Python has a class attribute __mro__ for every class.
It's a tuple, containing the class itself and all the superclasses.
>>> D.__mro__
(<class 'ABCD_trad.D'>,
<class 'ABCD_trad.B'>,
<class 'ABCD_trad.C'>,
<class 'ABCD_trad.A'>,
<type 'object'>)