如何使用类型提示来确保两个变量始终具有相同的类型？

Question

我有以下代码

from typing import Union

def add(a: Union[int, str], b: Union[int, str]) -> str:
    return str(a) + str(b)

a: int = 1
b: int = 2
out = add(a, b)  # nice

a: str = 'one'
b: str = 'two'
out = add(a, b)  # nice

a: int = 1
b: str = 'two'
out = add(a, b)  # I hope the static check can raise error here when a and b are different types

希望mypy第三种情况能报错。我该怎么办？

Answer 1

一种方法是 @overload 装饰器，它允许您定义可接受的特定参数组合（实现定义为更广泛的联合类型）：

from typing import Union, overload


@overload
def add(a: int, b: int) -> str:
    pass


@overload
def add(a: str, b: str) -> str:
    pass


def add(a: Union[int, str], b: Union[int, str]) -> str:
    return f"{a}{b}"


add(1, 2)           # nice
add("one", "two")   # nice
add(1, "two")       # error: No overload variant of "add" matches argument types "int", "str"

另一种选择是 TypeVar:

from typing import TypeVar

A = TypeVar("A", str, int)


def add(a: A, b: A) -> str:
    return f"{a}{b}"


add(1, 2)           # nice
add("one", "two")   # nice
add(1, "two")       # error: Value of type variable "A" of "add" cannot be "object"

错误有点神秘——它告诉我们两个参数的最近子类型（A 需要对应的）是 object，并且 object 过于宽泛，无法满足对 A.

定义的限制

如何使用类型提示来确保两个变量始终具有相同的类型？

How to use type hint to make sure two variables always have the same type?

python

type-hinting

mypy

python-typing