{"abstract":"The operation returns a result or retained state that violates this contract: Pop a valid index and return [removed value, remaining list].","category":"Container mutation","checks":4,"contract":"Pop a valid index and return [removed value, remaining list]. Inputs are the finite Python values shown by the executable fixtures; no concurrent execution is assumed.","contract_signature":"x, y=None","evaluation_group":"model-4324483b6c72054b","failed_approach":"Popping the final position ignores the requested index.","family":"xr-list-pop-returns-value-and-remainder","id":"FA-4351","implementations":{"attempt":{"sha256":"fc35937d0791e5c3f9e42f109f16b992b7d5f93fd2a0eb130e5e6ff8fcd66648","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    a=list(x); v=a.pop(); return [v,a]\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('pop second duplicate', solve(['a', 'b', 'a'], 2), ['a', ['a', 'b']])\ncheck('pop front', solve(['a', 'b'], 0), ['a', ['b']])\ncheck('negative index', solve(['a', 'b'], -1), ['b', ['a']])\ncheck('singleton', solve(['a'], 0), ['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":"df14a737014f5a95a48a0a66e2d76daee6de9c8442ec10a2c16dcd18f6f67d0b","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    a=list(x); v=a[y]; a.remove(v); return [v,a]\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('pop second duplicate', solve(['a', 'b', 'a'], 2), ['a', ['a', 'b']])\ncheck('pop front', solve(['a', 'b'], 0), ['a', ['b']])\ncheck('negative index', solve(['a', 'b'], -1), ['b', ['a']])\ncheck('singleton', solve(['a'], 0), ['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-pop-returns-value-and-remainder","generated_at":"2026-09-29T14:37:38.309969+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.","root_cause":"Removing by returned value deletes the first equal item instead of the requested index.","sha256":"10cfc25d5fd9128c8619d617d5f9f4585502e093f79167554d6adcdba8a91584","title":"List pop returns value and remainder · case 01","variant":1,"variant_policy":"Five execution reruns of a fixed adversarial fixture suite; variant number does not alter semantic inputs.","verified":true,"visibility":"public","verification":{"attempt":{"elapsed_ms":43.287,"exit_code":1,"observations":[{"actual":["a",["a","b"]],"check":"pop second duplicate","expected":["a",["a","b"]],"passed":true},{"actual":["b",["a"]],"check":"pop front","expected":["a",["b"]],"passed":false},{"actual":["b",["a"]],"check":"negative index","expected":["b",["a"]],"passed":true},{"actual":["a",[]],"check":"singleton","expected":["a",[]],"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"pop second duplicate\", \"actual\": [\"a\", [\"a\", \"b\"]], \"expected\": [\"a\", [\"a\", \"b\"]], \"passed\": true}, {\"check\": \"pop front\", \"actual\": [\"b\", [\"a\"]], \"expected\": [\"a\", [\"b\"]], \"passed\": false}, {\"check\": \"negative index\", \"actual\": [\"b\", [\"a\"]], \"expected\": [\"b\", [\"a\"]], \"passed\": true}, {\"check\": \"singleton\", \"actual\": [\"a\", []], \"expected\": [\"a\", []], \"passed\": true}], \"passed\": false}\n"},"broken":{"elapsed_ms":112.753,"exit_code":1,"observations":[{"actual":["a",["b","a"]],"check":"pop second duplicate","expected":["a",["a","b"]],"passed":false},{"actual":["a",["b"]],"check":"pop front","expected":["a",["b"]],"passed":true},{"actual":["b",["a"]],"check":"negative index","expected":["b",["a"]],"passed":true},{"actual":["a",[]],"check":"singleton","expected":["a",[]],"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"pop second duplicate\", \"actual\": [\"a\", [\"b\", \"a\"]], \"expected\": [\"a\", [\"a\", \"b\"]], \"passed\": false}, {\"check\": \"pop front\", \"actual\": [\"a\", [\"b\"]], \"expected\": [\"a\", [\"b\"]], \"passed\": true}, {\"check\": \"negative index\", \"actual\": [\"b\", [\"a\"]], \"expected\": [\"b\", [\"a\"]], \"passed\": true}, {\"check\": \"singleton\", \"actual\": [\"a\", []], \"expected\": [\"a\", []], \"passed\": true}], \"passed\": false}\n"}},"member_only":{"stages":["fixed"],"fields":["implementations.fixed","verification.fixed","harness","repair"],"note":"The verified repair, its recorded checks, the repair description, and the scoring harness are available to members."}}