Set default kwargs when overrriding in Python 3

Question

I have a class in Python that I want to instantiate with kwargs: e.g.

a = ClassA(happy=True)

I want to override this class with ClassB and set happy to True, I assumed this definition would work:

class ClassB(ClassA(happy=True))

However I get:

TypeError: 'ClassA' object is not callable

The workaround I have currently is to define ClassB as:

class ClassB(ClassA)

But then every time I instantiate ClassB, I have to do this:

b = ClassB(happy=True)

Anyone know how to do this?

Answer 1

This is how to do it:

class ClassB(ClassA):
    def __init__(self, happy=True):
        super().__init__(happy=happy)

When defining base classes of a class, that is all you do - define the base classes (here: (ClassA)).

What happens when you want create an instance of your class (e.g. ClassB()) is that its constructor (__init__) is called.

The base class apparently has a constructor which takes False as the default value, so you override the constructor and change its default argument value.

Then, call the base constructor with whatever the value is (super().__init__(happy=happy)).