Is it bad form to call a classmethod as a method from an instance?

Question

Ex.

If I have something like this:

class C(object):
    @classmethod
    def f(cls, x):
       return x + x

This will work:

c = C()

c.f(2)
4

But is that bad form? Should I only call

C.f()

or

c.__class__.f()

Obviously, this would only make sense in cases where f doesn't interact with self/cls expecting it to be class.

?

Answer 1

If you are tempted to call a class method from an instance you probably don't need a class method.

In the example you gave a static method would be more appropriate precisely because of your last remark (no self/cls interaction).

class C(object):     @staticmethod     def f(x):        return x + x 

this way it's "good form" to do both

c = C() c.f(2) 

and

C.f(2) 

Answer 2

I don't recall using a classmethod like this from outside the class, but it is certainly ok for an instance method to call a classmethod on itself (e.g. self.foo() where foo is a classmethod). This makes sure that inheritance acts as expected, and will call .foo() in the right subclass instead of the base class.