The built-in functions type() and isinstance() do not call 
__getattribute__ for instances of user-defined classes. 
Thus, for instance,

x.__class__ == sometype

and

type(x) == sometype

can give inconsistent results.

I ran into this problem in writing a transparent 
persistence system, where instances of proxy classes 
are used as stand-ins for unloaded objects and the 
proxy does just-in-time loading by overloading 
__getattribute__ and __setattr__ (which changes 
__class__).

(This applies to 2.4.1.)