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Summary

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CPython will automatically shutdown the event loop and cancel all background async tasks when the main task exits from asyncio.run(...). This is especially important if the background tasks need to perform some sort of cleanup and/or reset.

This adds support for asyncio.all_tasks() which returns a set object similar to the way CPython works and allows inspection of all running tasks. This may be used inside a running event loop to forcibly cancel all running tasks.

Testing

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This adds two tests in hopes of measuring compatibility with CPython. The first is asyncio_all_tasks which shows the collection of tasks returned from the asyncio.all_tasks() function. The second is asyncio_shutdown which shows the compatibility of shutting down background tasks as the main task exits for various reasons.

Trade-offs and Alternatives

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The spirit of this feature was originally implemented in #9870; however, it needs this concept to fully work correctly.

Micropython still suffers from the issue where a try block in the main task will not have its finally executed if it's interrupted from a KeyboardInterrupted (like eg. from mpremote); however, at least this will trigger the proper cancel and cleanup of background tasks.

As a caveat the Loop in Micropython can be reused after being close()d. In CPython, such is forbidden. CPython uses a Runner to manage the shutdown of the loop and tasks after the main task exits.