带有附加功能的序列或列表类型注释

Question

我有以下设置：

class A:
    pass

class B(A):
    pass

def add_element(lst: list[A], el: A) -> None:
    lst.append(el)

lst: list[B] = [B()]
add_element(lst, B())

这会导致 mypy 出现错误：

t.py:11: error: Argument 1 to "add_element" has incompatible type "List[B]"; expected "List[A]"
t.py:11: note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
t.py:11: note: Consider using "Sequence" instead, which is covariant

如果我尝试这个：

def add_element(lst: Sequence[A], el: A) -> None:
    lst.append(el)

我明白了

t.py:10: error: "Sequence[A]" has no attribute "append"

我可以想出强制其工作的方法，但是正确的方法是什么？

Answer 1

一种选择是使用

TypeVar

:

from typing import TypeVar

class A:
    pass

class B(A):
    pass

T = TypeVar('T', bound=A)
def add_element(lst: list[T], el: T) -> None:
    lst.append(el)

lst: list[B] = [B()]
add_element(lst, B())

如果我们尝试编写使用 A 子类以外的其他内容的代码：

lst: list[int] = []
add_element(lst, 0)

然后mypy会抱怨：

typetest.py:13: error: Value of type variable "T" of "add_element" cannot be "int"  [type-var]
Found 1 error in 1 file (checked 1 source file)