解决Python中Literal类型赋值引发的Mypy类型检查错误

from typing import Literal

ValidChar = Literal["A", "B"]

class MyClass:
    char: ValidChar = "A"

    def set_char(self, char: str) -> None:
        """Set only valid character."""
        # 错误：Literal类型没有__args__属性，且即使验证通过，Mypy也无法推断str到Literal的转换
        if char in ValidChar.__args__: # Mypy error: "<typing special form>" has no attribute "__args__"
            self.char = char # Mypy error: Incompatible types in assignment (expression has type "str", variable has type "Literal['A', 'B']")

obj = MyClass()
obj.set_char("B")
obj.set_char("C") # 运行时无报错，但Mypy会报错

from typing import Literal, cast, get_args

ValidChar = Literal["A", "B"]

class MyClass:
    char: ValidChar = "A"

    def set_char(self, char: str) -> None:
        """Set only valid character."""
        if char in get_args(ValidChar): # 使用get_args正确获取Literal的参数
            # OK to cast - validated above.
            # Mypy does not infer literals from strings this way.
            self.char = cast(ValidChar, char) # 使用cast明确告诉Mypy类型是安全的

# 示例使用
obj = MyClass()
print(f"{obj.char=}") # obj.char='A'

obj.set_char("B")
print(f"{obj.char=}") # obj.char='B'

obj.set_char("C")
print(f"{obj.char=}") # obj.char='B' (因为'C'不是有效字符，所以没有赋值)

from typing import Literal, TypeGuard, get_args

ValidChar = Literal["A", "B"]

def is_valid_char(char: str) -> TypeGuard[ValidChar]:
    """
    检查一个字符串是否是ValidChar类型。
    如果返回True，Mypy会将char的类型缩小为ValidChar。
    """
    return char in get_args(ValidChar)

class MyClass:
    char: ValidChar = "A"

    def set_char(self, char: str) -> None:
        """Set only valid character."""
        if is_valid_char(char): # 调用TypeGuard函数
            self.char = char # Mypy现在知道char在这里是ValidChar类型，无需cast

# 示例使用
obj = MyClass()
print(f"{obj.char=}") # obj.char='A'

obj.set_char("B")
print(f"{obj.char=}") # obj.char='B'

obj.set_char("C")
print(f"{obj.char=}") # obj.char='B'

from typing import Literal, TypeGuard, get_args

ValidChar = Literal["A", "B"]

def is_valid_char(char: str) -> TypeGuard[ValidChar]:
    return char in get_args(ValidChar)

class InvalidCharError(ValueError):
    """自定义异常：无效字符"""
    pass

class MyClass:
    char: ValidChar = "A"

    def set_char(self, char: str) -> None:
        """Set only valid character, otherwise raise an error."""
        if is_valid_char(char):
            self.char = char
        else:
            raise InvalidCharError(f"Invalid character '{char}'. Must be one of {get_args(ValidChar)}")

# 示例使用
obj = MyClass()
print(f"{obj.char=}")

try:
    obj.set_char("B")
    print(f"{obj.char=}")
    obj.set_char("C") # 这将引发异常
    print(f"{obj.char=}") # 这行不会被执行
except InvalidCharError as e:
    print(f"Error: {e}")