FA-4121 / Sequence semantics / Open access
Last element default · case 01
The operation returns a result or retained state that violates this contract: Return the last element, or None only when the list is empty.
ROOT CAUSE
The first positional element is selected instead of the last.
VERIFIED REPAIR
Return the last element, or None only when the list is empty.
Unsuccessful approach: The lexical maximum need not be the final element.
Case contract
Return the last element, or None only when the list 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
N = 1
observations = []
def solve(x, y=None):
return x[0] if x else None
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('empty collection', solve([]), None)
check('singleton collection', solve(['a']), 'a')
check('two distinct elements', solve(['a', 'b']), 'b')
check('nonadjacent duplicate and unsorted order', solve(['b', 'a', 'b', 'c']), 'c')
check('last is not maximum', solve(['c','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 |
|---|---|---|---|
| empty collection | None | None | Passed |
| singleton collection | a | a | Passed |
| two distinct elements | a | b | Failed |
| nonadjacent duplicate and unsorted order | b | c | Failed |
| last is not maximum | c | a | Failed |
SHA-256 / a2827f8d1dac9a69b3eb0da540143e21ac0706e95f0dbd4b59d74d863a18f050
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 max(x) if x else None
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('empty collection', solve([]), None)
check('singleton collection', solve(['a']), 'a')
check('two distinct elements', solve(['a', 'b']), 'b')
check('nonadjacent duplicate and unsorted order', solve(['b', 'a', 'b', 'c']), 'c')
check('last is not maximum', solve(['c','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 |
|---|---|---|---|
| empty collection | None | None | Passed |
| singleton collection | a | a | Passed |
| two distinct elements | b | b | Passed |
| nonadjacent duplicate and unsorted order | c | c | Passed |
| last is not maximum | c | a | Failed |
SHA-256 / c8ac35b7d3229507354698622752adf6de236e4e449aabce986c4cd139b7d5f4
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 x[-1] if x else None
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('empty collection', solve([]), None)
check('singleton collection', solve(['a']), 'a')
check('two distinct elements', solve(['a', 'b']), 'b')
check('nonadjacent duplicate and unsorted order', solve(['b', 'a', 'b', 'c']), 'c')
check('last is not maximum', solve(['c','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 |
|---|---|---|---|
| empty collection | None | None | Passed |
| singleton collection | a | a | Passed |
| two distinct elements | b | b | Passed |
| nonadjacent duplicate and unsorted order | c | c | Passed |
| last is not maximum | a | a | Passed |
SHA-256 / ef7a65e6bd22cd30d6727bb351c9da5fcc3f0cf9c1b17f53ee3a2baf83a8cbff
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 2026-09-29T14:37:35.783235+00:00.
Case digest / 51844c55763358cda4d8dac53d6bdca53752d82a729ab23ece1feb15532bf4b4