{"abstract":"The operation returns a result or retained state that violates this contract: Pop a key if present and return [value,remaining map], using fallback only when absent.","category":"Mapping semantics","checks":4,"contract":"Pop a key if present and return [value,remaining map], using fallback only when absent. Inputs are the finite Python values shown by the executable fixtures; no concurrent execution is assumed.","contract_signature":"x, y=None","evaluation_group":"model-fd95e939061e4c3c","failed_approach":"Boolean fallback discards legitimate falsy popped values.","family":"xr-map-pop-preserves-present-default-value","id":"FA-4391","implementations":{"attempt":{"sha256":"62d5c0040bb72bd994dca7716873cf1cc2bfd2350d4d57c0ff68307f7568624a","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    a=dict(x); v=a.pop(y[0],y[1]); return [v or y[1],a]\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('present value', solve({'a': 'v'}, ['a', 'd']), ['v', {}])\ncheck('present zero', solve({'a': 0}, ['a', 'd']), [0, {}])\ncheck('missing', solve({'b': 'v'}, ['a', 'd']), ['d', {'b': 'v'}])\ncheck('present None', solve({'a': None}, ['a', 'd']), [None, {}])\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":"51348de728d53d012fbe5c20c6317d0be2a797c3933e7752c4817b9173c3b9ec","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    a=dict(x); v=a.get(y[0],y[1]); return [v,a]\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('present value', solve({'a': 'v'}, ['a', 'd']), ['v', {}])\ncheck('present zero', solve({'a': 0}, ['a', 'd']), [0, {}])\ncheck('missing', solve({'b': 'v'}, ['a', 'd']), ['d', {'b': 'v'}])\ncheck('present None', solve({'a': None}, ['a', 'd']), [None, {}])\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-map-pop-preserves-present-default-value","generated_at":"2026-09-29T14:37:38.547194+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":"A lookup reads the value without removing its binding.","sha256":"092b781220416f6a63844e2b8e8836a5e3a7b8b10a145eb375ca6c21d4bf6380","title":"Map pop preserves present default value · 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":36.494,"exit_code":1,"observations":[{"actual":["v",{}],"check":"present value","expected":["v",{}],"passed":true},{"actual":["d",{}],"check":"present zero","expected":[0,{}],"passed":false},{"actual":["d",{"b":"v"}],"check":"missing","expected":["d",{"b":"v"}],"passed":true},{"actual":["d",{}],"check":"present None","expected":[null,{}],"passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"present value\", \"actual\": [\"v\", {}], \"expected\": [\"v\", {}], \"passed\": true}, {\"check\": \"present zero\", \"actual\": [\"d\", {}], \"expected\": [0, {}], \"passed\": false}, {\"check\": \"missing\", \"actual\": [\"d\", {\"b\": \"v\"}], \"expected\": [\"d\", {\"b\": \"v\"}], \"passed\": true}, {\"check\": \"present None\", \"actual\": [\"d\", {}], \"expected\": [null, {}], \"passed\": false}], \"passed\": false}\n"},"broken":{"elapsed_ms":37.31,"exit_code":1,"observations":[{"actual":["v",{"a":"v"}],"check":"present value","expected":["v",{}],"passed":false},{"actual":[0,{"a":0}],"check":"present zero","expected":[0,{}],"passed":false},{"actual":["d",{"b":"v"}],"check":"missing","expected":["d",{"b":"v"}],"passed":true},{"actual":[null,{"a":null}],"check":"present None","expected":[null,{}],"passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"present value\", \"actual\": [\"v\", {\"a\": \"v\"}], \"expected\": [\"v\", {}], \"passed\": false}, {\"check\": \"present zero\", \"actual\": [0, {\"a\": 0}], \"expected\": [0, {}], \"passed\": false}, {\"check\": \"missing\", \"actual\": [\"d\", {\"b\": \"v\"}], \"expected\": [\"d\", {\"b\": \"v\"}], \"passed\": true}, {\"check\": \"present None\", \"actual\": [null, {\"a\": null}], \"expected\": [null, {}], \"passed\": false}], \"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."}}