Python multiple inheritance constructor not called when using super()

Question

Consider the following code:

class A(object):
    def __init__(self):
        pass
class B(object):
    def __init__(self):
        self.something = 'blue'
    def get_something(self):
        return self.something
class C(A,B):
    def __init__(self):
        super().__init__()
        print(self.get_something())

and then do:

c = C()

which results in something like this:

AttributeError: 'C' object has no attribute 'something'

I suppose this happens due to the constructor of B not being called when using super(). Is there a way to achieve the correct behavior with Python 3?

wim · Accepted Answer · 2014-08-18 21:57:40Z

14

Superclasses should use super if their subclasses do. If you add the super().__init__() line into A and B your example should work again.

Check the method resolution order of C:

>>> C.mro()
[__main__.C, __main__.A, __main__.B, builtins.object]

This article should clear things up.

answered Aug 18, 2014 at 21:57

wim

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Comments

wim · Accepted Answer · 2023-12-16 18:23:06Z

4

As others have mentioned, the method resolution order is key here. If you want to call multiple superclass constructors, then you can call them directly.

class A(object):
    def __init__(self):
        pass
class B(object):
    def __init__(self):
        self.something = 'blue'
    def get_something(self):
        return self.something
class C(A,B):
    def __init__(self):
        A.__init__(self)
        B.__init__(self)
        print(self.get_something())

edited Dec 16, 2023 at 18:23

wim

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answered Aug 18, 2014 at 22:03

Andrew Johnson

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10 Comments

Andrew Johnson Over a year ago

Your choice. I just wanted to show this option as well, since it is my preference.

Blckknght Over a year ago

This style of multiple inheritance breaks down if your inheritance hierarchy is more complex. For instance, if A and B both inherited from some common Base class (the classic "diamond" pattern) and both called Base.__init__(self) from their own __init__ methods, the Base class would be initialized twice. If you used super instead, each __init__ method would be called exactly once.

Ned Batchelder Over a year ago

Hardcoding the base classes in the subclasses is definitely the wrong way to solve this.

Andrew Johnson Over a year ago

The superclasses are already "hardcoded" into the class definition. Creating a dependence from a superclass to other peers is more damaging in my opinion.

Andrew Johnson Over a year ago

What happens if one of your __init__ methods anywhere takes arguments? Or what if you inherit from a class that exists in a package that you don't maintain?

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