

def _create_payload(payload):

"""Return the string payload filled with zero bytes

up to the next 512 byte border.

"""

blocks, remainder = divmod(len(payload), BLOCKSIZE)

if remainder > 0:

payload += (BLOCKSIZE - remainder) * NUL

return payload

</pre></div>

</div>

<h2>Class Methods<a class="headerlink" href="#class-methods" title="Permalink to this headline"></a></h2>

A class method gets the class object, rather than an instance, as the first

argument

<div class="highlight-ipython notranslate"><div class="highlight"><pre>In [41]: class Classy:

....: x = 2

....: @classmethod

....: def a_class_method(cls, y):

....: print("in a class method: ", cls)

....: return y ** cls.x

....:

In [42]: Classy.a_class_method(4)

in a class method: <class '__main__.Classy'>

Out[42]: 16

</pre></div>

</div>

Unlike static methods, class methods are quite common.

They have the advantage of being friendly to subclassing.

Consider this:

<div class="highlight-ipython notranslate"><div class="highlight"><pre>In [44]: class SubClassy(Classy):

....: x = 3

....:

In [45]: SubClassy.a_class_method(4)

in a class method: <class '__main__.SubClassy'>

Out[45]: 64

</pre></div>

</div>

<code class="docutils literal notranslate">a_class_method</code> is defined in the <code class="docutils literal notranslate">Classy</code> class (see above).

And it prints the class that it is called on. But despite being defined in <code class="docutils literal notranslate">Classy</code>, it gets the <code class="docutils literal notranslate">SubClassy</code> class object as the first parameter (<code class="docutils literal notranslate">cls</code>).

So a classmethod will “do the right thing” when used in a subclass.

</div>

<h2>Alternate Constructors<a class="headerlink" href="#alternate-constructors" title="Permalink to this headline"></a></h2>

Because of this friendliness to subclassing, class methods are often used to

build alternate constructors.

Consider the case of wanting to build a dictionary with a given iterable of

keys:

<div class="highlight-ipython notranslate"><div class="highlight"><pre>In [57]: d = dict([1,2,3])

---------------------------------------------------------------------------

TypeError Traceback (most recent call last)

<ipython-input-57-50c56a77d95f> in <module>()

----> 1 d = dict([1,2,3])

TypeError: cannot convert dictionary update sequence element #0 to a sequence

</pre></div>

</div>

The stock constructor for a dictionary won’t work this way. So the dict object

implements an alternate constructor that can.

<div class="highlight-python notranslate"><div class="highlight"><pre>@classmethod

def fromkeys(cls, iterable, value=None):

'''OD.fromkeys(S[, v]) -> New ordered dictionary with keys from S.

If not specified, the value defaults to None.

'''

self = cls()

for key in iterable:

self[key] = value

return self

</pre></div>

</div>

(This is actually from the <code class="docutils literal notranslate">OrderedDict</code> implementation in <code class="docutils literal notranslate">collections.py</code>).

See also <code class="docutils literal notranslate">datetime.datetime.now()</code>, etc….

Properties, Static Methods and Class Methods are powerful features of Python’s OO model.

They are implemented using an underlying structure called descriptors

<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.

The cool part is that this mechanism is available to you, the programmer, as

well.

For the Circle Excercise: use a class method to make an alternate constructor that takes the diameter instead.

Ultimately, make a subclass of <code class="docutils literal notranslate">Circle</code>, called <code class="docutils literal notranslate">Sphere</code>. Check and see if the <code class="docutils literal notranslate">.from_diameter</code> alternate consructor still works!

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