Python3.9下用Union声明类型时mypy报不兼容错误

原始代码 #

from enum import Enum
from typing import Dict, Union, Type

class MyEnum(Enum):
    FIRST = 1, 'first'
    SECOND = 2 , 'second'

__myenum_int_ref: Dict[int, MyEnum] = {k.value[0]: k for k in MyEnum}
__myenum_str_ref: Dict[str, MyEnum] = {k.value[1]: k for k in MyEnum}
__myenum_str_ref.update({k.name: k for k in MyEnum})

__flags_ref: Dict[Union[Type[str], Type[int]], Dict[Union[str, int], MyEnum]] = {
    str: __myenum_str_ref,
    int: __myenum_int_ref,
}

Mypy报错信息 #

function.py:14: error: Dict entry 0 has incompatible type "Type[str]": "Dict[str, MyEnum]"; expected "Union[Type[str], Type[int]]": "Dict[Union[str, int], MyEnum]"
function.py:15: error: Dict entry 1 has incompatible type "Type[int]": "Dict[int, MyEnum]"; expected "Union[Type[str], Type[int]]": "Dict[Union[str, int], MyEnum]"

问题原因 #

Mypy对字典类型的协变检查严格：Dict[str, MyEnum]并非Dict[Union[str, int], MyEnum]的子类型——前者键只能是str，后者允许str或int，类型约束不匹配。即便你明确知道str键对应的值只会接收str键的字典，Mypy默认也不会做上下文推断。

解决方案 #

方案1：用TypedDict精确映射结构（最优解）

通过TypedDict明确指定每个键对应的字典类型，类型最严谨：

from enum import Enum
from typing import Dict, Type, TypedDict

class MyEnum(Enum):
    FIRST = 1, 'first'
    SECOND = 2 , 'second'

__myenum_int_ref: Dict[int, MyEnum] = {k.value[0]: k for k in MyEnum}
__myenum_str_ref: Dict[str, MyEnum] = {k.value[1]: k for k in MyEnum}
__myenum_str_ref.update({k.name: k for k in MyEnum})

class FlagsRef(TypedDict):
    str: Dict[str, MyEnum]
    int: Dict[int, MyEnum]

__flags_ref: FlagsRef = {
    str: __myenum_str_ref,
    int: __myenum_int_ref,
}

方案2：用cast做类型断言

不修改原有结构，手动告诉Mypy类型匹配：

from enum import Enum
from typing import Dict, Union, Type, cast

class MyEnum(Enum):
    FIRST = 1, 'first'
    SECOND = 2 , 'second'

__myenum_int_ref: Dict[int, MyEnum] = {k.value[0]: k for k in MyEnum}
__myenum_str_ref: Dict[str, MyEnum] = {k.value[1]: k for k in MyEnum}
__myenum_str_ref.update({k.name: k for k in MyEnum})

__flags_ref: Dict[Union[Type[str], Type[int]], Dict[Union[str, int], MyEnum]] = {
    str: cast(Dict[Union[str, int], MyEnum], __myenum_str_ref),
    int: cast(Dict[Union[str, int], MyEnum], __myenum_int_ref),
}

方案3：调整子字典类型注解

直接将子字典声明为Dict[Union[str, int], MyEnum]，牺牲子字典的类型精确性换兼容性：

from enum import Enum
from typing import Dict, Union, Type

class MyEnum(Enum):
    FIRST = 1, 'first'
    SECOND = 2 , 'second'

__myenum_int_ref: Dict[Union[str, int], MyEnum] = {k.value[0]: k for k in MyEnum}
__myenum_str_ref: Dict[Union[str, int], MyEnum] = {k.value[1]: k for k in MyEnum}
__myenum_str_ref.update({k.name: k for k in MyEnum})

__flags_ref: Dict[Union[Type[str], Type[int]], Dict[Union[str, int], MyEnum]] = {
    str: __myenum_str_ref,
    int: __myenum_int_ref,
}

内容的提问来源于stack exchange，提问作者Lucas Coppio