I don't understand the order of methods in python

Question

I don't understand why the code uses the print_me method from Class D, and not the method in class A.

I did some testing using print-statements and can see that it reads the print_me-method of class D before initialising class A, but I don't understand why it doesn't do the same for class A.

class A:
    name = "Alfa"

    def __init__(self, foo):
        self.foo = foo
        foo = 100
        self.print_me()

    def print_me(self):
        print(self.name, self.foo)

class B(A):
    name = "Beta"

    def __init__(self, bar = 40):
        self.bar = bar
        print(self.name, bar)

class C:
    name = "Charlie"

class D(A, C):
    name = "Delta"

    def __init__(self, val):
        A.__init__(self, val)

    def print_me(self):
        print(self.name, "says", self.foo)

d = D(60)

The output is: Delta says 60

I thought it would be: Delta 60

Because the lookup starts from the "lowest" class. I'm pretty sure this behavior is the same in any other major OO language. Try to do print(D.mro()) — DeepSpace
– DeepSpace, Commented May 13, 2019 at 18:34
So when it looks for the print_me method, it starts looking in class D cause that is the lowest class? And because it finds one, it's okay with it and uses that? — Djaff
– Djaff, Commented May 13, 2019 at 18:37
Possible duplicate of How to call Base Class's __init__ method from the child class? — sakhunzai
– sakhunzai, Commented May 13, 2019 at 19:15

blue note · Accepted Answer · 2019-05-13 18:36:10Z

3

Because the self you are passing to the __init__ of A is still an instance of D, not A. And the function A.__init__ is calling self.print_me, which belongs to D.

If you do a = A(); a.print_me() you'd get what you expect.

Important note: The __init__ method in python is not the actual constructor, it is just a method that is automatically called after the actual construction of the object. However, when you call it yourself, it works just like any other method.

answered May 13, 2019 at 18:36

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1 Comment

DeepSpace Over a year ago

This answer could use an explanation (or a link to an answer that explains) the concept of MRO. If D.print_me will be removed then A.print_me will be called. Similarly, if A.print_me will be removed then D.print_me will be called.

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