我有如下代码

from typing import Protocol, runtime_checkable

@runtime_checkable
class A(Protocol):
    def f(self): ...

class B:
    def f(self): ...


if __name__ == '__main__':
    assert isinstance(B(), A)

如果我移除runtime_checkable装饰器，会得到如下异常：

TypeError: Instance and class checks can only be used with @runtime_checkable protocols

然而，如果我为类A 增加一个属性，只要我使用的是Python 3.9或更早版本，就不需要这个装饰器。

class A(Protocol):
    x: int
    def f(self): ...

结果：

>>> assert isinstance(B(), A)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError

包含至少一个非方法成员的协议被称为数据协议（typing.python.org）。为什么数据协议不需要runtime_checkable装饰器？我在文档中找不到相关内容。

没有属性时（3.9）：

Python 3.9.13 (tags/v3.9.13:6de2ca5, May 17 2022, 16:36:42) [MSC v.1929 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import Protocol
>>> class A(Protocol):
...     def f(self): ...
...
>>> class B:
...     def f(self): ...
...
>>> assert isinstance(B(), A)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.9_3.9.3568.0_x64__qbz5n2kfra8p0\lib\typing.py", line 1146, in __instancecheck__
    issubclass(instance.__class__, cls)):
  File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.9_3.9.3568.0_x64__qbz5n2kfra8p0\lib\abc.py", line 123, in __subclasscheck__
    return _abc_subclasscheck(cls, subclass)
  File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.9_3.9.3568.0_x64__qbz5n2kfra8p0\lib\typing.py", line 1208, in _proto_hook
    raise TypeError("Instance and class checks can only be used with"
TypeError: Instance and class checks can only be used with @runtime_checkable protocols

有属性时（3.9）：

Python 3.9.13 (tags/v3.9.13:6de2ca5, May 17 2022, 16:36:42) [MSC v.1929 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import Protocol
>>> class A(Protocol):
...     x: int
...     def f(self): ...
...
>>> class B:
...     def f(self): ...
...
>>> assert isinstance(B(), A)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError

使用Python版本 ≥ 3.10时，我得到TypeError：

Python 3.10.11 (tags/v3.10.11:7d4cc5a, Apr  5 2023, 00:38:17) [MSC v.1929 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import Protocol
>>> class A(Protocol):
...     x: int
...     def f(self): ...
...
>>> class B:
...     def f(self): ...
...
>>> assert isinstance(B(), A)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.10_3.10.3056.0_x64__qbz5n2kfra8p0\lib\typing.py", line 1498, in __instancecheck__
    raise TypeError("Instance and class checks can only be used with"
TypeError: Instance and class checks can only be used with @runtime_checkable protocols

解决方案

恭喜，你刚刚重新发现了Python的 typing模块中的一个六年前的bug。它在Python 3.9及更新版本中被修复，但随后在3.9中被回滚，因为该变更与补丁版本的语义化版本号要求不兼容。

你可以在 这个GitHub问题 中追踪这个bug的历史，它在3.10的发行说明中也有提及（这里的第10点）。

Python 3.9已正式退休（即EOL），所以这个修复不会回移植到3.9或更早的版本。目前受支持的Python版本都不受此bug影响。