This construct isn't traced properly:

    except ExceptionName as var:
        if something:
            raise

Here's a reproducer:

-- 8< ---------------------------------
import linecache, sys

def trace(frame, event, arg):
    # The weird globals here is to avoid a NameError on shutdown...
    if frame.f_code.co_filename == globals().get("__file__"):
        lineno = frame.f_lineno
        print("{} {}: {}".format(event[:4], lineno, linecache.getline(__file__, lineno).rstrip()))
    return trace

def f(x):
    try:
        1/0
    except ZeroDivisionError as error:
        if x:
            raise
    return 12

print(sys.version)
sys.settrace(trace)

for x in [0, 1]:
    try:
        print(f(x))
    except:
        print("oops")
-----------------------------------

When run with 3.10.0b4, it produces this output:

  3.10.0b4 (default, Jul 11 2021, 13:51:53) [Clang 12.0.0 (clang-1200.0.32.29)]
  call 10: def f(x):
  line 11:     try:
  line 12:         1/0
  exce 12:         1/0
  line 13:     except ZeroDivisionError as error:
  line 14:         if x:
* line 15:             raise
  line 16:     return 12
  retu 16:     return 12
  12
  call 10: def f(x):
  line 11:     try:
  line 12:         1/0
  exce 12:         1/0
  line 13:     except ZeroDivisionError as error:
  line 14:         if x:
  line 15:             raise
  retu 15:             raise
  oops

The starred line claims that raise is being run, but it is not run at that point.

The variable on the except clause is important.  If you change that line to "except ZeroDivisionError:", then the output is correct:

  3.10.0b4 (default, Jul 11 2021, 13:51:53) [Clang 12.0.0 (clang-1200.0.32.29)]
  call 10: def f(x):
  line 11:     try:
  line 12:         1/0
  exce 12:         1/0
  line 13:     except ZeroDivisionError:
  line 14:         if x:
  line 16:     return 12
  retu 16:     return 12
  12
  call 10: def f(x):
  line 11:     try:
  line 12:         1/0
  exce 12:         1/0
  line 13:     except ZeroDivisionError:
  line 14:         if x:
  line 15:             raise
  retu 15:             raise
  oops

Based on https://github.com/nedbat/coveragepy/issues/1187, btw

Marking as release blocker for the first release candidate.

Ned, I assume this comes from some recent bug report in coverage, but it would be great if you could ask your users to test the latest beta and report any issue before we release the first candidate. That way we can be more sure that we won't find any future surprise

Thanks in advance

I assume this is due to some artificial bytecode for cleaning the variable in the exception

Pablo, this is triggered by my bug report here: https://github.com/nedbat/coveragepy/issues/1187

I tested this again today with the 3.10 git head and still got the coverage misses.  Happy to try any other combinations.

Thanks Ned, you're doing a fantastic job of finding these issues.
Sorry for keeping you so busy with this.

This one was a latent bug, exposed by fixing https://bugs.python.org/issue44570.

New changeset e5862f79c16e28f1ec51d179698739a9b2d8c1d2 by Mark Shannon in branch 'main':
bpo-44616: Mark all clean up instructions at end of named exception block as artificial (GH-27109)
https://github.com/python/cpython/commit/e5862f79c16e28f1ec51d179698739a9b2d8c1d2

New changeset 794ff7d505f852dc4e0f94901dc7387afaead3bb by Mark Shannon in branch '3.10':
bpo-44616: Mark all clean up instructions at end of named exception block as artificial (GH-27109) (GH-27135)
https://github.com/python/cpython/commit/794ff7d505f852dc4e0f94901dc7387afaead3bb

I just retested my test case (see the coveragepy link below) with Python 3.10 git head and coveragepy git head, and I'm still seeing the misses only in 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     80      3    99%   412->415, 418->420, 535->exit
---------------------------------------------------------------------


The lines in question:

        if is_locked:
            try:
                os.unlink(self._lockfile)
            except OSError as error:
                if error.errno not in ERRORS:
                    raise
        # Remove our claim file.
        try:
            os.unlink(self._claimfile)
        except OSError as error:
            if error.errno not in ERRORS:
                raise

I'm not sure whether coveragepy needs to be updated or whether there are other lurking bugs in Python 3.10.  I'm reopening this issue until this is resolved.

Barry, is it possible you accidentally used the beta 4?  I ran the test suite with a freshly built 3.10, but by mistake was still using the beta, and was surprised.  Once I really got the latest build in place (Python 3.10.0b4+ (heads/3.10:47695e3c88, Jul 15 2021, 11:55:52)), I got these results:

platform darwin -- Python 3.10.0b4+, pytest-6.2.4, py-1.10.0, pluggy-0.13.1
cachedir: .tox/py310/.pytest_cache
rootdir: /System/Volumes/Data/root/src/foo/bug1187/flufl.lock, configfile: setup.cfg, testpaths: test, docs
plugins: cov-2.12.1
collected 108 items

test/test_api.py .                                                                                                                                          [  0%]
test/test_lock.py ...........................                                                                                                               [ 25%]
docs/using.rst ................................................................................                                                             [100%]

======================================================================== warnings summary =========================================================================
.tox/py310/lib/python3.10/site-packages/sybil/integration/pytest.py:58: 80 warnings
  /System/Volumes/Data/root/src/foo/bug1187/flufl.lock/.tox/py310/lib/python3.10/site-packages/sybil/integration/pytest.py:58: PytestDeprecationWarning: A private pytest class or function was used.
    self._request = fixtures.FixtureRequest(self)

-- Docs: https://docs.pytest.org/en/stable/warnings.html

---------- 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     80      0   100%
---------------------------------------------------------------------
TOTAL                       258      0     80      0   100%
Coverage XML written to file coverage.xml

Required test coverage of 100.0% reached. Total coverage: 100.00%

Thanks Ned.  Seems you might be right.  I thought I'd done a clean test, but in any event, I just pulled 3.10 head and verified that it's producing 100% coverage.  Thanks all for the fix!