Python 2.3 introduced new-style classes (classes inheriting from object). New-style classes use
the C3 linearization method to determine a method resolution ordering (MRO) for each class. The C3
linearization method ensures that for a class C, if a class C1 precedes a class C2 in the MRO of C
then C1 should also precede C2 in the MRO of all subclasses of C. It is possible to create a
situation where it is impossible to achieve this consistency and this will guarantee that a
TypeError will be raised at runtime.
Use a class hierarchy that is not ambiguous.
The MRO of class X is just X, object. The program will fail when the MRO
of class Y needs to be calculated because object precedes X in
the definition of Y but the opposite is true in the MRO of X.