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Two years on, I think about this again.  Still here? :)

The motivating thought is that:

class A(object): pass
class B(object): pass
B.__bases__ = (A,)

and 

class A(object): pass
class B(A): pass

should be equivalent.

An issue that hadn't occurred to me before is that in the first example both 
A and B have a __dict__ (and __weakref__) descriptor, and in the second 
B doesn't.  Should B's __dict__ descriptor be removed on the 
__bases__ assignment?