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?

I would comment out super().__init__() call in A constructor method — Wizard.Ritvik
– Wizard.Ritvik, Commented Aug 26, 2022 at 15:02
X calls A with the parameter, but then A calls B without it. Your parent classes need to be written to collaborate in the inheritance. — jonrsharpe
– jonrsharpe, Commented Aug 26, 2022 at 15:02
so A and B are not related to each other? i.e. I assume it wouldn't make sense to inherit one from the other? — Wizard.Ritvik
– Wizard.Ritvik, Commented Aug 26, 2022 at 15:04

Barmar · Accepted Answer · 2022-08-26 22:03:59Z

15

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)

edited Aug 26, 2022 at 22:03

answered Aug 26, 2022 at 15:14

Barmar

789k57 gold badges555 silver badges669 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Andreas Over a year ago

Thank you for your answer!, Unfortunately when I try this, I get the error: ´TypeError: object.__init__() takes exactly one argument (the instance to initialize)` I

Andreas Over a year ago

ahhh, thank you so much for the explanation! I tried to solve it in the meantime by creating an abstract class shared by A and B but it feels not really clean, I definetly can use your solution and learned something new on the way, thank you again.

Wizard.Ritvik · Accepted Answer · 2022-08-26 15:09:39Z

0

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 Aug 26, 2022 at 15:09

Wizard.Ritvik

12.1k5 gold badges38 silver badges75 bronze badges

3 Comments

Barmar Over a year ago

This class hierarchy may not make sense for the rest of the behavior of B.

Wizard.Ritvik Over a year ago

Totally right, I did make an assumption that since B and A share the same field, it might make sense to introduce some sort of inheritance. But I agree that this might not be the best idea, depending on the use case or scenario.

Andreas Over a year ago

@rv.kvetch thank you for your answer, class A and B are Mixins, they provided different functunalities e.g. search by value (A) and search by id (B). Some childs of A and B only need A as parent, some only B and some both. But both classes share some attributes.

Collectives™ on Stack Overflow

Python Multiple Inheritance super().init()

2 Answers 2

2 Comments

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related