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This PR is stale because it has been open for 30 days with no activity.

After investigation I think that actual problem is in asyncio.iscoroutine function.

From iscoroutine docs:

This method is different from inspect.iscoroutine() because it returns True for generator-based coroutines.

But actually this function returns true for generators that is not coroutines:

from asyncio import iscoroutine, run
from types import coroutine


@coroutine
def coro():
    yield


def not_coro():
    yield


print(iscoroutine(coro()))     # True
print(iscoroutine(not_coro())) # True

But if we will try to await not_coro() it will fail:

async def main():
    await coro()      # ok
    await not_coro()  # will fail with TypeError: object generator can't be used in 'await' expression

In my opinion asyncio.iscoroutine should check that generator is actually coroutine like inspect.isawaitable do:

@Fidget-Spinner What is your opinion regarding this problem?

Should we ping Yury and Andrew regarding this?

UPD: While I was playing with as_completed I have found interesting behevior:

def not_coro():
    yield


async def main():
    await gather(not_coro()) # will not fail
    await not_coro() # will fail

It looks like a bug, but I should do more investigation.

@uriyyo great investigation! That's a really interesting find.

IMO, changing the behavior of asyncio.iscoroutine is backwards incompatible. There is a StackOverflow question on this, and the linked bug report seems to me that Yury was already aware of this behavior. So we should probably stick to changing just as_completed.

I think we can ping Yury and Andrew politely on the bug tracker once OP adds tests like you mentioned.

BTW, I use asyncio, but you probably have more knowledge about asyncio internals than me :). So this is a learning experience for me too (ie I might make mistakes).

@Fidget-Spinner Ideal solution will be to change behavior of asyncio.iscoroutine to not return true for a generators that is not coroutines. It will fix this issue and will not allow to use non-coroutine generators with asyncio.gather and others.

For instance currently, code bellow will not fail:

await gather(None for _ in range(10))

When we have generator:

def not_coro():
    yield

It's not valid to await it:

await not_coro()  # TypeError: object generator can't be used in 'await' expression

But all cases below will not fail:

await create_task(not_coro())
await ensure_future(not_coro())
await gather(not_coro())
await wait([not_coro()])
await wait([not_coro()])
await wait_for(not_coro(), 10)
await shield(not_coro())

for f in as_completed([not_coro()]):
    print(await f)

We can fix it be changing asyncio.iscoroutine to:

_COROUTINE_TYPES = (types.CoroutineType, collections.abc.Coroutine)
_iscoroutine_typecache = set()


def iscoroutine(obj):
    """Return True if obj is a coroutine object."""
    if type(obj) in _iscoroutine_typecache:
        return True

    if isinstance(obj, _COROUTINE_TYPES):
        # Just in case we don't want to cache more than 100
        # positive types.  That shouldn't ever happen, unless
        # someone stressing the system on purpose.
        if len(_iscoroutine_typecache) < 100:
            _iscoroutine_typecache.add(type(obj))
        return True
    elif isinstance(obj, types.GeneratorType) and obj.gi_code.co_flags & inspect.CO_ITERABLE_COROUTINE:
        return True
    else:
        return False

But this solution is not backward compatible(

Just ran into this issue myself and managed to find this PR. Looking forward to it being merged.