范围元类；或在包含从 A 继承的嵌套类的 B 类的 __init__ 期间更改 A 类的类变量？

Question

考虑下面的代码：

class A(object):
    attr = None

    def __init__(self):
        assert A.attr is not None


class B(object):
    def __init__(self, attr):
        A.attr = attr

    class C(A):
        def __init__(self):
            super().__init__()

    class D(A):
        def __init__(self):
            super().__init__()

    nested_classes = {cls.__name__: cls for cls in {C, D}}

上面似乎没有按我的预期工作，因为：

>>> first_class = B("first")

>>> first_sub_class = first_class.C()

>>> first_sub_class.attr
'first'

>>> second_class = B("second")

>>> second_sub_class = second_class.C()

>>> second_sub_class.attr
'second'

>>> first_sub_class.attr
'second'

有没有办法让first_sub_class.attr 成为first 而让second_sub_class.attr 成为second？可能是因为有一个范围在 B 内的元类？

几点：

1. 我不想传递 attr，我想在 B 初始化时设置它。
2. 我不想通过使用partial 来绕过上面的点，因为它会破坏依赖__name__ 或__qualname__ 或类似的代码的其余部分。
3. 我想尽可能忠实于当前的结构。

Answer 1

要解决这个问题，只需在A 的__init__ 函数内添加行self.attr = self.attr。由于您不想更改A 的属性，因此您必须进行以下更改：

class A(object):
    attr = None

    def __init__(self):
        assert self.attr is not None # Don't refer to A, but to self to get the correct value
        self.attr = self.attr


class B(object):
    def __init__(self, attr):
        self.attr = attr # Don't edit A, just save the value in own instance

    def __getattribute__(self, item): # completely added, does some magic to ensure class.attr is set correctly 
        if item in B.nested_classes:
            c = B.nested_classes[item]
            c.attr = self.attr
            return c
        return super().__getattribute__(item)

    class C(A):
        def __init__(self):
            super().__init__()

    class D(A):
        def __init__(self):
            super().__init__()

    nested_classes = {cls.__name__: cls for cls in {C, D}}


first_class = B("first")
first_sub_class = first_class.C()
print(first_sub_class.attr)
second_class = B("second")
second_sub_class = second_class.C()
print(second_sub_class.attr)
print(first_sub_class.attr)

Answer 2

你太复杂了：

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

class C(A):
    pass

class D(A):
    pass

class B:
    def __init__(self, attr):
        self.attr = attr

    def C(self):
        return C(self.attr)

    def D(self):
        return D(self.attr)

行为完全符合要求：

>>> first_class = B("first")

>>> first_sub_class = first_class.C()

>>> first_sub_class.attr
'first'

>>> second_class = B("second")

>>> second_sub_class = second_class.C()

>>> second_sub_class.attr
'second'

>>> first_sub_class.attr
'first'