Python multiple inheritance and MRO

Question

I'm currently studying for a python exam but I don't understand MRO and linearization in Python 3 yet.

class F: pass 
class G: pass 
class H: pass
class E(G,H): pass
class D(E,F): pass 
class C(E,G): pass
class B(C,H): pass
class A(D,B,E): pass

For example in one assignment theres a question whether it is possible for E to occur BEFORE C in the linearization of class A.

How do I determine whether it is possible? How could one describe the linearization algorithm (C3) as easy as possible? I'd be really thankful for all kinds of explanations and resources on this, as I'm having a hard time understanding linearization in Python.

Thanks a lot in advance!

Answer 1

Python looks for parents or child classes in DFS order NOT IN BFS

# This order is called Method Resolution Order (`mro()`)

class A(object):
    def dothis(self):
        print('Doing this in A')

class B(A):
    pass

class C(object):
    def dothis(self):
        print('Doing this in C')

class D(B, C):
    pass

d_instance = D()
d_instance.dothis()
# METHOD RESOLUTION ORDER
print(D.mro())

'''
OUTPUT:

# Doing this in A
# [<class '__main__.D'>, <class '__main__.B'>, <class '__main__.A'>, <class '__main__.C'>, <class 'object'>]

    A

    |               C

    B(A)          /
        \
            D(B, C)

RESOLUTION ORDER IS (DFS) = D-B-A-C



             A

        /       \

    B(A)         C(A)
        \       /
            D(B, C)

If the same class appears in mro, the earlier occurrences get removed
D-B-A-C-A -> D-B-C-A, hence still a DFS
RESOLUTION ORDER IS (In Diamond shape) = D-B-C-A


'''

Answer 2

Class E inherit from G and H.

classes G and H dependent (they don't inherit from anybody).

Because class E has no dependency on class C, he can be initialize before him