如何在Python中正确调节Union类型？

Question

我编写了一个python函数，用于处理int列表或int列表列表，即[1,2,3]或[[1,2],[3,4]，如下所示：

from typing import Sequence, Union
IntSeq = Sequence[int]

def foo(a: Union[IntSeq, Sequence[IntSeq]]):
    if isinstance(a, Sequence) and isinstance(a[0], int):
        # here type of a equals IntSeq
        b: IntSeq = a

但是，mypy告诉我以下错误：

$ mypy bar.py

test.py:6: error: Incompatible types in assignment (expression has type "Union[Sequence[int], Sequence[Sequence[int]]]", variable has type "Sequence[int]")

如何消除此错误？