classmethod_descriptor instances such as vars(dict)['fromkeys'] are callable. The first argument has to be a subclass of __objclass__:

    >>> vars(dict)['fromkeys'].__objclass__
    <class 'dict'>

Calling the descriptor creates a bound built-in method; slices the args to remove the class; and calls it with the args and kwds.

    >>> vars(dict)['fromkeys'](dict, 'abc')
    {'a': None, 'b': None, 'c': None}

source: classmethoddescr_call
http://hg.python.org/cpython/file/04f714765c13/Objects/descrobject.c#l256

While the classmethod and staticmethod types that are defined in funcobject.c aren't callable, they do expose a __func__ member.