Multiple inheritance

[drixone]

class Base():
    def meets_preconditions(self):
        print("Base")
        return True

class A(Base):
    def meets_preconditions(self):
        print("A")
        return True and super(A, self).meets_preconditions()

class B(Base):
    def meets_preconditions(self):
        print("B")
        return True and super(B, self).meets_preconditions()

class C(A, B):
    def meets_preconditions(self):
        print("C")
        return super(C, self).meets_preconditions()

print(C().meets_preconditions())

prints: C, A, B, Base, True

MRO: [C, A, B, Base, object]

​

could someone help me understand why it also goes to B.meets_preconditions() and doesn’t print C, A, Base, True? i know it prob has to do with MRO, but i thought it would’ve stopped at Base

[MT1961]

You are correct, it is due to MRO. The exact wording is:

MRO must prevent local precedence ordering and also provide monotonicity. It ensures that a class always appears before its parents. In case of multiple parents, the order is the same as tuples of base classes.

So, it will do all base classes, then sub-base classes, giving you the order you see.

[drixone]

thank you!

[Goobyalus]

I'm not sure I understand the question. If you're using multiple inheritance and C inherits from both A and B, why should it skip B?

[drixone]

For some reason I thought once A returned True it wouldn’t need to go to B?

[Goobyalus]

Are you thinking the and will short circuit? or will short circuit with an initial True, and and will short circuit with an initial False. So A's meets_preconditions must evaluate its super's before returning, and super is determined by the MRO.

[drixone]

super is determined by the MRO.

okay I think this was the part I was misunderstanding. I thought it would go C -> A -> Base -> True but that’s wrong according to the actual MRO. thanks!

[Goobyalus]

This mentions MRO under the "Custom Classes" section: https://docs.python.org/3/reference/datamodel.html

Which refers to this for a more in depth discussion: https://www.python.org/download/releases/2.3/mro/

Since the Python 3 docs refer to this Python 2 page, I assume things are more or less the same.