clear_out_obsolete_test_names never persists its cleanup: counters iterate keys instead of values

[t-roi33]

ListAllTestsResult.clear_out_obsolete_test_names (src/mutmut/__main__.py, lines 421–433 on main) filters obsolete test names out of tests_by_mangled_function_name correctly, but the guard around save_stats() can never be true, so the cleaned state is never written back to mutants/mutmut-stats.json.

count_before = sum(len(x) for x in mutmut.tests_by_mangled_function_name)   # line 422
...
count_after  = sum(len(x) for x in mutmut.tests_by_mangled_function_name)   # line 430
if count_before != count_after:
    print(f"Removed {count_before - count_after} obsolete test names")
    save_stats()

tests_by_mangled_function_name is a defaultdict(set) (src/mutmut/__init__.py, line 15). Iterating it yields keys, so len(x) measures the length of each mangled function name, not the size of its test set. The dict comprehension in between preserves every key, so both sums are computed over an identical key set and count_before == count_after always — not intermittently. The branch is dead: the message never prints and save_stats() never runs from here.

Impact

Limited but persistent. The in-memory filtering happens before tests_for_mutant_names is used, so runs are unaffected and no verdict is wrong. What lingers is the file on disk: obsolete node IDs stay in mutmut-stats.json indefinitely, unless something else happens to call save_stats() in the same process (for example run_stats_collection when new tests are found). Anyone reading the stats file therefore sees a state no run has ever used.

Reproduce

1. Run mutmut run on a project until stats are cached.
2. Delete or move one test file.
3. Run mutmut run again.
4. mutants/mutmut-stats.json still contains that file's node IDs under duration_by_test and tests_by_mangled_function_name, and Removed N obsolete test names is never printed.

Suggested fix

count_before = sum(len(x) for x in mutmut.tests_by_mangled_function_name.values())
count_after  = sum(len(x) for x in mutmut.tests_by_mangled_function_name.values())

in both places, which also makes the reported number mean what the message says.

Observed on 3.7.0; the code is unchanged on main. Happy to send a PR if that is easier than fixing it directly.

[nicklafleur]

feel free to open a fix pr, nice find