Incomplete coverage reported with python 3.11.0-alpha.1

[jepler]

A minimized test case shows a branch with only partial coverage when using python 3.11.0-alpha.0, but 100% coverage with a wide range of other python versions.

To Reproduce
How can we reproduce the problem? Please be specific. Don't just link to a failing CI job. Answer the questions below:

1. python3.11 from https://github.com/actions/python-versions/releases/download/3.11.0-alpha.1-117932/python-3.11.0-alpha.1-linux-20.04-x64.tar.gz

2.  $ coverage debug sys
    -- sys -------------------------------------------------------
               coverage_version: 6.1.1
                coverage_module: /tmp/311env/lib/python3.11/site-packages/coverage/__init__.py
                         tracer: -none-
                        CTracer: unavailable
           plugins.file_tracers: -none-
            plugins.configurers: -none-
      plugins.context_switchers: -none-
              configs_attempted: .coveragerc
                                 setup.cfg
                                 tox.ini
                                 pyproject.toml
                   configs_read: -none-
                    config_file: None
                config_contents: -none-
                      data_file: -none-
                         python: 3.11.0a1 (default, Oct 18 2021, 14:24:01) [GCC 9.3.0]
                       platform: Linux-5.10.0-8-amd64-x86_64-with-glibc2.31
                 implementation: CPython
                     executable: /tmp/311env/bin/python3
                   def_encoding: utf-8
                    fs_encoding: utf-8
                            pid: 2221646
                            cwd: /tmp/co
                           path: /tmp/311env/bin
                                 /tmp/py311/lib/python311.zip
                                 /tmp/py311/lib/python3.11
                                 /tmp/py311/lib/python3.11/lib-dynload
                                 /tmp/311env/lib/python3.11/site-packages
                    environment: HOME = /home/jepler
                   command_line: /tmp/311env/bin/coverage debug sys
                sqlite3_version: 2.6.0
         sqlite3_sqlite_version: 3.34.1
             sqlite3_temp_store: 0
        sqlite3_compile_options: COMPILER=gcc-10.2.1 20210110; ENABLE_COLUMN_METADATA; ENABLE_DBSTAT_VTAB
                                 ENABLE_FTS3; ENABLE_FTS3_PARENTHESIS; ENABLE_FTS3_TOKENIZER
                                 ENABLE_FTS4; ENABLE_FTS5; ENABLE_JSON1
                                 ENABLE_LOAD_EXTENSION; ENABLE_PREUPDATE_HOOK; ENABLE_RTREE
                                 ENABLE_SESSION; ENABLE_STMTVTAB; ENABLE_UNLOCK_NOTIFY
                                 ENABLE_UPDATE_DELETE_LIMIT; HAVE_ISNAN; LIKE_DOESNT_MATCH_BLOBS
                                 MAX_SCHEMA_RETRY=25; MAX_VARIABLE_NUMBER=250000; OMIT_LOOKASIDE
                                 SECURE_DELETE; SOUNDEX; TEMP_STORE=1
                                 THREADSAFE=1; USE_URI
   

Code:

import unittest

class ExceptionCoverage(unittest.TestCase):
    def test_exception_coverage(self) -> None:
        with self.assertRaises(ZeroDivisionError):
            1/0

if __name__ == '__main__': # pragma no cover
    unittest.main()

Running:

$ python3 -mcoverage run --branch test_assertraises.py && python3 -mcoverage report --fail-under=100
.
----------------------------------------------------------------------
Ran 1 test in 0.001s

OK
Name                   Stmts   Miss Branch BrPart  Cover
--------------------------------------------------------
test_assertraises.py       5      0      2      1    86%
--------------------------------------------------------
TOTAL                      5      0      2      1    86%
Coverage failure: total of 86 is less than fail-under=100

Expected behavior
When running under Python 3.11-alpha.0, the same 100% branch coverage should be shown as under 3.9 and other Python versions

Additional context
This is a minimized version of a problem affecting https://github.com/jepler/wwvbpy. As with

• Apparently inaccurate coverage under pypy3.8-v7.3.6-linux64 #1251

this is probably a bug in the underlying Python implementation, but hopefully you are able to better engage with the python devs than I can.

[domdfcoding]

I don't know whether the two are related, but as well as the incorrect coverage I'm seeing CoverageWarning: Trace function changed, measurement is likely wrong: None (trace-changed) after the pytest suite finishes.The warning isn't present on other Python versions.

[nedbat]

Python 3.11 is still early days, and in particular, the coverage.py C code doesn't compile on 3.11 yet (#1241). But I will investigate to see why it's behaving like this.

[nedbat]

I've reported a bug in 3.11: https://bugs.python.org/issue45709

[nedbat]

@domdfcoding The "Trace function changed" message is partially due to not have a compiled C-language tracer available, but I'm not seeing that message. Can you show how you are producing it?