[New-bugs-announce] [issue36295] Need to yield (sleep(0)) twice in asyncio

[Assaf Dayan]

New submission from Assaf Dayan <asfdyn at gmail.com>:

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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<https://bugs.python.org/issue36295>
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