Document optional 'task'/'asyncgen' fields in call_exception_handler

[ShaneHarvey]

This change documents the optional "task" field in call_exception_handler. For example I noticed this context while working on Motor:

>>> context
{'message': 'Task was destroyed but it is pending!', 'task': <Task pending name='Task-2' coro=<TestAsyncIOCursor.test_cancelled_error_fetch_next_aggregate() running at /Users/shane/git/motor/test/asyncio_tests/test_asyncio_cursor.py:261> wait_for=<Future finished result={'$clusterTime': {'clusterTime': Timestamp(1596567456, 202), 'signature': {'hash': b'\x00\x00\x0...0\x00\x00\x00', 'keyId': 0}}, 'count': 7, 'ok': 1.0, 'operationTime': Timestamp(1596567456, 202)} created at /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/asyncio/base_events.py:418> created at /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/asyncio/tasks.py:470>, 'source_traceback': [<FrameSummary file setup.py, line 128 in <module>>, <FrameSummary file /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/site-packages/setuptools/__init__.py, line 145 in setup>, <FrameSummary file /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/distutils/core.py, line 148 in setup>, <FrameSummary file /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/distutils/dist.py, line 966 in run_commands>, <FrameSummary file /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/distutils/dist.py, line 985 in run_command>, <FrameSummary file setup.py, line 120 in run>, <FrameSummary file /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/unittest/runner.py, line 176 in run>, <FrameSummary file /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/unittest/suite.py, line 84 in __call__>, <FrameSummary file /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/unittest/suite.py, line 122 in run>, <FrameSummary file /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/unittest/case.py, line 736 in __call__>, <FrameSummary file /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/unittest/case.py, line 676 in run>, <FrameSummary file /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/unittest/case.py, line 633 in _callTestMethod>, <FrameSummary file /Users/shane/git/motor/test/asyncio_tests/__init__.py, line 48 in __call__>, <FrameSummary file /Users/shane/git/motor/test/test_environment.py, line 283 in wrap>, <FrameSummary file /Users/shane/git/motor/test/asyncio_tests/__init__.py, line 214 in wrapped>, <FrameSummary file /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/asyncio/base_events.py, line 595 in run_until_complete>, <FrameSummary file /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/asyncio/base_events.py, line 563 in run_forever>, <FrameSummary file /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/asyncio/base_events.py, line 1836 in _run_once>, <FrameSummary file /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/asyncio/events.py, line 81 in _run>, <FrameSummary file /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/asyncio/tasks.py, line 470 in wait_for>]}

Most PRs will require an issue number. Trivial changes, like fixing a typo, do not need an issue.

This change seems trivial enough to not need an issue. Let me know if you disagree and I'll create one.

[rougeth]

Hey @ShaneHarvey, thank you for your PR. There's another key missing at the docs, would you mind adding it as well?

cpython/Lib/asyncio/base_events.py

Lines 1740 to 1755 in 374d998

	def call_exception_handler(self, context):
	"""Call the current event loop's exception handler.
	The context argument is a dict containing the following keys:
	- 'message': Error message;
	- 'exception' (optional): Exception object;
	- 'future' (optional): Future instance;
	- 'task' (optional): Task instance;
	- 'handle' (optional): Handle instance;
	- 'protocol' (optional): Protocol instance;
	- 'transport' (optional): Transport instance;
	- 'socket' (optional): Socket instance;
	- 'asyncgen' (optional): Asynchronous generator that caused
	the exception.

[ShaneHarvey]

@rougeth thanks for the the quick review. Updated to include the asyncgen field as well.

[bedevere-bot]

@asvetlov: Please replace # with GH- in the commit message next time. Thanks!

[asvetlov]

Thanks!

[graingert]

this needs a backport to python3.6

[ShaneHarvey]

@graingert 3.6 is EOL so it will not be backported: https://devguide.python.org/devcycle/#end-of-life-branches

[graingert]

@graingert 3.6 is EOL so it will not be backported: https://devguide.python.org/devcycle/#end-of-life-branches

Ah sorry I mean 3.6-3.9 are missing this change - so it should be backported to 3.9

[ShaneHarvey]

Oh that's a good point. I think a CPython maintainer just needs to add the "needs backport to 3.7/8/9" labels. Either that or one of us can create the backport PR manually.

[AlexWaygood]

Oh that's a good point. I think a CPython maintainer just needs to add the "needs backport to 3.7/8/9" labels. Either that or one of us can create the backport PR manually.

3.7 & 3.8 are only accepting security-related patches. But 3.9 is still accepting bugfixes and docs patches :)

[miss-islington]

Thanks @ShaneHarvey for the PR, and @asvetlov for merging it 🌮🎉.. I'm working now to backport this PR to: 3.9.
🐍🍒⛏🤖

[bedevere-bot]

GH-30727 is a backport of this pull request to the 3.9 branch.