mypy结合overload与contextmanager实现文件打开器抽象类的类型报错如何解决

问题原因 #

• 重载签名上误加了@contextmanager装饰器：该装饰器仅需要作用于最终的方法实现，不需要标注在重载的桩函数上。
• 子类方法返回值类型与父类不匹配：@contextmanager装饰器会将返回Iterator[T]的生成器函数包装为返回ContextManager[T]的可调用对象，因此子类的方法签名需要和父类保持一致，使用ContextManager作为返回值类型，而非Iterator。
• 装饰器顺序问题：@abstractmethod需要放在最靠近方法定义的位置，避免装饰器执行顺序错误导致抽象方法识别异常。

修复后的代码 #

仅需要修改子类FileOpener的定义即可，完整可通过检查的代码如下：

from abc import ABC, abstractmethod
from contextlib import contextmanager
from typing import ContextManager, Iterator, Literal, Union, overload


class Reader:
    """A file reader."""
    def __init__(self, path: str) -> None:
        self.path = path


class Writer:
    """A file writer."""
    def __init__(self, path: str) -> None:
        self.path = path


class FileOpenerAbstract(ABC):

    @overload
    @abstractmethod
    def open(self,
             path: str,
             mode: Literal['r']
    ) -> ContextManager[Reader]:
        ...

    @overload
    @abstractmethod
    def open(self,
             path: str,
             mode: Literal['x', 'w', 'a']
    ) -> ContextManager[Writer]:
        ...

    @abstractmethod
    def open(
        self,
        path: str,
        mode: Literal['r', 'x', 'w', 'a'] = 'r'
    ) -> Union[ContextManager[Reader], ContextManager[Writer]]:
        """Open an archive"""


class FileOpener(FileOpenerAbstract):

    @overload
    def open(self,
             path: str,
             mode: Literal['r']
    ) -> ContextManager[Reader]:
        ...

    @overload
    def open(self,
             path: str,
             mode: Literal['x', 'w', 'a']
    ) -> ContextManager[Writer]:
        ...

    @contextmanager
    def open(
        self,
        path: str,
        mode: Literal['r', 'x', 'w', 'a'] = 'r',
    ) -> Union[ContextManager[Reader], ContextManager[Writer]]:
        if mode == 'r':
            yield Reader(path)
        else:
            yield Writer(path)

以上代码在Python 3.8 + mypy 0.910环境下可正常通过类型检查，同时Pylance、pylint均可正常解析。

内容的提问来源于stack exchange，提问作者Chris Sewell