Python Multiple Inheritance super().init()

Question

I have two classes, with the same parameter initialized by their __init__ method.
I would like to inherit both classes in class "X". But I will get: TypeError: B.__init__() missing 1 required positional argument: 'my_param'

Reproducible Example:

class A:
    def __init__(self, my_param):
        super().__init__()
        self.my_param = my_param


class B:
    def __init__(self, my_param):
        super().__init__()
        self.my_param = my_param * 2


class X(A, B):
    def __init__(self, my_param):
        super().__init__(my_param=my_param)

a = X(my_param=1)
print(a.my_param)

A and B are Mixins, they provide additional functionality for Child Classes. They can be used separetly or together. Lets’s say class A provides searhc functionality for search by ID, where class B provides search by value.

Is there a way to set my_param for each of A and B or to set it without getting the error?

Asked By: Andreas

||

Source

Answer 1

Since A and B share the same fields, I think it makes sense to make one inherit from the other – in this case, B inherit from A. That way, you’ll only need to subclass from B in class X.

For example:

class A:
    def __init__(self, my_param):
        self.my_param = my_param


class B(A):
    def __init__(self, my_param):
        super().__init__(my_param * 2)
        # or:
        #   A.__init__(self, my_param * 2)


class X(B):
    def __init__(self, my_param):
        super().__init__(my_param=my_param)


a = X(my_param=1)
print(a.my_param)  # 2

Answered By: rv.kvetch

Answer 2

This can’t be done just using super() calls, because the superclass of the last class in the chain will be object, which doesn’t take a my_param parameter.

See python multiple inheritance passing arguments to constructors using super for more discussion of parameter passing to __init__() methods with multiple inheritance.

So you need to change X to call the superclass init methods explicitly, rather than using super(). And A and B shouldn’t call super().__init__() because they’re subclasses of object, which doesn’t do anything in its __init__().

class A:
    def __init__(self, my_param):
        self.my_param = my_param

class B:
    def __init__(self, my_param):
        self.my_param = my_param * 2

class X(A, B):
    def __init__(self, my_param):
        A.__init__(self, my_param=my_param)
        B.__init__(self, my_param=my_param)

Answered By: Barmar

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