How can you test that a python typing Protocol is a subclass of another Protocol?

The obvious solution to the problem is to use issubclass, but this raises TypeError (using Python 3.6.7), e.g.

>>> 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

Answer

For more on the topic of python Protocols, see

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

In Python 3.6.7, one way to solve this is to use the @runtime_checkable decorator:

>>> 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