Coverage misses with Python 3.10

[warsaw]

Describe the bug
I think there are still Python 3.10 compatibility problems, despite #1184 and #1106 and possibly other bugs. When testing flufl.lock against Python 3.10 (in CI and locally), I'm seeing line misses only in Python 3.10. These misses still appear to exist after testing locally with CPython 3.10 head and coveragepy head.

To Reproduce

1. git clone https://gitlab.com/warsaw/flufl.lock.git
2. cd flufl.lock
3. git switch qa
4. tox -e py39 -e py310

Expected behavior (Python 3.9)

---------- coverage: platform darwin, python 3.9.6-final-0 -----------
Name                      Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------------------------
flufl/lock/__init__.py        5      0      0      0   100%
flufl/lock/_lockfile.py     251      0     70      0   100%
---------------------------------------------------------------------
TOTAL                       256      0     70      0   100%
Coverage XML written to file coverage.xml

Actual behavior (Python 3.10)

---------- coverage: platform darwin, python 3.10.0-beta-4 -----------
Name                      Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------------------------
flufl/lock/__init__.py        5      0      0      0   100%
flufl/lock/_lockfile.py     253      0     70      3    99%   412->415, 418->420, 535->exit
---------------------------------------------------------------------
TOTAL                       258      0     70      3    99%
Coverage XML written to file coverage.xml

FAIL Required test coverage of 100.0% not reached. Total coverage: 99.09%

From local testing the lines are covered AFAICT, so there should be no functional difference between 3.9 and 3.10. The commonality is that these are conditionals inside try/excepts with raises in the true branch of the conditional.

[nedbat]

Thanks, this is reported against CPython here: https://bugs.python.org/issue44616

[warsaw]

Indeed, thanks @nedbat - that looks exactly like my case.

[warsaw]

The latest 3.10 head fixes this. Thanks!

[nedbat]

bpo 44616 is now fixed in the next 3.10 release.

[DerRidda]

@nedbat Is this actually fully resolved? I have just updated a project to Python 3.10(.8) using coverage through pytest-cov (4.0.0 using coverage 6.5.0) with no logic changes and suddenly get very weird coverage results with what looks like - to me - impossible misses. Such as:

If it is of note this code runs through the Apache beam SDK 2.43.0.
Needless to say, coverage was as expected under Python 3.9.

[nedbat]

@DerRidda Do you have a way for me to reproduce these failures?

[DerRidda]

I will try to extract an minimal example for this behavior.

[Nik-09]

Hi @nedbat, I'm encountering a similar issue where trivial lines are being marked as missing in the coverage report. However, when I add print statements for debugging, those lines are clearly being executed as they appear in the console output.

I'm attaching screenshots below to illustrate the difference — one from Python 3.9 and the other from 3.10 — showing changes in coverage without any modifications to the source files.

Python 3.10.16 and Code coverage for Python, version 7.8.0 with C extension:

Python 3.9.20 and Code coverage for Python, version 7.2.3 with C extension: