FA-4746 / Scoped mappings / Open access

Chainmap setdefault observes outer binding · case 01

The operation returns a result or retained state that violates this contract: setdefault returns an existing binding from any scope without inserting into the first scope; only an absent key is inserted.

Verified by executionVariant 1 · 4 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Applying setdefault directly to the first map ignores inherited bindings.

VERIFIED REPAIR

setdefault returns an existing binding from any scope without inserting into the first scope; only an absent key is inserted.

Unsuccessful approach: Boolean fallback overwrites falsy inherited values and inserts unnecessary shadows.

Case contract

setdefault returns an existing binding from any scope without inserting into the first scope; only an absent key is inserted. Inputs are the finite Python values shown by the executable fixtures; no concurrent execution is assumed.

Why this case matters

A controlled local-runtime regression for collection APIs, language semantics, or ownership wrappers. Fixtures include boundary and interaction cases.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
from collections import Counter, ChainMap, deque
import heapq
N = 1
observations = []
def solve(x, y=None):
    a=[dict(m) for m in x]; v=a[0].setdefault(y[0],y[1]); return [v,a]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('outer binding remains outer', solve([{}, {'a': 1}], ['a', 2]), [1, [{}, {'a': 1}]])
check('outer None retained', solve([{}, {'a': None}], ['a', 2]), [None, [{}, {'a': None}]])
check('absent inserted', solve([{}, {}], ['a', 2]), [2, [{'a': 2}, {}]])
check('inner existing', solve([{'a': 1}], ['a', 2]), [1, [{'a': 1}]])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
outer binding remains outer	`[2, [{'a': 2}, {'a': 1}]]`	`[1, [{}, {'a': 1}]]`	Failed
outer None retained	`[2, [{'a': 2}, {'a': None}]]`	`[None, [{}, {'a': None}]]`	Failed
absent inserted	`[2, [{'a': 2}, {}]]`	`[2, [{'a': 2}, {}]]`	Passed
inner existing	`[1, [{'a': 1}]]`	`[1, [{'a': 1}]]`	Passed

SHA-256 / 06d30ce4a95345efce083816c7a15539f74f513074b624d63a19ec70ebb6e954

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
from collections import Counter, ChainMap, deque
import heapq
N = 1
observations = []
def solve(x, y=None):
    c=ChainMap(*[dict(m) for m in x]); v=c.get(y[0]) or y[1]; c[y[0]]=v; return [v,c.maps]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('outer binding remains outer', solve([{}, {'a': 1}], ['a', 2]), [1, [{}, {'a': 1}]])
check('outer None retained', solve([{}, {'a': None}], ['a', 2]), [None, [{}, {'a': None}]])
check('absent inserted', solve([{}, {}], ['a', 2]), [2, [{'a': 2}, {}]])
check('inner existing', solve([{'a': 1}], ['a', 2]), [1, [{'a': 1}]])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
outer binding remains outer	`[1, [{'a': 1}, {'a': 1}]]`	`[1, [{}, {'a': 1}]]`	Failed
outer None retained	`[2, [{'a': 2}, {'a': None}]]`	`[None, [{}, {'a': None}]]`	Failed
absent inserted	`[2, [{'a': 2}, {}]]`	`[2, [{'a': 2}, {}]]`	Passed
inner existing	`[1, [{'a': 1}]]`	`[1, [{'a': 1}]]`	Passed

SHA-256 / 0072b7eeeee66ce77d7c344334f24d2529b8a080f3e0c1823efcf362db817d77

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json
from collections import Counter, ChainMap, deque
import heapq
N = 1
observations = []
def solve(x, y=None):
    c=ChainMap(*[dict(m) for m in x]); v=c.setdefault(y[0],y[1]); return [v,c.maps]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('outer binding remains outer', solve([{}, {'a': 1}], ['a', 2]), [1, [{}, {'a': 1}]])
check('outer None retained', solve([{}, {'a': None}], ['a', 2]), [None, [{}, {'a': None}]])
check('absent inserted', solve([{}, {}], ['a', 2]), [2, [{'a': 2}, {}]])
check('inner existing', solve([{'a': 1}], ['a', 2]), [1, [{'a': 1}]])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
outer binding remains outer	`[1, [{}, {'a': 1}]]`	`[1, [{}, {'a': 1}]]`	Passed
outer None retained	`[None, [{}, {'a': None}]]`	`[None, [{}, {'a': None}]]`	Passed
absent inserted	`[2, [{'a': 2}, {}]]`	`[2, [{'a': 2}, {}]]`	Passed
inner existing	`[1, [{'a': 1}]]`	`[1, [{'a': 1}]]`	Passed

SHA-256 / aea5378df51cf0e0dca683bc37113a6676049bb3831418522d62c1a599435417

Verification & scope

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.

Observations recorded using Python 3.12.14 at 2026-09-29T14:37:42.190518+00:00.

Case digest / ac20850c4730ae9a24f2de0a762fd7ac4ce1dc85a2653595d084e9afbef36335