python - 是否有 List 的协变可变版本？

Question

我已经将我真正想要注释的代码归结为这个最小版本:

def print_it(numbers_or_nones):
    for i, number in enumerate(numbers_or_nones):
        if number is None:
            numbers_or_nones[i] = 0
            print("NOOOO")
        else:
            print(number)


numbers = [1, 2, 3, 4]
print_it(numbers)

我要注释参数 numbers_or_nones的 print_it .它必须是...

... 元素为 Optional[int] 的泛型类型

... 可迭代

...支持索引分配

这个案例的正确类型是什么？ 请注意，无法更改 numbers : List[int] 的类型.我能看到的唯一选择是使用 typing.overload .
列表
最简单的就是 List[Optional[int]] .但是，这给出了:

error: Argument 1 to "print_it" has incompatible type "List[int]"; expected "List[Optional[int]]"
note: "List" is invariant -- see http://mypy.readthedocs.io/en/latest/common_issues.html#variance
note: Consider using "Sequence" instead, which is covariant

序列

Unsupported target for indexed assignment ("Sequence[Optional[int]]")

可变序列

error: Argument 1 to "print_it" has incompatible type "List[int]"; expected "MutableSequence[Optional[int]]"

Answer 1

简短回答:没有协变可变集合，此处列出了处理这种情况的可能策略 https://mypy.readthedocs.io/en/stable/common_issues.html#invariance-vs-covariance

为什么可变集合不能是协变的？
因为这会导致严重的问题，例如:

def make_first_None(numbers_or_nones: MutableSequence[Optional[int]]):
    numbers_or_nones[0] = None


numbers: List[int] = [1, 2, 3, 4]
make_first_None(numbers) # Error!! Numbers is not a List[int] anymore!!!

关于为什么可变集合必须保持不变的详细解释在这里 https://mypy.readthedocs.io/en/stable/generics.html#variance-of-generics

对于这种情况，文档中列出的三种策略如下所示:

使用显式类型注释:

numbers: List[Optional[int]] = [1, 2, 3, 4]
print_it(numbers)

复制并传递

print_it(list(numbers))

使用不可变集合

def print_it(numbers_or_nones: Sequence[Optional[int]]) -> List[int]:
    for number in numbers:
        if number is None:
            print("NOOOO")
        else:
            print(number)
    return [number or 0 for number in numbers]


numbers = [1, 2, 3, 4]
print_it(numbers)