FA-4656 / Iterator consumption / Open access

Islice step consumes stop prefix · case 01

The operation returns a result or retained state that violates this contract: Exhausting islice(start,stop,step) consumes through stop regardless of the last yielded position; return [yielded,remaining].

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

ROOT CAUSE

Consumption stops after the last emitted position rather than the requested stop boundary.

THE FAILURE

Consumption stops after the last emitted position rather than the requested stop boundary.

Unsuccessful approach: Returning the suffix from start ignores every position consumed by slicing.

Case contract

Exhausting islice(start,stop,step) consumes through stop regardless of the last yielded position; return [yielded,remaining]. 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
import itertools as it
from collections import deque
N = 1
observations = []
def solve(x, y=None):
    return [x[y[0]:y[1]:y[2]],x[(y[0]+((max(0,y[1]-y[0])-1)//y[2])*y[2]+1) if y[1]>y[0] else y[0]:]]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('stop beyond last yield', solve(['a', 'b', 'c', 'd', 'e'], [0, 4, 3]), [['a', 'd'], ['e']])
check('skipped final position consumed', solve(['a', 'b', 'c', 'd', 'e'], [0, 4, 2]), [['a', 'c'], ['e']])
check('empty slice consumes start', solve(['a', 'b', 'c'], [2, 2, 1]), [[], ['c']])
check('empty input', solve([], [0, 3, 2]), [[], []])
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
stop beyond last yield	`[['a', 'd'], ['e']]`	`[['a', 'd'], ['e']]`	Passed
skipped final position consumed	`[['a', 'c'], ['d', 'e']]`	`[['a', 'c'], ['e']]`	Failed
empty slice consumes start	`[[], ['c']]`	`[[], ['c']]`	Passed
empty input	`[[], []]`	`[[], []]`	Passed

SHA-256 / beb784bff714fee722ec85c590de768f2b699232ac7eef2960999e92c9141858

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import itertools as it
from collections import deque
N = 1
observations = []
def solve(x, y=None):
    return [x[y[0]:y[1]:y[2]],x[y[0]:]]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('stop beyond last yield', solve(['a', 'b', 'c', 'd', 'e'], [0, 4, 3]), [['a', 'd'], ['e']])
check('skipped final position consumed', solve(['a', 'b', 'c', 'd', 'e'], [0, 4, 2]), [['a', 'c'], ['e']])
check('empty slice consumes start', solve(['a', 'b', 'c'], [2, 2, 1]), [[], ['c']])
check('empty input', solve([], [0, 3, 2]), [[], []])
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
stop beyond last yield	`[['a', 'd'], ['a', 'b', 'c', 'd', 'e']]`	`[['a', 'd'], ['e']]`	Failed
skipped final position consumed	`[['a', 'c'], ['a', 'b', 'c', 'd', 'e']]`	`[['a', 'c'], ['e']]`	Failed
empty slice consumes start	`[[], ['c']]`	`[[], ['c']]`	Passed
empty input	`[[], []]`	`[[], []]`	Passed

SHA-256 / 795892bfdc7cebfa5bf17617fe55dc94fffdf0e6e5a715693a8953dd8c12dd2a

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 4 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

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 2026-09-29T14:37:41.473162+00:00.

Case digest / 169fa116af7d23dfd9258f2c9cc928fbabb042ea2fff3ce8bf0e6b09698f7a1e