Missing branch coverage with multiline `case` statement in Python 3.14

[burritoatspoton]

Describe the bug
When using multiline case statements in match..case (e.g. with a long list of variants) coveragepy reports incomplete branch coverage (missing jump from a long case statement to the following one, to be exact). I haven't managed to reproduce the issue using Python versions other than 3.14.

To Reproduce
Installed Python: 3.14.0
Installed coverage: 7.11.0

.coveragerc

[run]
branch = true
[report]
show_missing = true

match.py

def foo(x: int) -> int:
    match x:
        case (
            1
            | 2
        ):
            return 1
        case _:
            return 0


print(foo(1))
print(foo(2))
print(foo(3))

Output of coverage erase && coverage run match.py && coverage report:

1
1
0
Name       Stmts   Miss Branch BrPart  Cover   Missing
------------------------------------------------------
match.py       9      0      2      1    91%   3->8
------------------------------------------------------
TOTAL          9      0      2      1    91%

Same output after we collapse the long case statement to a single line (case 1 | 2:):

1
1
0
Name       Stmts   Miss Branch BrPart  Cover   Missing
------------------------------------------------------
match.py       9      0      2      0   100%
------------------------------------------------------
TOTAL          9      0      2      0   100%

Answer the questions below:

1. What version of Python are you using? 3.14.0

2. What version of coverage.py shows the problem? The output of coverage debug sys is helpful.

   coverage debug sys

   -- sys -------------------------------------------------------
                  coverage_version: 7.11.0
                   coverage_module: /Users/burrito/coverage-report/.venv/lib/python3.14/site-packages/coverage/__init__.py
                              core: -none-
                           CTracer: available from /Users/burrito/coverage-report/.venv/lib/python3.14/site-packages/coverage/tracer.cpython-314-darwin.so
              plugins.file_tracers: -none-
               plugins.configurers: -none-
         plugins.context_switchers: -none-
                 configs_attempted: /Users/burrito/coverage-report/.coveragerc
                      configs_read: /Users/burrito/coverage-report/.coveragerc
                       config_file: /Users/burrito/coverage-report/.coveragerc
                   config_contents: b'[run]\nbranch = true\n\n[report]\nshow_missing = true\n'
                         data_file: -none-
                            python: 3.14.0 (main, Oct 10 2025, 12:54:13) [Clang 20.1.4 ]
                          platform: macOS-15.7.1-arm64-arm-64bit-Mach-O
                    implementation: CPython
                             build: main
                                    Oct 10 2025 12:54:13
                       gil_enabled: True
                        executable: /Users/burrito/coverage-report/.venv/bin/python
                      def_encoding: utf-8
                       fs_encoding: utf-8
                               pid: 36117
                               cwd: /Users/burrito/coverage-report
                              path: /Users/burrito/coverage-report/.venv/bin
                                    /Users/burrito/.local/share/uv/python/cpython-3.14.0-macos-aarch64-none/lib/python314.zip
                                    /Users/burrito/.local/share/uv/python/cpython-3.14.0-macos-aarch64-none/lib/python3.14
                                    /Users/burrito/.local/share/uv/python/cpython-3.14.0-macos-aarch64-none/lib/python3.14/lib-dynload
                                    /Users/burrito/coverage-report/.venv/lib/python3.14/site-packages
                       environment: HOME = /Users/burrito
                      command_line: /Users/burrito/coverage-report/.venv/bin/coverage debug sys
            sqlite3_sqlite_version: 3.50.4
                sqlite3_temp_store: 0
           sqlite3_compile_options: ATOMIC_INTRINSICS=1, COMPILER=clang-20.1.4, DEFAULT_AUTOVACUUM,
                                    DEFAULT_CACHE_SIZE=-2000, DEFAULT_FILE_FORMAT=4,
                                    DEFAULT_JOURNAL_SIZE_LIMIT=-1, DEFAULT_MMAP_SIZE=0, DEFAULT_PAGE_SIZE=4096,
                                    DEFAULT_PCACHE_INITSZ=20, DEFAULT_RECURSIVE_TRIGGERS,
                                    DEFAULT_SECTOR_SIZE=4096, DEFAULT_SYNCHRONOUS=2,
                                    DEFAULT_WAL_AUTOCHECKPOINT=1000, DEFAULT_WAL_SYNCHRONOUS=2,
                                    DEFAULT_WORKER_THREADS=0, DIRECT_OVERFLOW_READ, ENABLE_DBSTAT_VTAB,
                                    ENABLE_FTS3, ENABLE_FTS3_PARENTHESIS, ENABLE_FTS4, ENABLE_FTS5,
                                    ENABLE_GEOPOLY, ENABLE_MATH_FUNCTIONS, ENABLE_RTREE,
                                    MALLOC_SOFT_LIMIT=1024, MAX_ATTACHED=10, MAX_COLUMN=2000,
                                    MAX_COMPOUND_SELECT=500, MAX_DEFAULT_PAGE_SIZE=8192, MAX_EXPR_DEPTH=1000,
                                    MAX_FUNCTION_ARG=1000, MAX_LENGTH=1000000000,
                                    MAX_LIKE_PATTERN_LENGTH=50000, MAX_MMAP_SIZE=0x7fff0000,
                                    MAX_PAGE_COUNT=0xfffffffe, MAX_PAGE_SIZE=65536, MAX_SQL_LENGTH=1000000000,
                                    MAX_TRIGGER_DEPTH=1000, MAX_VARIABLE_NUMBER=32766, MAX_VDBE_OP=250000000,
                                    MAX_WORKER_THREADS=8, MUTEX_PTHREADS, SYSTEM_MALLOC, TEMP_STORE=1,
                                    THREADSAFE=1
   

3. What versions of what packages do you have installed? The output of pip freeze is helpful.

   uv pip freeze

   coverage==7.11.0
   

4. What code shows the problem? Give us a specific commit of a specific repo that we can check out. If you've already worked around the problem, please provide a commit before that fix.

   I don't have such commit at hand, only the minimal example posted above.

5. What commands should we run to reproduce the problem? Be specific. Include everything, even git clone, pip install, and so on. Explain like we're five!

   I've ran the example in a uv-managed venv.

   pyproject.toml

   [project]
   name = "coverage-bug-report"
   requires-python = ">=3.14"
   version = "0.0.0"
   dependencies = [
       "coverage==7.11.0",
   ]

   Then installed coverage and activated the venv:

   uv sync
   source .venv/bin/activate

   Then proceeded with the example as described above.

Expected behavior
coverage report should show complete branch coverage regardless of the formatting.

Additional context
N/A

[nedbat]

Thanks. This seems to be due to the use of sys.monitoring as the default for 3.14. If you set COVERAGE_CORE=ctrace, it reports 100%. I'm investigating...

[prauscher]

Probably the same issue happens with multiline for loops:

# full coverage
def run_a(iterator):
    for (i) in iterator:
        print(i)

# missing coverage from for-line to print-line
def run_b(iterator):
    for (i
         ) in iterator:
        print(i)

Indeed, setting COVERAGE_CORE=ctrace leads to 100% coverage again :)