A tool to help debug inheritance failures.

from inheritance_graph import mro_check

class Dec:
    pass

class Dif:
    pass

class E:
    pass

class R(E):
    pass

class Err(Dif, R):
    pass

class B(E, Dec, Dif):
    pass

Suppose, you now try:

class Z(B, Err):
    pass

Then, Python merely says:

TypeError: Cannot create a consistent method resolution order (MRO) for bases E, Dif

But if you call:

mro_check((B, Err))

it displays

Cycle found:
   Dif precedes R in Err's MRO (and Err was in the proposed based class list) because
     Dif is a base class of Err
     R is a base class of Err
   R precedes E because it inherits from it directly.
   E precedes Dec in B's MRO (and B was in the proposed based class list) because
     E is a base class of B
     Dec is a base class of B
   Dec precedes Dif in B's MRO (and B was in the proposed based class list) because
     Dec is a base class of B
     Dif is a base class of B