[issue36295] Need to yield (sleep(0)) twice in asyncio

[Andrew Svetlov]

Andrew Svetlov  added the comment:

In asyncio `await asyncio.sleep(0)` is for switching execution context from the 
current task to other code. 
There is no guarantee for finishing already running tasks before returning from 
`asyncio.sleep(0)` call etc.

Also, your code snippet has a logical error. It should not call regular 
`time.sleep(123)` from a coroutine but use `loop.run_in_executor()` for 
performing blocking calls

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[issue36295] Need to yield (sleep(0)) twice in asyncio

[Zachary Ware]

Change by Zachary Ware :


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components: +asyncio
nosy: +asvetlov, yselivanov

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[issue36295] Need to yield (sleep(0)) twice in asyncio

[Assaf Dayan]

New submission from Assaf Dayan :

In asyncio, 'await asyncio.sleep(0)' is used to relinquish control and check 
for finished awaitables before continuing. 
In my example, an awaitable returns but is only handled if 'await 
asyncio.sleep(0)' is called twice (adjacently) and only then the returned 
'await' statements are handled correctly.

The attached script generates tasks which do the following: compute for 0.8 
seconds; do an async op for 0.5 seconds; and compute some more for 0.2 seconds.
Before generating each new task, we relinquish control so that existing pending 
tasks will resume operation, if their 'await' statements returned.
We simulate a business-case in which we give precedence to already-processing 
tasks over new ones.

When running the attached script, Task 1 goes async at d=0.8 and should be 
ready by d=1.3 (asyncio.sleep(0.5)). Because Task 2 started at d=0.8 (when T1 
went async) it will relinquish control at d=1.6. By relinquishing control at 
d=1.6, Task 1 should resume, but it doesn't, unless asyncio.sleep(0) is called 
once more.
If asyncio.sleep(0) is not called again (as a fix), Task 1 will only resume at 
d=2.4, which is after Task 3 relinquished control.

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files: async_yield.py
messages: 337950
nosy: Assaf Dayan
priority: normal
severity: normal
status: open
title: Need to yield (sleep(0)) twice in asyncio
type: behavior
versions: Python 3.7
Added file: https://bugs.python.org/file48207/async_yield.py

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