CODEWITHSUNDEEP
pythonselfclass-method
Yes - that Jake.
Yes - that Jake.

Reputation: 17119

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

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?

Upvotes: 24

Views: 24452

Answers (4)

boatcoder
boatcoder

Reputation: 18107

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'

Upvotes: 0

Jason Baker
Jason Baker

Reputation: 198577

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.

Upvotes: 0

SilentGhost
SilentGhost

Reputation: 319601

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

Upvotes: 38

Torsten Marek
Torsten Marek

Reputation: 86502

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

Upvotes: 16

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