在类继承中使用默认值传递参数的最安全方法是什么？

Question

Suppose I have some Python class:

class A:
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z

And now, I want to define a child class whose y value is set by default, say with value 1. The only change in this new class is this default for y , then I guess this formulation will work

class B(A):
    def __init__(self, **kwargs):
        y = kwargs.pop("y", 1)
        assert y == 1
        super.__init__(y=y, **kwargs)

What is the classic form to pass defaults in inheritance?

Answer 1

The second code sample won't work. It requires keyword arguments which is surprising compared to the first sample and with assert it requires y == 1 , it doesn't default to it. Also super must be called.

Usual way (with reordering of parameters) is:

class B(A):
    def __init__(self, x, z, y=1):
        super().__init__(x, y, z)

Alternatively without reordering:

class B(A):
    def __init__(self, x, yz, z=None):
        if z is None:
            super().__init__(x, 1, yz)
        else:
            super().__init__(x, yz, z)

Answer 2

It depends on what you want. Your current code throws an error if y is anything other than 1 , so it's pointless to make it a parameter at all. Just do:

class B(A):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, y=1, **kwargs)

This passes all parameters through to A , whether passed positionally or as keywords, and fixes y to 1 . You'll get an error if you try to pass y to B .

Alternatively, if you want y to default to 1 but still be able to supply it as an argument to B :

class B(A):
    def __init__(self, *args, y=1, **kwargs):
        super().__init__(*args, y=y, **kwargs)