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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.