FA-5061 / Descriptor lookup / Open access

Special method type lookup: Bypasses the instance shadow for an explicit len call · case 01

Bypasses the instance shadow for an explicit __len__ call and changes the required Python-language result.

Verified by executionVariant 1 · 6 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Bypasses the instance shadow for an explicit len call. The executable reproducer isolates this operation from the rest of the contract.

VERIFIED REPAIR

Unsuccessful approach: The alternative still fails because it uses the instance override for an explicit class method invocation.

Case contract

A per-instance __len__ shadows direct attribute calls but does not change implicit len or truth testing. Return the selected native result; declared is an integer from 0 through sys.maxsize, and shadow is any nonnegative integer. Inputs use the finite Python types shown in the fixtures. An error string denotes the specified caught exception, not an unhandled process failure.

Why this case matters

Runs Python standard-library language operations in a deterministic local reproducer. Literal expected answers are authored independently. Related defects share an evaluation group to avoid benchmark split leakage.

1 / The failure

Exit 1

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

N = 1
observations = []
def solve(declared, shadow, operation):
    class C:
        def __len__(self): return declared
    obj=C()
    obj.__len__=lambda: shadow
    if operation=='builtin': return len(obj)
    if operation=='direct': return C.__len__(obj)
    if operation=='class': return C.__len__(obj)
    return bool(obj)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('implicit ignores shadow', solve(2, 9, 'builtin'), 2)
check('direct sees shadow', solve(2, 9, 'direct'), 9)
check('class explicit sees declared', solve(2, 9, 'class'), 2)
check('zero native length false', solve(0, 9, 'truth'), False)
check('positive native length true', solve(2, 0, 'truth'), True)
check('zero shadow direct', solve(2, 0, 'direct'), 0)
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
implicit ignores shadow	`2`	`2`	Passed
direct sees shadow	`2`	`9`	Failed
class explicit sees declared	`2`	`2`	Passed
zero native length false	`False`	`False`	Passed
positive native length true	`True`	`True`	Passed
zero shadow direct	`2`	`0`	Failed

SHA-256 / 9906f478106ae22d65b731a2a8ebea255b6e48092b30bcd41f16560e31c1e0bc

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(declared, shadow, operation):
    class C:
        def __len__(self): return declared
    obj=C()
    obj.__len__=lambda: shadow
    if operation=='builtin': return len(obj)
    if operation=='direct': return obj.__len__()
    if operation=='class': return obj.__len__()
    return bool(obj)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('implicit ignores shadow', solve(2, 9, 'builtin'), 2)
check('direct sees shadow', solve(2, 9, 'direct'), 9)
check('class explicit sees declared', solve(2, 9, 'class'), 2)
check('zero native length false', solve(0, 9, 'truth'), False)
check('positive native length true', solve(2, 0, 'truth'), True)
check('zero shadow direct', solve(2, 0, 'direct'), 0)
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
implicit ignores shadow	`2`	`2`	Passed
direct sees shadow	`9`	`9`	Passed
class explicit sees declared	`9`	`2`	Failed
zero native length false	`False`	`False`	Passed
positive native length true	`True`	`True`	Passed
zero shadow direct	`0`	`0`	Passed

SHA-256 / ab20839fd246573bce950b5308886889e6d7edf7c47332aaa6fb9f288633f8f1

3 / The verified repair

Exit 0

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

N = 1
observations = []
def solve(declared, shadow, operation):
    class C:
        def __len__(self): return declared
    obj=C()
    obj.__len__=lambda: shadow
    if operation=='builtin': return len(obj)
    if operation=='direct': return obj.__len__()
    if operation=='class': return C.__len__(obj)
    return bool(obj)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('implicit ignores shadow', solve(2, 9, 'builtin'), 2)
check('direct sees shadow', solve(2, 9, 'direct'), 9)
check('class explicit sees declared', solve(2, 9, 'class'), 2)
check('zero native length false', solve(0, 9, 'truth'), False)
check('positive native length true', solve(2, 0, 'truth'), True)
check('zero shadow direct', solve(2, 0, 'direct'), 0)
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
implicit ignores shadow	`2`	`2`	Passed
direct sees shadow	`9`	`9`	Passed
class explicit sees declared	`2`	`2`	Passed
zero native length false	`False`	`False`	Passed
positive native length true	`True`	`True`	Passed
zero shadow direct	`0`	`0`	Passed

SHA-256 / a74ebdffc72f8ea8cbc50318adb0514f88f5314a1629dc175b55ea6dea6f20ba

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 2026-09-29T14:37:45.809604+00:00.

Case digest / d704c099bd97cef9e188d03997c19ef5f8a24da17010a3f1e21502240c8b6b1a

Special method type lookup: Bypasses the instance shadow for an explicit __len__ call · case 01

Bypasses the instance shadow for an explicit __len__ call. The executable reproducer isolates this operation from the rest of the contract.