FA-4371 / Container mutation / Open access

List extend expands one level · case 01

The operation returns a result or retained state that violates this contract: Extend by the iterable elements, without recursively flattening nested elements.

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

ROOT CAUSE

Appending keeps the iterable container as a single element.

VERIFIED REPAIR

Extend by the iterable elements, without recursively flattening nested elements.

Unsuccessful approach: Recursive flattening destroys nested elements below the requested level.

Case contract

Extend by the iterable elements, without recursively flattening nested elements. 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

N = 1
observations = []
def solve(x, y=None):
    return list(x)+[y]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('flat extension', solve(['a'], ['b', 'c']), ['a', 'b', 'c'])
check('nested stays nested', solve([], [['a'], 'b']), [['a'], 'b'])
check('empty extension', solve(['a'], []), ['a'])
check('string iteration', solve([], 'ab'), ['a', 'b'])
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
flat extension	`['a', ['b', 'c']]`	`['a', 'b', 'c']`	Failed
nested stays nested	`[[['a'], 'b']]`	`[['a'], 'b']`	Failed
empty extension	`['a', []]`	`['a']`	Failed
string iteration	`['ab']`	`['a', 'b']`	Failed

SHA-256 / c2ea590cc9bb5384dae1b61b8e7cdd3558252e47881d31340adeafae5bfd7cbb

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x, y=None):
    return list(x)+[z for v in y for z in (v if isinstance(v,list) else [v])]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('flat extension', solve(['a'], ['b', 'c']), ['a', 'b', 'c'])
check('nested stays nested', solve([], [['a'], 'b']), [['a'], 'b'])
check('empty extension', solve(['a'], []), ['a'])
check('string iteration', solve([], 'ab'), ['a', 'b'])
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
flat extension	`['a', 'b', 'c']`	`['a', 'b', 'c']`	Passed
nested stays nested	`['a', 'b']`	`[['a'], 'b']`	Failed
empty extension	`['a']`	`['a']`	Passed
string iteration	`['a', 'b']`	`['a', 'b']`	Passed

SHA-256 / 0454d543326d85b03ae49819e002873977810b0f6ee9b2835043c3e47a8ec126

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x, y=None):
    return list(x)+list(y)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('flat extension', solve(['a'], ['b', 'c']), ['a', 'b', 'c'])
check('nested stays nested', solve([], [['a'], 'b']), [['a'], 'b'])
check('empty extension', solve(['a'], []), ['a'])
check('string iteration', solve([], 'ab'), ['a', 'b'])
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
flat extension	`['a', 'b', 'c']`	`['a', 'b', 'c']`	Passed
nested stays nested	`[['a'], 'b']`	`[['a'], 'b']`	Passed
empty extension	`['a']`	`['a']`	Passed
string iteration	`['a', 'b']`	`['a', 'b']`	Passed

SHA-256 / 5051166d6f7b402769b2a001aeb1ca39fa39b57fd6bd5187210fe43702cb8010

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 2026-09-29T14:37:38.470273+00:00.

Case digest / 5259e57c92bb59fea69c37d050f4c9f5002b19602f9922b00fddb22836cbc302