Preserve cached mutation results on rerun of unchanged source

src/mutmut/__main__.py

@@ -1,4 +1,3 @@
-from typing import Iterable
 from mutmut.type_checking import TypeCheckingError
 from mutmut.type_checking import run_type_checker
 from typing import Any
@@ -55,7 +54,6 @@
 
 import click
 import libcst as cst
-import libcst.matchers as m
 from rich.text import Text
 from setproctitle import setproctitle
 
@@ -137,7 +135,7 @@ def guess_paths_to_mutate():
 
 
 def record_trampoline_hit(name):
-    assert not name.startswith('src.'), f'Failed trampoline hit. Module name starts with `src.`, which is invalid'
+    assert not name.startswith('src.'), 'Failed trampoline hit. Module name starts with `src.`, which is invalid'
     if mutmut.config.max_stack_depth != -1:
         f = inspect.currentframe()
         c = mutmut.config.max_stack_depth
@@ -293,13 +291,7 @@ def create_mutants_for_file(filename: Path, output_path: Path) -> FileMutationRe
         # source_mtime == mutant_mtime: only copied, otherwise the mutant file is untouched
         # source_mtime < mutant_mtime: the mutations have been saved after copying; source file untouched
         if source_mtime < mutant_mtime:
-            # reset the mutation stats
-            source_file_mutation_data = SourceFileMutationData(path=filename)
-            source_file_mutation_data.load()
-            for key in source_file_mutation_data.exit_code_by_key:
-                source_file_mutation_data.exit_code_by_key[key] = None
-            source_file_mutation_data.save()
-
+            # Source unchanged, mutations already generated — preserve cached results
             return FileMutationResult(unmodified=True)
     except OSError:
         pass
@@ -626,7 +618,6 @@ def pytest_deselected(self, items):
 
         collector = TestsCollector()
 
-        tests_dir = mutmut.config.tests_dir
         pytest_args = ['-x', '-q', '--collect-only'] + self._pytest_add_cli_args_test_selection
 
         with change_cwd('mutants'):
@@ -1127,7 +1118,7 @@ def print_time_estimates(mutant_names):
 
     for time, key in sorted(times_and_keys):
         if not time:
-            print(f'<no tests>', key)
+            print('<no tests>', key)
         else:
             print(f'{int(time*1000)}ms', key)
 
@@ -1620,7 +1611,7 @@ def on_data_table_row_highlighted(self, event):
                 duration = source_file_mutation_data.durations_by_key.get(mutant_name, '?')
                 type_check_error = source_file_mutation_data.type_check_error_by_key.get(mutant_name, '?')
 
-                view_tests_description = f'(press t to view tests executed for this mutant)'
+                view_tests_description = '(press t to view tests executed for this mutant)'
 
                 match status:
                     case 'killed':

tests/e2e/test_e2e_incremental_cache.py

@@ -0,0 +1,53 @@
+import shutil
+from pathlib import Path
+
+from tests.e2e.e2e_utils import change_cwd, read_all_stats_for_project, write_json_file, read_json_file
+
+import mutmut
+from mutmut.__main__ import _run
+
+
+def test_rerun_preserves_cached_results():
+    """Rerunning mutmut on unchanged source must not reset cached exit codes to None.
+
+    Strategy: run mutmut once, then inject a sentinel exit code (99) into the
+    meta file. If the second run preserves the cache, the sentinel survives.
+    If it resets and re-tests, 99 gets replaced with a real exit code.
+    """
+    project_path = Path("..").parent / "e2e_projects" / "my_lib"
+    mutants_path = project_path / "mutants"
+    shutil.rmtree(mutants_path, ignore_errors=True)
+
+    # First run: generate and test all mutants
+    mutmut._reset_globals()
+    with change_cwd(project_path):
+        _run([], None)
+
+    # Inject sentinel exit code (99) into every mutant result
+    meta_files = list(mutants_path.rglob("*.meta"))
+    assert meta_files, "Expected .meta files after first run"
+
+    sentinel = 99
+    for meta_file in meta_files:
+        meta = read_json_file(meta_file)
+        for key in meta["exit_code_by_key"]:
+            meta["exit_code_by_key"][key] = sentinel
+        write_json_file(meta_file, meta)
+
+    # Second run: source unchanged, sentinel values should survive
+    mutmut._reset_globals()
+    with change_cwd(project_path):
+        _run([], None)
+
+    second_run_stats = read_all_stats_for_project(project_path)
+
+    # Every result should still be the sentinel — not None, not a real exit code
+    for meta_path, results in second_run_stats.items():
+        for mutant_name, exit_code in results.items():
+            assert exit_code == sentinel, (
+                f"Cached result for {mutant_name} in {meta_path} was not preserved. "
+                f"Expected sentinel {sentinel}, got {exit_code}."
+            )
+
+    # Cleanup
+    shutil.rmtree(mutants_path, ignore_errors=True)