[Python] What is the Diamond Problem in Python and why its not appear in python2?

I got this code:

class A:
pass

class B(A):
pass

class C(A):
pass

class D(A,B):
pass

d = D()


In Python3 i get a MRO Error. I mean it appears because the Diamond Problem. In Python2 its no Problem. Why is this the case, and what is this Diamond Problem exactly ?

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