你如何测试一个 python 类型的协议是另一个协议的子类？答案

【问题标题】：How can you test that a python typing Protocol is a subclass of another Protocol?你如何测试一个 python 类型的协议是另一个协议的子类？
【发布时间】：2020-04-15 02:50:22
【问题描述】：

该问题的明显解决方案是使用issubclass，但这会引发TypeError（使用Python 3.6.7），例如

>>> from typing_extensions import Protocol
>>> class ProtoSubclass(Protocol):
...     pass
... 
>>> issubclass(ProtoSubclass, Protocol)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/conda/envs/airflow/lib/python3.6/site-packages/typing_extensions.py", line 1265, in __subclasscheck__
    raise TypeError("Instance and class checks can only be used with"
TypeError: Instance and class checks can only be used with @runtime protocols
>>> from typing_extensions import runtime
>>> @runtime
... class ProtoSubclass(Protocol):
...     pass
... 
>>> issubclass(ProtoSubclass, Protocol)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/conda/envs/airflow/lib/python3.6/site-packages/typing_extensions.py", line 1265, in __subclasscheck__
    raise TypeError("Instance and class checks can only be used with"
TypeError: Instance and class checks can only be used with @runtime protocols

【问题讨论】：

标签： python inheritance protocols typing

【解决方案1】：

选择的答案虽然没有错，但并不能反映Protocol 的实际用途，它提供了结构子类型。 Python 3 始终可以使用名义子类型。

from typing import Protocol, runtime_checkable

@runtime_checkable
class Closable(Protocol):
    def close(self):
        ...

class AnyClass:
    def close(self):
        ...

issubclass(AnyClass, Closable)
#> True

此外，runtime_checkable 仅在基类中需要，即使被检查的类是 Protocol 子类。

class ExtendedProtocol(Closable, Protocol):
    ...

class AnotherProtocol(Protocol):
    def close(self):
        ...

class NotAProtocol(Closable):
    # just inherits the protocol's default implementation
    ...

issubclass(ExtendedProtocol, Closable)
#> True
issubclass(AnotherProtocol, Closable)
#> True
issubclass(NotAProtocol, Closable)
#> True

【讨论】：

【解决方案2】：

有关 python Protocols 的更多信息，请参阅

https://mypy.readthedocs.io/en/latest/protocols.html#using-isinstance-with-protocols

在 Python 3.6.7 中，解决此问题的一种方法是使用 @runtime_checkable 装饰器：

>>> from typing_extensions import Protocol
>>> from typing_extensions import runtime_checkable

>>> @runtime_checkable
... class CustomProtocol(Protocol):
...     def custom(self):
...         ...
... 

>>> @runtime_checkable
... class ExtendedCustomProtocol(CustomProtocol, Protocol):
...     def extended(self):
...         ...
... 

>>> issubclass(ExtendedCustomProtocol, CustomProtocol)
True

【讨论】：