“Python Expert” Newsletter (Sep 8, 2021): Learning Corner

mro.py

class A:
    def foo(self):
        return 'A.foo',

class B(A):
    def foo(self):
        return 'B.foo', *super().foo()

class C(A):
    def foo(self):
        return 'C.foo', *super().foo()

class D(B, C):
    def foo(self):
        return 'D.foo', *super().foo()

if __name__ == '__main__':
    print(
        f'{A.__mro__ = }',
        f'{A().foo() = }',

        f'{B.__mro__ = }',
        f'{B().foo() = }',

        f'{C.__mro__ = }',
        f'{C().foo() = }',

        f'{D.__mro__ = }',
        f'{D().foo() = }',

        sep='\n'
    )

As you can see:

• Python 2's “new-style classes” introduced the “method resolution order”—a linearised ordering of base classes
• this “MRO" (accessible via __mro__) determines the order in which super() will operate
• super() is one of the few cases where “what you see is not what you get” in Python (another is class-attribute name-mangling)

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