FA-4356 / Container mutation / Open access

Extended slice assignment stride · case 01

The operation returns a result or retained state that violates this contract: Replace values at the selected positive stride positions; replacement length matches selected length.

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

ROOT CAUSE

A contiguous assignment overwrites intervening positions outside the stride.

VERIFIED REPAIR

Replace values at the selected positive stride positions; replacement length matches selected length.

Unsuccessful approach: Beginning the stride at position one shifts every replacement target.

Case contract

Replace values at the selected positive stride positions; replacement length matches selected length. 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):
    a=list(x); a[:len(y[1])]=y[1]; return a
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('alternating positions', solve(['a', 'b', 'c', 'd'], [2, ['x', 'y']]), ['x', 'b', 'y', 'd'])
check('stride one', solve(['a', 'b'], [1, ['x', 'y']]), ['x', 'y'])
check('empty selection', solve([], [2, []]), [])
check('large stride', solve(['a', 'b'], [8, ['x']]), ['x', '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
alternating positions	`['x', 'y', 'c', 'd']`	`['x', 'b', 'y', 'd']`	Failed
stride one	`['x', 'y']`	`['x', 'y']`	Passed
empty selection	`[]`	`[]`	Passed
large stride	`['x', 'b']`	`['x', 'b']`	Passed

SHA-256 / b4457f1f55abb66736627adb6110ff7351dcfa6f6c76a76546242c5c372b2483

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(x, y=None):
    a=list(x); a[1::y[0]]=y[1][:len(a[1::y[0]])]; return a
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('alternating positions', solve(['a', 'b', 'c', 'd'], [2, ['x', 'y']]), ['x', 'b', 'y', 'd'])
check('stride one', solve(['a', 'b'], [1, ['x', 'y']]), ['x', 'y'])
check('empty selection', solve([], [2, []]), [])
check('large stride', solve(['a', 'b'], [8, ['x']]), ['x', '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
alternating positions	`['a', 'x', 'c', 'y']`	`['x', 'b', 'y', 'd']`	Failed
stride one	`['a', 'x']`	`['x', 'y']`	Failed
empty selection	`[]`	`[]`	Passed
large stride	`['a', 'x']`	`['x', 'b']`	Failed

SHA-256 / bdd3c1387d0f186d306870098b767578ed7a19b288ddf28472c88691b9ab19ed

3 / The verified repair

Exit 0

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

N = 1
observations = []
def solve(x, y=None):
    a=list(x); a[::y[0]]=y[1]; return a
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('alternating positions', solve(['a', 'b', 'c', 'd'], [2, ['x', 'y']]), ['x', 'b', 'y', 'd'])
check('stride one', solve(['a', 'b'], [1, ['x', 'y']]), ['x', 'y'])
check('empty selection', solve([], [2, []]), [])
check('large stride', solve(['a', 'b'], [8, ['x']]), ['x', '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
alternating positions	`['x', 'b', 'y', 'd']`	`['x', 'b', 'y', 'd']`	Passed
stride one	`['x', 'y']`	`['x', 'y']`	Passed
empty selection	`[]`	`[]`	Passed
large stride	`['x', 'b']`	`['x', 'b']`	Passed

SHA-256 / d1b4ceb33f98c9d8dfcca3ee5bdba7a983318550a96eae4298e607205fb4b88c

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 2026-09-29T14:37:38.349479+00:00.

Case digest / a871eae9ad7fb235cf5943ec422c0e094a40ef5c206098611780caba899a2329