Summary

On Python 3.14 (the default sys.monitoring core), a one-line class body immediately followed by another statement is wrongly reported as an uncovered branch, e.g. line 1 didn't jump to line 4. This is class Foo: ... (or any one-line body such as x = 1) on its own line, followed by any statement. Reported in #2167.

class Foo: ...


pass

Before: 1->4 reported as a missing branch (75%). After: 100%, matching the ctrace core, Python 3.13, EOF, and multi-line class forms.

Fix

The sysmon core records sequential (non-branch) transitions as self-arcs (n, n) and relies on a later pass to resolve them to the real destination, but only when the line has a single possible destination. A one-line class body's exit arc (1, -1) gives the class line a second possible destination, so the fall-through self-arc (1, 1) was never resolved to (1, 4), and 1->4 was reported missing.

The fix records the true sequential transition between consecutive lines in a frame (like the ctrace and pytracer cores already do), in sysmon_line_arcs. last_line is tracked per CodeInfo and cleared on PY_RETURN so recursive frames of the same code object aren't joined by a bogus arc. The change is localized to coverage/sysmon.py and touches neither the parser's arc analysis nor the C tracer.

Tests

Added SimpleArcTest.test_bug_2167 in tests/test_arcs.py using the existing arc-testing helpers. It covers the exact 3-line repro, a non-... one-line body, and a following branch statement. The test fails on main (Wrong missing branches: [] != [(1, 4)]) and passes with the fix.

Verified locally on CPython 3.14.6:

• New test passes; test_arcs.py, test_coverage.py, test_results.py, test_parser.py, test_api.py, test_html.py all green.
• A differential check across 23 constructs (recursion, loops, generators, match, comprehensions, try/except, decorators, with) confirms sysmon now matches both the ctrace core and Python 3.13 exactly, with no new false positives or negatives.
• ruff format --check clean; pylint clean; mypy --python-version=3.14 --strict coverage clean.
• Added a CHANGES.rst entry.

Disclosure: prepared with AI assistance; reviewed and verified locally.