gh-95882: fix regression in the traceback of exceptions propagated from inside a contextlib context manager

[graingert]

…lib.asynccontextmanager

• Issue: Trackstack includes __aexit__ in contextlib which is not in previous versions #95882

[iritkatriel]

Some tests are failing.

[graingert]

Oh dear, missing import. I probably need to port more tests over too I also have this as a drive by fix https://github.com/python/cpython/pull/95888/files to save a bit of __context__ mutation and import time

[graingert]

needs a test for this case

[graingert]

@iritkatriel it doesn't look like this line does anything. If I delete it I get the same behavior, same with L176:

cpython/Lib/contextlib.py

Lines 166 to 177 in adf2bf3

	# Avoid suppressing if a StopIteration exception
	# was passed to throw() and later wrapped into a RuntimeError
	# (see PEP 479 for sync generators; async generators also
	# have this behavior). But do this only if the exception wrapped
	# by the RuntimeError is actually Stop(Async)Iteration (see
	# issue29692).
	if (
	isinstance(value, StopIteration)
	and exc.__cause__ is value
	):
	exc.__traceback__ = traceback
	return False

[graingert]

@iritkatriel do you expect to see this yield here?

[iritkatriel]

What does 3.10 do?

[graingert]

the yield is new, incorrect, behavior in 3.11:

import contextlib
import logging

logger = logging.getLogger("__name__")

@contextlib.contextmanager
def f():
    yield


try:
    with f():
        raise StopIteration
except StopIteration:
    logger.exception("stop iteration")



print("================ overwriting StopIteration via globals!==============")


class StopIteration(Exception):
    pass


try:
    with f():
        raise StopIteration
except StopIteration:
    logger.exception("stop iteration")

 graingert@conscientious  testing311  ~/projects/trio   master ± python3.10 demo.py
stop iteration
Traceback (most recent call last):
  File "/home/graingert/projects/trio/demo.py", line 13, in <module>
    raise StopIteration
StopIteration
================ overwriting StopIteration via globals!==============
stop iteration
Traceback (most recent call last):
  File "/home/graingert/projects/trio/demo.py", line 28, in <module>
    raise StopIteration
StopIteration

Notice the extra yield in the first run before overwriting StopIteration via globals happens:

 graingert@conscientious  testing311  ~/projects/trio   master ± python3.11 demo.py 
stop iteration
Traceback (most recent call last):
  File "/home/graingert/projects/trio/demo.py", line 8, in f
    yield
  File "/home/graingert/projects/trio/demo.py", line 13, in <module>
    raise StopIteration
StopIteration
================ overwriting StopIteration via globals!==============
stop iteration
Traceback (most recent call last):
  File "/home/graingert/projects/trio/demo.py", line 28, in <module>
    raise StopIteration
StopIteration

[graingert]

fixed it!

[iritkatriel]

Could you make a PR against 3.10 to backport only the new tests, so we can make sure this is not changing anything?

[graingert]

Could you make a PR against 3.10 to backport only the new tests, so we can make sure this is not changing anything?

Yeah, I'll see how far I can backport it in contextlib2 too

[graingert]

@iritkatriel can you re-review this please?

[iritkatriel]

Is this done?

[graingert]

Once it's in a release I'll apply it to the backport

[iritkatriel]

Once it's in a release I'll apply it to the backport

It has to be a separate PR anyway because it will only contain the tests. Why not do it as part of the review process for this PR?

[iritkatriel]

This looks fine but I think we should see the tests pass on all 3.10 buildbots before merging this. It’s the same amount of work, we might as well do it in the right order.

[bedevere-bot]

GH-100715 is a backport of this pull request to the 3.10 branch.

[miss-islington]

Thanks @graingert for the PR, and @iritkatriel for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11.
🐍🍒⛏🤖

[bedevere-bot]

GH-100718 is a backport of this pull request to the 3.11 branch.

[bedevere-bot]

GH-100715 is a backport of this pull request to the 3.10 branch.