Mypy无法正确解析复合条件下的Union类型，求优化处理方案

当mypy解析Union类型时，会通过代码流推导类型，如下代码可以正常通过mypy检查：

import typing as typ


def foo(x: typ.Union[int, None] = None, y: typ.Union[int, None] = None) -> int:
    if x is None and y is None:
        raise ValueError
    if x is None:
        x = 1
    if y is None:
        y = 2
    return x + y


foo(1, 2)

但当调整为复合条件（逻辑仍正确）时，mypy会报错：

import typing as typ


def foo(x: typ.Union[int, None] = None, y: typ.Union[int, None] = None) -> int:
    if x is None and y is None:
        raise ValueError
    if x is None and y is not None:
        x = 1
    if y is None and x is not None:
        y = 2
    return x + y


foo(1, 2)

报错信息：

main.py:15: error: Unsupported operand types for + ("int" and "None")
main.py:15: error: Unsupported operand types for + ("None" and "int")
main.py:15: error: Unsupported left operand type for + ("None")
main.py:15: note: Both left and right operands are unions
Found 3 errors in 1 file (checked 1 source file)

针对必须使用复合条件、不愿添加冗余代码的需求，以下是几种更优的处理方式：

1. 重构条件分支为互斥结构 #

将后续的if改为elif，让分支逻辑更清晰，帮助mypy完成类型推导：

import typing as typ


def foo(x: typ.Union[int, None] = None, y: typ.Union[int, None] = None) -> int:
    if x is None and y is None:
        raise ValueError
    elif x is None:
        # 此时y必然不为None（已被前面的条件排除）
        x = 1
    elif y is None:
        # 此时x必然不为None（已被前面的条件排除）
        y = 2
    return x + y

这种方式无需额外代码，通过优化分支逻辑让mypy自然识别变量类型，是最推荐的方案。

2. 使用assert辅助类型推导 #

在return前添加assert语句，既可以辅助mypy推导类型，又能保留运行时的逻辑校验（并非冗余代码）：

import typing as typ


def foo(x: typ.Union[int, None] = None, y: typ.Union[int, None] = None) -> int:
    if x is None and y is None:
        raise ValueError
    if x is None and y is not None:
        x = 1
    if y is None and x is not None:
        y = 2
    # 断言x和y均不为None，帮助mypy推导类型
    assert x is not None and y is not None, "x或y不能为None"
    return x + y

assert在运行时会触发检查，若逻辑出错能及时发现，同时mypy会将其视为类型断言，认定后续x和y均为int类型。

3. 使用typing.cast强制类型标注 #

通过cast直接告知mypy变量的实际类型，绕过类型推导限制：

import typing as typ


def foo(x: typ.Union[int, None] = None, y: typ.Union[int, None] = None) -> int:
    if x is None and y is None:
        raise ValueError
    if x is None and y is not None:
        x = 1
    if y is None and x is not None:
        y = 2
    return typ.cast(int, x) + typ.cast(int, y)

注意：cast仅用于类型检查阶段，不会对实际运行时的变量做转换，因此必须确保逻辑上x和y此时确实不为None，否则会引发运行时错误。

内容的提问来源于stack exchange，提问作者norok2