Hi! I've met strange behaviour related to the coroutine execution. Probably it's somehow related to the asyncio.Queue get() method. I have the following lines of code:
class WeightSource:
    def __init__(self):
        self._queue = asyncio.Queue(maxsize=1)
    def __aiter__(self):
        return self
    async def __anext__(self):
        return await self._queue.get()

async def _preconfigure(app: web.Application) -> None:
    setup_db()
    # asyncio.create_task(WeightSource.listen_scales('/dev/ttyACM0', 9600)
    asyncio.create_task(handle_weight_event_creation()) # coroutine prints WEIGHT <int>
    await create_track_tasks()

async def create_track_tasks():
    asyncio.create_task(track_scales_availability())
    asyncio.create_task(track_crm_availability())

async def track_scales_availability():
    async for weight in WeightSource():
        print(weight)

When I'm trying to run _preconfigure coroutine (automatically started by aiohttp), i see the message: ERROR:asyncio:Task was destroyed but it is pending. The strange things two:
The process and loop remains alive (so by the logic error should be not shown) and the second thing is when I export PYTHONASYNCIODEBUG=1 everything works well without any error. 
On unset this variable the error returns. When I don't use asyncio.Queue and just place asyncio.sleep(), coroutine doesn't fall. Error happens inside the `async for weight in WeightSource()` statement. Why PYTHONASYNCIODEBUG changes behaviour?

I understand the issue as the following:

1. You don't save the result of `asyncio.create_task()` somewhere.
The task object is dereferenced and thus you see a warning.

2. With PYTHONASYNCIODEBUG on the task is referenced also by internal debug structure, that's why you don't see the warning.

Perhaps we should store a weak reference to task in _source_traceback

Minor thing but nice to have it fixed.

Thanks! That's resolved my problem, but this thing wasn't really obvious as for me. Probably it would be better to write about this explicitly in docs for the create_task. All in all this issue can be closed.

While filed later, issue44665 correctly describes the problem reported here: asyncio is behaving as expected, but the docs don't make it clear that you need to save a reference to the result of ensure_future()/create_task() if you don't want to risk the task unexpectedly disappearing mid execution.

So marking this as a duplicate of that docs issue.

I opened a documentation PR agreeing with @ncoghlan, that adds this behavior. See https://bugs.python.org/issue44665

I meant, "that describes the behaviour"