[FIXED] When will/won't Python suspend execution of a coroutine?

Issue

When I run it on cpython 3.6, the following program prints hello world a single time and then spins forever.

As a side note, uncommenting the await asyncio.sleep(0) line causes it to print hello world every second, which is understandable.

import asyncio

async def do_nothing():
    # await asyncio.sleep(0)
    pass

async def hog_the_event_loop():
    while True:
        await do_nothing()

async def timer_print():
    while True:
        print("hello world")
        await asyncio.sleep(1)

loop = asyncio.get_event_loop()
loop.create_task(timer_print())
loop.create_task(hog_the_event_loop())
loop.run_forever()

This behavior (printing hello world a single time) makes sense to me, because hog_the_event_loop never blocks and therefore has no need to suspend execution. Can I rely on this behavior? When the line await do_nothing() runs, is it possible that rather than entering the do_nothing() coroutine, execution will actually suspend and resume timer_print(), causing the program to print hello world a second time?

Put more generally: When will python suspend execution of a coroutine and switch to another one? Is it potentially on any use of the await keyword? or is it only in cases where this results in an underlying select call (such as I/O, sleep timers, etc)?

Additional Clarification

I understand that if hog_the_event_loop looked like this, it would certainly never yield execution to another coroutine:

async def hog_the_event_loop():
    while True:
        pass

I'm trying to specifically get at the question of whether await do_nothing() is any different than the above.

Solution

This behavior (printing hello world a single time) makes sense to me, because hog_the_event_loop never blocks and therefore has no need to suspend execution. Can I rely on this behavior?

Yes. This behavior directly follows from how await is both specified and implemented by the language. Changing it to suspend without the awaitable object having itself suspended would certainly be a breaking change, both for asyncio and other Python libraries based in async/await.

Put more generally: When will python suspend execution of a coroutine and switch to another one? Is it potentially on any use of the await keyword?

From the caller's perspective, any await can potentially suspend, at the discretion of the awaited object, also known as an awaitable. So the final decision of whether a particular await will suspend is on the awaitable (or on awaitables it awaits itself if it's a coroutine, and so on). Awaiting an awaitable that doesn't choose to suspend is the same as running ordinary Python code - it won't pass control to the event loop.

The leaf awaitables that actually suspend are typically related to IO readiness or timeout events, but not always. For example, awaiting a queue item will suspend if the queue is empty, and awaiting run_in_executor will suspend until the function running in the other thread completes.

It is worth mentioning that asyncio.sleep is explicitly guaranteed to suspend execution and defer to the event loop, even when the specified delay is 0 (in which case it will immediately resume on the next event loop pass).

Answered By - user4815162342