FA-4646 / Iterator consumption / Open access

Iterator membership consumes through match · case 01

The operation returns a result or retained state that violates this contract: Membership testing on a one-pass iterator consumes through the matched item and leaves only later items.

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

ROOT CAUSE

Membership is tested on the original reusable list rather than advancing the iterator.

VERIFIED REPAIR

Membership testing on a one-pass iterator consumes through the matched item and leaves only later items.

Unsuccessful approach: Materializing before membership consumes the entire stream beyond the match.

Case contract

Membership testing on a one-pass iterator consumes through the matched item and leaves only later items. 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 [y in x,list(x)]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('middle match', solve(['a', 'b', 'c'], 'b'), [True, ['c']])
check('first match', solve(['a', 'b'], 'a'), [True, ['b']])
check('missing exhausts', solve(['a'], 'z'), [False, []])
check('empty', solve([], 'z'), [False, []])
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
middle match	`[True, ['a', 'b', 'c']]`	`[True, ['c']]`	Failed
first match	`[True, ['a', 'b']]`	`[True, ['b']]`	Failed
missing exhausts	`[False, ['a']]`	`[False, []]`	Failed
empty	`[False, []]`	`[False, []]`	Passed

SHA-256 / cdb95e047570cf236b2518a0ea52c60b6d187226cc22ca74b9a1c529274d70d5

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):
    g=iter(x); a=y in list(g); return [a,list(g)]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('middle match', solve(['a', 'b', 'c'], 'b'), [True, ['c']])
check('first match', solve(['a', 'b'], 'a'), [True, ['b']])
check('missing exhausts', solve(['a'], 'z'), [False, []])
check('empty', solve([], 'z'), [False, []])
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
middle match	`[True, []]`	`[True, ['c']]`	Failed
first match	`[True, []]`	`[True, ['b']]`	Failed
missing exhausts	`[False, []]`	`[False, []]`	Passed
empty	`[False, []]`	`[False, []]`	Passed

SHA-256 / 757ebb3544c5b6c196e59c0251aa5d84720ed66cc455c608d7697454ced5096f

3 / The verified repair

Exit 0

"""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):
    g=iter(x); a=y in g; return [a,list(g)]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('middle match', solve(['a', 'b', 'c'], 'b'), [True, ['c']])
check('first match', solve(['a', 'b'], 'a'), [True, ['b']])
check('missing exhausts', solve(['a'], 'z'), [False, []])
check('empty', solve([], 'z'), [False, []])
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
middle match	`[True, ['c']]`	`[True, ['c']]`	Passed
first match	`[True, ['b']]`	`[True, ['b']]`	Passed
missing exhausts	`[False, []]`	`[False, []]`	Passed
empty	`[False, []]`	`[False, []]`	Passed

SHA-256 / 63c3a820246230d378ae83770fa4d5d90e48bea6dcdaa2ea71f712ef3c45dce2

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 2026-09-29T14:37:41.433695+00:00.

Case digest / 67688627af32774bcc90cf3d12b8e525df462d5feb5741849d85d2e825d4f8e0