>>> from typing import *
>>> @runtime_checkable
... class Foo(Protocol):
...     x = 1
...     @classmethod
...     def __subclasshook__(cls, other):
...         return hasattr(other, 'x')
...
>>> issubclass(object, Foo)
False

>>> from typing import *
>>> @runtime_checkable
... class Foo(Protocol):
...     x = 1
...     @classmethod
...     def __subclasshook__(cls, other):
...         return hsattr(cls, 'x')
...
>>> issubclass(object, Foo)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1829, in __subclasscheck__
    raise TypeError(
TypeError: Protocols with non-method members don't support issubclass()