Accept generator created in Cython

[methane]

Problem was reported here: https://groups.google.com/forum/#!topic/cython-users/g146SZHxRyM
I think supporting Cython is very important for asyncio performance.

[gvanrossum]

Agreed, I would gladly accept patches. I'm not a Cython user myself so I can't fix this without help.

[ludovic-gasc]

Hi @methane, thank you for the link.
On the mailing-list, Stefan seems to find a solution: cython/cython@4af4244
I've played my role of messenger: I've invited him to open a pull request.

[scoder]

I had already created a CPython ticket for this:

https://bugs.python.org/issue24004

Here's what Cython currently (as in "since yesterday") does in order to circumvent the type checks in asyncio and inspect:

https://github.com/cython/cython/blob/4af42443bd37f6207143f8870904f780a65bb3e3/Cython/Utility/Generator.c#L824

https://github.com/cython/cython/blob/4af42443bd37f6207143f8870904f780a65bb3e3/Cython/Utility/Generator.c#L906

It's not clear to me what the best fix is here, but my gut feeling tells me that the correct way to deal with this is to create a Generator ABC and do the type checking based on that. A "Generator" is an object that implements the Generator protocol, not necessarily something of type types.GeneratorType. Special casing that type is fine, obviously, but excluding everything else isn't.

The fact that this crash bug here went unnoticed since Py3.3 makes it obvious, though, that no-one has ever tried this at the C level:

https://bugs.python.org/issue23996

[methane]

I think adding Coroutine ABC to somewhere is good way to solve this problem.
It may be in the stdlib. But I think we should wait discussion of async/await syntax.

For now, how about asyncio.coroutines.Coroutine ?

from abc import ABCMeta, abstractmethod
import types


class Coroutine(metaclass=ABCMeta):
    def __iter__(self):
        return self

    def __next__(self):
        raise StopIteration

    @abstractmethod
    def send(self):
        raise StopIteration


Coroutine.register(types.GeneratorType)
Coroutine.register(CoroWrapper)


def iscoroutine(obj):
    """Return True if obj is a coroutine object."""
    return isinstance(obj, Coroutine)

[1st1]

@scoder You can now use collections.abc.Coroutine for your purposes (3a09a93). Closing this issue.