{"abstract":"The operation returns a result or retained state that violates this contract: Append the provided value as one item, even if it is a list.","category":"Container mutation","checks":4,"contract":"Append the provided value as one item, even if it is a list. Inputs are the finite Python values shown by the executable fixtures; no concurrent execution is assumed.","evaluation_group":"model-cc5d1fa131f7cabe","failed_approach":"A truthiness guard drops valid empty values.","family":"xr-list-append-keeps-nested-value","id":"FA-4366","implementations":{"attempt":{"sha256":"3c6a5e76959dfea4c35a080b1c51c27a99533e5c263ffef49bc239858a9c9b0e","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    return list(x)+([y] if y else [])\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('nested list', solve(['a'], ['b', 'c']), ['a', ['b', 'c']])\ncheck('empty nested list', solve([], []), [[]])\ncheck('string is one item', solve([], 'ab'), ['ab'])\ncheck('empty string retained', solve(['a'], ''), ['a', ''])\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"},"broken":{"sha256":"bb99dd780344825a3b4efc3f208cd9646c060fd82cd3370a2c3a1595f1706986","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    return list(x)+list(y)\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('nested list', solve(['a'], ['b', 'c']), ['a', ['b', 'c']])\ncheck('empty nested list', solve([], []), [[]])\ncheck('string is one item', solve([], 'ab'), ['ab'])\ncheck('empty string retained', solve(['a'], ''), ['a', ''])\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"},"fixed":{"sha256":"ad346d558945e4bf761aa42a8f72b81a36a79497599d39e129cde67c7bce4534","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    return list(x)+[y]\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('nested list', solve(['a'], ['b', 'c']), ['a', ['b', 'c']])\ncheck('empty nested list', solve([], []), [[]])\ncheck('string is one item', solve([], 'ab'), ['ab'])\ncheck('empty string retained', solve(['a'], ''), ['a', ''])\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"}},"limitations":" This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.","method":"Deterministic executable model with adversarial boundary fixtures.","provenance":{"created_by":"Failure Map","dependencies":"Python standard library","family":"xr-list-append-keeps-nested-value","generated_at":"2026-09-29T14:37:38.454095+00:00","license":"CC0-1.0","python":"3.12.14","seed":1,"split":"open-access"},"relevance":"A controlled local-runtime regression for collection APIs, language semantics, or ownership wrappers. Fixtures include boundary and interaction cases.","repair":"Append the provided value as one item, even if it is a list.","root_cause":"Extending expands a nested value that should remain one list item.","sha256":"3808ff7d6ccfe2cc8b19f54f2bc4d88a5ce23293b6637ebc05ca921eebe52f73","title":"List append keeps nested value · case 01","variant":1,"variant_policy":"Five execution reruns of a fixed adversarial fixture suite; variant number does not alter semantic inputs.","verification":{"attempt":{"elapsed_ms":37.425,"exit_code":1,"observations":[{"actual":["a",["b","c"]],"check":"nested list","expected":["a",["b","c"]],"passed":true},{"actual":[],"check":"empty nested list","expected":[[]],"passed":false},{"actual":["ab"],"check":"string is one item","expected":["ab"],"passed":true},{"actual":["a"],"check":"empty string retained","expected":["a",""],"passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"nested list\", \"actual\": [\"a\", [\"b\", \"c\"]], \"expected\": [\"a\", [\"b\", \"c\"]], \"passed\": true}, {\"check\": \"empty nested list\", \"actual\": [], \"expected\": [[]], \"passed\": false}, {\"check\": \"string is one item\", \"actual\": [\"ab\"], \"expected\": [\"ab\"], \"passed\": true}, {\"check\": \"empty string retained\", \"actual\": [\"a\"], \"expected\": [\"a\", \"\"], \"passed\": false}], \"passed\": false}\n"},"broken":{"elapsed_ms":37.971,"exit_code":1,"observations":[{"actual":["a","b","c"],"check":"nested list","expected":["a",["b","c"]],"passed":false},{"actual":[],"check":"empty nested list","expected":[[]],"passed":false},{"actual":["a","b"],"check":"string is one item","expected":["ab"],"passed":false},{"actual":["a"],"check":"empty string retained","expected":["a",""],"passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"nested list\", \"actual\": [\"a\", \"b\", \"c\"], \"expected\": [\"a\", [\"b\", \"c\"]], \"passed\": false}, {\"check\": \"empty nested list\", \"actual\": [], \"expected\": [[]], \"passed\": false}, {\"check\": \"string is one item\", \"actual\": [\"a\", \"b\"], \"expected\": [\"ab\"], \"passed\": false}, {\"check\": \"empty string retained\", \"actual\": [\"a\"], \"expected\": [\"a\", \"\"], \"passed\": false}], \"passed\": false}\n"},"fixed":{"elapsed_ms":38.264,"exit_code":0,"observations":[{"actual":["a",["b","c"]],"check":"nested list","expected":["a",["b","c"]],"passed":true},{"actual":[[]],"check":"empty nested list","expected":[[]],"passed":true},{"actual":["ab"],"check":"string is one item","expected":["ab"],"passed":true},{"actual":["a",""],"check":"empty string retained","expected":["a",""],"passed":true}],"passed":true,"stderr":"","stdout":"{\"observations\": [{\"check\": \"nested list\", \"actual\": [\"a\", [\"b\", \"c\"]], \"expected\": [\"a\", [\"b\", \"c\"]], \"passed\": true}, {\"check\": \"empty nested list\", \"actual\": [[]], \"expected\": [[]], \"passed\": true}, {\"check\": \"string is one item\", \"actual\": [\"ab\"], \"expected\": [\"ab\"], \"passed\": true}, {\"check\": \"empty string retained\", \"actual\": [\"a\", \"\"], \"expected\": [\"a\", \"\"], \"passed\": true}], \"passed\": true}\n"}},"verified":true,"visibility":"public"}