{"abstract":"The operation returns a result or retained state that violates this contract: Pop removes only a top-scope binding; an outer-only binding returns the supplied fallback.","category":"Scoped mappings","checks":4,"contract":"Pop removes only a top-scope binding; an outer-only binding returns the supplied fallback. Inputs are the finite Python values shown by the executable fixtures; no concurrent execution is assumed.","contract_signature":"x, y=None","evaluation_group":"model-b1b3be1be54f650e","failed_approach":"Reading inherited values returns a successful-looking pop without actually removing a local binding.","family":"xr-chainmap-pop-is-top-local","id":"FA-4751","implementations":{"attempt":{"sha256":"70ddcc5c56077d2b96812bd8b595be4f6d118e47a3fe7ab4b46de6ef4c22977d","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\nfrom collections import Counter, ChainMap, deque\nimport heapq\nN = 1\nobservations = []\ndef solve(x, y=None):\n    c=ChainMap(*[dict(m) for m in x]); v=c.get(y[0],y[1]); c.maps[0].pop(y[0],None); return [v,c.maps]\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('outer-only returns default', solve([{}, {'a': 1}], ['a', 'missing']), ['missing', [{}, {'a': 1}]])\ncheck('inner removes only inner', solve([{'a': 2}, {'a': 1}], ['a', 'missing']), [2, [{}, {'a': 1}]])\ncheck('None pops', solve([{'a': None}], ['a', 'missing']), [None, [{}]])\ncheck('absent fallback', solve([{}], ['a', 'missing']), ['missing', [{}]])\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":"f20895b4d8192feb2cb11556bf879ff92b83253d97bef85442a462d1de77ecab","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\nfrom collections import Counter, ChainMap, deque\nimport heapq\nN = 1\nobservations = []\ndef solve(x, y=None):\n    a=[dict(m) for m in x]; m=next((m for m in a if y[0] in m),a[0]); v=m.pop(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('outer-only returns default', solve([{}, {'a': 1}], ['a', 'missing']), ['missing', [{}, {'a': 1}]])\ncheck('inner removes only inner', solve([{'a': 2}, {'a': 1}], ['a', 'missing']), [2, [{}, {'a': 1}]])\ncheck('None pops', solve([{'a': None}], ['a', 'missing']), [None, [{}]])\ncheck('absent fallback', solve([{}], ['a', 'missing']), ['missing', [{}]])\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-chainmap-pop-is-top-local","generated_at":"2026-09-29T14:37:42.334330+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":"Searching outer scopes lets a local pop remove inherited bindings.","sha256":"24152e0229e53845df28a254b21002abf6ff294474ff236374ccf4fdd91c66fe","title":"Chainmap pop is top local · 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":35.902,"exit_code":1,"observations":[{"actual":[1,[{},{"a":1}]],"check":"outer-only returns default","expected":["missing",[{},{"a":1}]],"passed":false},{"actual":[2,[{},{"a":1}]],"check":"inner removes only inner","expected":[2,[{},{"a":1}]],"passed":true},{"actual":[null,[{}]],"check":"None pops","expected":[null,[{}]],"passed":true},{"actual":["missing",[{}]],"check":"absent fallback","expected":["missing",[{}]],"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"outer-only returns default\", \"actual\": [1, [{}, {\"a\": 1}]], \"expected\": [\"missing\", [{}, {\"a\": 1}]], \"passed\": false}, {\"check\": \"inner removes only inner\", \"actual\": [2, [{}, {\"a\": 1}]], \"expected\": [2, [{}, {\"a\": 1}]], \"passed\": true}, {\"check\": \"None pops\", \"actual\": [null, [{}]], \"expected\": [null, [{}]], \"passed\": true}, {\"check\": \"absent fallback\", \"actual\": [\"missing\", [{}]], \"expected\": [\"missing\", [{}]], \"passed\": true}], \"passed\": false}\n"},"broken":{"elapsed_ms":37.853,"exit_code":1,"observations":[{"actual":[1,[{},{}]],"check":"outer-only returns default","expected":["missing",[{},{"a":1}]],"passed":false},{"actual":[2,[{},{"a":1}]],"check":"inner removes only inner","expected":[2,[{},{"a":1}]],"passed":true},{"actual":[null,[{}]],"check":"None pops","expected":[null,[{}]],"passed":true},{"actual":["missing",[{}]],"check":"absent fallback","expected":["missing",[{}]],"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"outer-only returns default\", \"actual\": [1, [{}, {}]], \"expected\": [\"missing\", [{}, {\"a\": 1}]], \"passed\": false}, {\"check\": \"inner removes only inner\", \"actual\": [2, [{}, {\"a\": 1}]], \"expected\": [2, [{}, {\"a\": 1}]], \"passed\": true}, {\"check\": \"None pops\", \"actual\": [null, [{}]], \"expected\": [null, [{}]], \"passed\": true}, {\"check\": \"absent fallback\", \"actual\": [\"missing\", [{}]], \"expected\": [\"missing\", [{}]], \"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."}}