FA-4651 / Iterator consumption / Open access
Iterator second pass is exhausted · case 01
The operation returns a result or retained state that violates this contract: Materializing a one-pass iterator exhausts it; a second materialization is empty.
ROOT CAUSE
The second pass constructs a fresh iterator instead of reusing the exhausted cursor.
VERIFIED REPAIR
Materializing a one-pass iterator exhausts it; a second materialization is empty.
Unsuccessful approach: Returning the first materialization again fabricates a second traversal.
Case contract
Materializing a one-pass iterator exhausts it; a second materialization is empty. 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 [list(x),list(x)]
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('two values', solve(['a', 'b']), [['a', 'b'], []])
check('singleton', solve(['a']), [['a'], []])
check('empty', solve([]), [[], []])
check('duplicates', solve(['a', 'a']), [['a', 'a'], []])
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 |
|---|---|---|---|
| two values | [['a', 'b'], ['a', 'b']] | [['a', 'b'], []] | Failed |
| singleton | [['a'], ['a']] | [['a'], []] | Failed |
| empty | [[], []] | [[], []] | Passed |
| duplicates | [['a', 'a'], ['a', 'a']] | [['a', 'a'], []] | Failed |
SHA-256 / e3dd2cd179aba948fd51a93b93650074e2e15c16404a8bfef5b8b76d93ed0cd1
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=list(g); return [a,a]
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('two values', solve(['a', 'b']), [['a', 'b'], []])
check('singleton', solve(['a']), [['a'], []])
check('empty', solve([]), [[], []])
check('duplicates', solve(['a', 'a']), [['a', 'a'], []])
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 |
|---|---|---|---|
| two values | [['a', 'b'], ['a', 'b']] | [['a', 'b'], []] | Failed |
| singleton | [['a'], ['a']] | [['a'], []] | Failed |
| empty | [[], []] | [[], []] | Passed |
| duplicates | [['a', 'a'], ['a', 'a']] | [['a', 'a'], []] | Failed |
SHA-256 / dd179f402ddde9c628159e86a9d99d970d09529faadd096839ffc3080e8db851
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=list(g); return [a,list(g)]
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('two values', solve(['a', 'b']), [['a', 'b'], []])
check('singleton', solve(['a']), [['a'], []])
check('empty', solve([]), [[], []])
check('duplicates', solve(['a', 'a']), [['a', 'a'], []])
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 |
|---|---|---|---|
| two values | [['a', 'b'], []] | [['a', 'b'], []] | Passed |
| singleton | [['a'], []] | [['a'], []] | Passed |
| empty | [[], []] | [[], []] | Passed |
| duplicates | [['a', 'a'], []] | [['a', 'a'], []] | Passed |
SHA-256 / e78889562d24f5bc79bdb3d598b9a984230da2ca38f46135599d915ea3dd949a
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 2026-09-29T14:37:41.471231+00:00.
Case digest / 6dde0d65623c0ca3c5b4e64ba0fe32e76534fd9f2d59d2119b718be11ce91b81