如何用mypy安全检查Protocol子类的Python函数签名？

当定义带有特定方法签名的Protocol时，隐式实现该协议的类会被mypy正确检测出签名不兼容问题，但显式继承该Protocol的类即使方法签名不匹配，mypy默认也不会报错，具体场景如下：

隐式实现的情况（mypy正确报错） #

# protocols.py
from abc import abstractmethod
from dataclasses import dataclass
from typing import Protocol

class SupportsPublish(Protocol):
    @abstractmethod
    def publish(self, topic: str):
        ...

def publish(m: SupportsPublish):
    m.publish("topic")

@dataclass
class Publishable:
    foo: str = "bar"

    def publish(self):
        print(self)

publish(Publishable())

运行mypy protocols.py会得到错误：

protocols.py:24: error: Argument 1 to "publish" has incompatible type "Publishable"; expected "SupportsPublish" [arg-type]
protocols.py:24: note: Following member(s) of "Publishable" have conflicts:
protocols.py:24: note: Expected:
protocols.py:24: note: def publish(self, topic: str) -> Any
protocols.py:24: note: Got:
protocols.py:24: note: def publish(self) -> Any
Found 1 error in 1 file (checked 1 source file)

显式继承的情况（mypy默认不报错） #

将Publishable改为显式继承SupportsPublish：

@dataclass
class Publishable(SupportsPublish):
    foo: str = "bar"

    def publish(self):
        print(self)

运行mypy protocols.py会提示Success: no issues found in 1 source file，但这不符合PEP 0544中类型检查器应验证显式实现正确性的要求。

解决方案 #

1. 启用mypy的协议成员检查选项 #

mypy默认关闭了对显式继承Protocol的类的成员签名检查，只需启用--protocol-member-checking选项（或包含该选项的--strict模式）即可触发检查：

mypy --protocol-member-checking protocols.py

此时mypy会正确检测出签名不匹配的错误，输出与隐式实现时一致的报错信息。

2. 结合运行时检查（可选） #

若需要运行时也能验证类是否符合Protocol，可以给SupportsPublish添加@runtime_checkable装饰器：

from typing import Protocol, runtime_checkable

@runtime_checkable
class SupportsPublish(Protocol):
    @abstractmethod
    def publish(self, topic: str):
        ...

之后可以用isinstance(Publishable(), SupportsPublish)在运行时检查，但注意运行时检查仅验证方法是否存在，无法检测签名兼容性，仍需配合静态类型检查确保签名正确。

内容的提问来源于stack exchange，提问作者Andrew Stewart