Class method differences in Python: bound, unbound and static

Question

What is the difference between the following class methods?

Is it that one is static and the other is not?

class Test(object):
  def method_one(self)         


        

Answer 1

Methods in Python are a very, very simple thing once you understood the basics of the descriptor system. Imagine the following class:

class C(object):
    def foo(self):
        pass

Now let's have a look at that class in the shell:

>>> C.foo

>>> C.__dict__['foo']

As you can see if you access the foo attribute on the class you get back an unbound method, however inside the class storage (the dict) there is a function. Why's that? The reason for this is that the class of your class implements a __getattribute__ that resolves descriptors. Sounds complex, but is not. C.foo is roughly equivalent to this code in that special case:

>>> C.__dict__['foo'].__get__(None, C)

That's because functions have a __get__ method which makes them descriptors. If you have an instance of a class it's nearly the same, just that None is the class instance:

>>> c = C()
>>> C.__dict__['foo'].__get__(c, C)
>

Now why does Python do that? Because the method object binds the first parameter of a function to the instance of the class. That's where self comes from. Now sometimes you don't want your class to make a function a method, that's where staticmethod comes into play:

 class C(object):
  @staticmethod
  def foo():
   pass

The staticmethod decorator wraps your class and implements a dummy __get__ that returns the wrapped function as function and not as a method:

>>> C.__dict__['foo'].__get__(None, C)

Hope that explains it.