python 3 class inheritance issue

Question

class A(object):
    def __init__(self):
        self.a = 1

class B(A):
    def __init__(self):
        A.__init__(self)
        self.a = 2
        self.b = 3


class C(object):
    def __init__(self):
        self.a = 4
        self.c = 5

class D(C, B):
    def __init__(self):
        C.__init__(self)
        B.__init__(self)
        self.d = 6

obj = D()
print(obj.a)

My understanding is that python will first search class C then B then A to get a. So print(obj.a) will print out 4 when searching class C. But the answer is 2. This means that Python got self.a = 2 from class B instead of self.a = 4 from class C. Can anyone explain the reasons? Thank you

You explicitly call B.__init__ after C.__init__. You should be using super to manage the multiple inheritance, and read up on the MRO (method resolution order) to see the order superclasse method implementations will be called in. — jonrsharpe
– jonrsharpe, Commented Sep 5, 2017 at 20:36
You manually called the parent __init__ methods; since B.__init__() is called last, self.a = 2 is executed last (after A.__init__() was called). — Martijn Pieters
– Martijn Pieters, Commented Sep 5, 2017 at 20:38

Martijn Pieters · Accepted Answer · 2017-09-05 20:42:51Z

1

There is no search going on here. You are making explicit, direct calls to unbound methods, passing in self manually. These are simple function calls, nothing more.

So this is just a question of tracing the execution order:

D() -> D.__init__(self)
    C.__init__(self)
        self.a = 4
    B.__init__(self)
        A.__init__(self)
            self.a = 1
        self.a = 2

So a is assigned 4, then 1, then 2.

answered Sep 5, 2017 at 20:42

Martijn Pieters

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