Make `Protocol`s inheritable.

[jymchng]

Feature or enhancement

Proposal:

Currently, typing.runtime_checkable does not work with 'Protocols' that were inherited from typing.Protocol.

Consider:

from typing import Protocol, runtime_checkable

@runtime_checkable
class Reader(Protocol):

    def read(self, source: object):
        ...

@runtime_checkable
class Writer(Protocol):

    def write(self, output: object):
        ...

@runtime_checkable
class ReaderWriter(Reader, Writer):
    ...

Playground

Writing the following does make it work:

from typing import _ProtocolMeta, TypeVar, Generic, runtime_checkable

_T = TypeVar("_T")

@runtime_checkable
class BaseProtocol(Generic[_T], metaclass=_ProtocolMeta):

    _is_protocol = True  

@runtime_checkable
class Reader(BaseProtocol):

    def read(self, source: object):
        ...

@runtime_checkable
class Writer(BaseProtocol):

    def write(self, output: object):
        ...

@runtime_checkable
class ReaderWriter(Reader, Writer):
    ...

Playground

Very often, we might want to inherit from protocols and run runtime checks on them. However, this workaround of setting _is_protocol = True on the base class does not seem reliable.

Any plans to consider having @runtime_checkable to work on subclasses of Protocol?

Thank you.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

[JelleZijlstra]

This is correct and will not change. Only classes that inherit directly from Protocol are protocols; this is essential because you can inherit from a protocol class in a concrete class to implement that protocol.