I was searching for a way to "yield" from task/coroutinue back to the event loop (not yielding a value in terms of a generator) and not finding anything documented, I found this bug report and PR: 
   https://github.com/python/asyncio/issues/284

It states that asyncio.sleep(0) should cause the coroutine to send control back to the event loop without wastefully doing other work.  That makes sense and a perfectly good way to do that.

And, the code appears to handle a value of <= 0 specifically: https://github.com/python/cpython/blob/3.8/Lib/asyncio/tasks.py#L632

However, using sleep(0) to yield back does not cause it to raise a CancelledError if the task has been cancelled as cancel()'s documentation indicates it should.  But sleeping for anything >0 does (e.g. 0.001)

The below code snippet will demonstrate the problem:

TIA

----

import asyncio
import time

async def cancel_me():
    print('cancel_me(): before sleep')

    try:
        while True:
            print("doing some really intensive cpu stuff")
            time.sleep(2)

            # now I want to yield control back to the event loop in order to determine if we've been cancelled 
            await asyncio.sleep(0) # I'm expecting this to throw CancelledError, but it never does.. it DOES throw if the delay > 0 (e.g. 0.001)

    except asyncio.CancelledError:
        print('cancel_me(): cancelled!')
        raise



async def main():
    task = asyncio.create_task(cancel_me())

    await asyncio.sleep(1)

    task.cancel()
    try:
        await task
    except asyncio.CancelledError:
        print("main(): cancel_me is cancelled now")

asyncio.run(main())

It relinquishes control for exactly one event loop cycle.

You'll see that it takes two or three cycles and then gets cancelled correctly.

The reason being is that recovery from your `await asyncio.sleep(1)` takes two or three cycles. (Under the hood, a few `asyncio.call_soon()` are called in a chain and only 1 is executed each cycle).

This is not a bug, but rather intended behavior. You should probably use `asyncio.to_thread()` or preferably run cpu-intensive code on a different process.

@triage Please close as not-a-bug.

Agree, not a bug.