asyncio.ensure_future() breaks implicit exception chaining

[cjerdonek]

BPO	40466
Nosy	@asvetlov, @cjerdonek, @1st1
PRs	bpo-29587: Enable exception chaining for gen.throw() with "yield from" #19858

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-05-06.10:36:11.392>
created_at = <Date 2020-05-01.09:57:11.661>
labels = ['3.8', 'type-bug', '3.7', '3.9', 'expert-asyncio']
title = 'asyncio.ensure_future() breaks implicit exception chaining'
updated_at = <Date 2020-05-06.10:36:11.391>
user = 'https://github.com/cjerdonek'

bugs.python.org fields:

activity = <Date 2020-05-06.10:36:11.391>
actor = 'chris.jerdonek'
assignee = 'none'
closed = True
closed_date = <Date 2020-05-06.10:36:11.392>
closer = 'chris.jerdonek'
components = ['asyncio']
creation = <Date 2020-05-01.09:57:11.661>
creator = 'chris.jerdonek'
dependencies = []
files = []
hgrepos = []
issue_num = 40466
keywords = ['patch']
message_count = 3.0
messages = ['367832', '367925', '368232']
nosy_count = 3.0
nosy_names = ['asvetlov', 'chris.jerdonek', 'yselivanov']
pr_nums = ['19858']
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40466'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9']

[cjerdonek]

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.

[cjerdonek]

My PR is ready for review: #19858

[cjerdonek]

Closing this as a duplicate of https://bugs.python.org/issue29587

It turns out that, as Nathaniel first suggested, this is really just another special case of that issue (the "yield from" case as opposed to the "yield" case). It's just that the particular examples provided in that issue were for "yield" and not "yield from", so it wasn't immediately evident.