gistfile1.txt

>>> class X:
...     pass
... 
>>> @classmethod
... def mysum(cls, x, y):
...     return x + y
... 
>>> X.setattr('s', mysum)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: class X has no attribute 'setattr'
>>> X.__dict__['s'] = mysum
>>> X.s(1,2)
3