bpo-44176: Allow asyncio.as_completed()'s first parameter to be a generator yielding awaitables

[alexdelorenzo]

Allow asyncio.as_completed()'s first parameter to be a generator yielding awaitables

bpo-44176: asyncio.as_completed() raises TypeError when the first supplied parameter is a generator that yields awaitables

https://bugs.python.org/issue44176

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[github-actions]

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

[uriyyo]

LGTM. Could you please also add tests to cover case when you pass generator to asyncio.as_completed and asyncio.wait function call?

[uriyyo]

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

From iscoroutinedocs:

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:

fromasyncioimportiscoroutine, runfromtypesimportcoroutine@coroutinedefcoro(): yielddefnot_coro(): yieldprint(iscoroutine(coro())) # Trueprint(iscoroutine(not_coro())) # True

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

asyncdefmain(): awaitcoro() # okawaitnot_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:

cpython/Lib/inspect.py

Lines 351 to 352 in 863154c

	isinstance(object, types.GeneratorType) and
	bool(object.gi_code.co_flags&CO_ITERABLE_COROUTINE) or

@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:

defnot_coro(): yieldasyncdefmain(): awaitgather(not_coro()) # will not failawaitnot_coro() # will fail

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

[Fidget-Spinner]

@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).

[uriyyo]

@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:

awaitgather(Nonefor_inrange(10))

When we have generator:

defnot_coro(): yield

It's not valid to await it:

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

But all cases below will not fail:

awaitcreate_task(not_coro()) awaitensure_future(not_coro()) awaitgather(not_coro()) awaitwait([not_coro()]) awaitwait([not_coro()]) awaitwait_for(not_coro(), 10) awaitshield(not_coro()) forfinas_completed([not_coro()]): print(awaitf)

We can fix it be changing asyncio.iscoroutine to:

_COROUTINE_TYPES= (types.CoroutineType, collections.abc.Coroutine) _iscoroutine_typecache=set() defiscoroutine(obj): """Return True if obj is a coroutine object."""iftype(obj) in_iscoroutine_typecache: returnTrueifisinstance(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.iflen(_iscoroutine_typecache) <100: _iscoroutine_typecache.add(type(obj)) returnTrueelifisinstance(obj, types.GeneratorType) andobj.gi_code.co_flags&inspect.CO_ITERABLE_COROUTINE: returnTrueelse: returnFalse

But this solution is not backward compatible(

[DevilXD]

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