What does 'self' refer to in a @classmethod?

Question

I thought I was starting to get a grip on "the Python way" of programming. Methods of a class accept self as the first parameter to refer to the instance of the class whose context the method is being called in. The @classmethod decorator refers to a method whose functionality is associated with the class, but which doesn't reference a specific instance.

So, what does the first parameter of a @classmethod (canonically 'self') refer to if the method is meant to be called without an instance reference?

Magical

Answer 1

class itself:

A class method receives the class as implicit first argument, just like an instance method receives the instance.

class C:
    @classmethod
    def f(cls):
        print(cls.__name__, type(cls))

>>> C.f()
C <class 'type'>

and it's cls canonically, btw

Genteelism

Answer 2

The first parameter of a classmethod is named cls by convention and refers to the the class object on which the method it was invoked.

>>> class A(object):
...     @classmethod
...     def m(cls):
...         print cls is A
...         print issubclass(cls, A)

>>> class B(A): pass
>>> a = A()
>>> a.m()
True
True
>>> b = B()
>>> b.m()
False 
True

Backcross

Answer 3

The class object gets passed as the first parameter. For example:

class Foo(object):
    @classmethod
    def bar(self):
        return self()

Would return an instance of the Foo class.

EDIT:

Note that the last line would be self() not self. self would return the class itself, while self() returns an instance.

Appurtenance

This is correct, but it is important to note that cls is more commonly used as the first parameter to a classmethod. – Celiaceliac

Answer 4

Django does some strange stuff with a class method here:

class BaseFormSet(StrAndUnicode):
    """
    A collection of instances of the same Form class.
    """
    def __init__(self, data=None, files=None, auto_id='id_%s', prefix=None,
                 initial=None, error_class=ErrorList):
        ...
        self.prefix = prefix or self.get_default_prefix()
        ...

Even though get_default_prefix is declared this way (in the same class):

    @classmethod
    def get_default_prefix(cls):
        return 'form'

Epithelioma