Python：确保 class 属性不在常见 inheritance 树之间共享

Question

I have the feeling I've seen this somewhere, but am not longer sure whether that's a re-constructed memory or if it really exists. In any case I cannot find a reference to this situation:

class A:
    a = 1

class B(A):  # should have it's own class attribute 'a'
    pass

class C(A):  # should have it's own class attribute 'a' and newly defined 'c'
    c = 3

In the above example, Aa is Ba is C.a . But, I actually wanted them to be separate while keeping this structure. There is a good reason for this and I don't want to discuss instance variables. The reason for that is that in my application all these classes are used as 'namespaces'. To solve the 'issue', I replace A inherited parts by a copy of itself as (pseudo code):

class A:
    a = 1

class B(A):  # B.a resolves to A.a
    pass

class C(A):  # C.a resolves to A.a
    c = 3

copy_func(B.A, A)  # investigates public attrs of A and B and create copies, now B.a is not A.a
copy_func(C.A, A)  # investigates public attrs of A and B and create copies, now C.a is not A.a

Answer 1

Define __init_subclass__ to automatically copy superclass-attributes when a childclass is defined.

import copy

class A:
    a = []

    def __init_subclass__(cls, **kwargs):
        for base in cls.mro():
            for attr in base.__dict__:
                if attr not in cls.__dict__ and not attr.startswith("_"):
                    setattr(cls, attr, copy.copy(getattr(cls, attr)))

This will copy everything from the superclass namespace that does not start with _ . In practice, one might want to apply more restrictive rules.

The __init_subclass__ is automatically run when a subclass is defined. This means subclasses gain the copied attributes without having to run code explicitly.

class B(A):
    pass

class C(A):
    c = 3

assert B.a is not A.a
assert C.a is not A.a