gistfile1.txt

>>> A.mro()
[<class '__main__.A'>, <type 'object'>]
>>> B.mro()
[<class '__main__.B'>, <type 'object'>]
>>> set.mro()
[<type 'set'>, <type 'object'>]
>>> dict.mro()
[<type 'dict'>, <type 'object'>]
>>> class C(A, B): pass
... 
>>> C.mro()
[<class '__main__.C'>, <class '__main__.A'>, <class '__main__.B'>, <type 'object'>]
>>> class C(set, dict): pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Error when calling the metaclass bases
    multiple bases have instance lay-out conflict