[issue40466] asyncio.ensure_future() breaks implicit exception chaining

[Chris Jerdonek]

New submission from Chris Jerdonek <chris.jerdonek at gmail.com>:

This issue is about how if a coroutine is wrapped in a Task with asyncio.ensure_future(), then portions of the exception chain can be lost.

Specifically, if you run the following code, then ValueError will be raised with exc.__context__ equal to the KeyError:

    import asyncio

    async def raise_error():
        raise ValueError

    async def main():
        try:
            raise KeyError
        except Exception as exc:
            future = raise_error()
            # Uncommenting the next line makes exc.__context__ None below.
            # future = asyncio.ensure_future(future)

            try:
                await future
            except Exception as exc:
                print(f'error: {exc!r}, context: {exc.__context__!r}')
                raise

    asyncio.get_event_loop().run_until_complete(main())

However, if you uncomment the `asyncio.ensure_future()` line, then the ValueError will be raised with no __context__.

I originally raised this issue a couple years ago here:
https://mail.python.org/pipermail/async-sig/2017-November/000403.html

There it was suggested that this was a special case of this issue:
https://bugs.python.org/issue29587

However, after writing code to fix that, this issue still exists.

----------
components: asyncio
messages: 367832
nosy: asvetlov, chris.jerdonek, yselivanov
priority: normal
severity: normal
status: open
title: asyncio.ensure_future() breaks implicit exception chaining
type: behavior
versions: Python 3.6, Python 3.7, Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue40466>
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