如何让Mypy意识到iInstance已被调用?

有没有办法让Mypy不打电话就知道我在处理什么类型的对象 isinstance 在每个if语句中?我希望有助手函数来做这类事情,但即使我有 存在 在helper函数中,Mypy抱怨。我试过使用 typing.Union 为了避开这个问题,但也有类似的问题。

import typing


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

    def is_b(self):
        return isinstance(self, B)

    def is_c(self):
        return isinstance(self, C)


class B(A):
    def __init__(self, a, b):
        self.b = b

        super().__init__(a)


class C(A):
    def __init__(self, a, c):
        self.c = c

        super().__init__(a)


a_list: typing.List[A] = []
for i in range(0, 10):
    b_or_c: A
    if i % 2 == 0:
        b_or_c = B('a' + str(i), 'b' + str(i))
    else:
        b_or_c = C('a' + str(i), 'c' + str(i))

    a_list.append(b_or_c)

for b_or_c in a_list:
    print(type(b_or_c))
    if b_or_c.is_b():
        print(b_or_c.b)  # Mypy Error: "A" has no attribute "b"

    if b_or_c.is_c():
        print(b_or_c.c)  # Mypy Error: "A" has no attribute "c"

    if isinstance(b_or_c, B):
        print(b_or_c.b)  # No Mypy Error

    if isinstance(b_or_c, C):
        print(b_or_c.c)  # No Mypy Error