FA-4956 / Descriptor lookup / Open access

Data descriptor storage: Lets an instance shadow override a data descriptor · case 01

Lets an instance shadow override a data descriptor and changes the required Python-language result.

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

ROOT CAUSE

Lets an instance shadow override a data descriptor. The executable reproducer isolates this operation from the rest of the contract.

VERIFIED REPAIR

Unsuccessful approach: The alternative still fails because it writes to the shadow field instead of the descriptor backing key.

Case contract

A data descriptor field takes precedence over an instance field shadow. Its backing key is _value, missing reads return 7, writes update that key, deletion removes it, and class access returns class-descriptor. Return [read results,final instance dictionary]. 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(initial, commands):
    class D:
        def __get__(self, obj, owner):
            if obj is None: return 'class-descriptor'
            return obj.__dict__.get('field',obj.__dict__.get('_value',7))
        def __set__(self, obj, value):
            obj.__dict__['_value']=value
        def __delete__(self, obj):
            obj.__dict__.pop('_value',None)
    class C:
        field=D()
    c=C(); c.__dict__.update(initial); out=[]
    for op,value in commands:
        if op=='get': out.append(c.field)
        elif op=='class': out.append(C.field)
        elif op=='set': c.field=value
        elif op=='delete': del c.field
    return [out,dict(c.__dict__)]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('descriptor beats shadow', solve({'field': 99, '_value': 2}, [['get', None], ['class', None]]), [[2, 'class-descriptor'], {'field': 99, '_value': 2}])
check('set and delete dispatch', solve({}, [['set', 5], ['get', None], ['delete', None], ['get', None]]), [[5, 7], {}])
check('zero is stored', solve({'_value': 0}, [['get', None]]), [[0], {'_value': 0}])
check('shadow survives descriptor delete', solve({'field': 4, '_value': 8}, [['delete', None], ['get', None]]), [[7], {'field': 4}])
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
descriptor beats shadow	`[[99, 'class-descriptor'], {'_value': 2, 'field': 99}]`	`[[2, 'class-descriptor'], {'_value': 2, 'field': 99}]`	Failed
set and delete dispatch	`[[5, 7], {}]`	`[[5, 7], {}]`	Passed
zero is stored	`[[0], {'_value': 0}]`	`[[0], {'_value': 0}]`	Passed
shadow survives descriptor delete	`[[4], {'field': 4}]`	`[[7], {'field': 4}]`	Failed

SHA-256 / ada02fbe7f08ad82d9e23e56846a5b21f8c04b22ab639bce1e4c7f9254467411

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(initial, commands):
    class D:
        def __get__(self, obj, owner):
            if obj is None: return 'class-descriptor'
            return obj.__dict__.get('_value',7)
        def __set__(self, obj, value):
            obj.__dict__['field']=value
        def __delete__(self, obj):
            obj.__dict__.pop('_value',None)
    class C:
        field=D()
    c=C(); c.__dict__.update(initial); out=[]
    for op,value in commands:
        if op=='get': out.append(c.field)
        elif op=='class': out.append(C.field)
        elif op=='set': c.field=value
        elif op=='delete': del c.field
    return [out,dict(c.__dict__)]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('descriptor beats shadow', solve({'field': 99, '_value': 2}, [['get', None], ['class', None]]), [[2, 'class-descriptor'], {'field': 99, '_value': 2}])
check('set and delete dispatch', solve({}, [['set', 5], ['get', None], ['delete', None], ['get', None]]), [[5, 7], {}])
check('zero is stored', solve({'_value': 0}, [['get', None]]), [[0], {'_value': 0}])
check('shadow survives descriptor delete', solve({'field': 4, '_value': 8}, [['delete', None], ['get', None]]), [[7], {'field': 4}])
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
descriptor beats shadow	`[[2, 'class-descriptor'], {'_value': 2, 'field': 99}]`	`[[2, 'class-descriptor'], {'_value': 2, 'field': 99}]`	Passed
set and delete dispatch	`[[7, 7], {'field': 5}]`	`[[5, 7], {}]`	Failed
zero is stored	`[[0], {'_value': 0}]`	`[[0], {'_value': 0}]`	Passed
shadow survives descriptor delete	`[[7], {'field': 4}]`	`[[7], {'field': 4}]`	Passed

SHA-256 / 76f6a5aba90662e6a90051b42d16a63253d1825d83c06e7c9ffb7f162ba8d1ac

3 / The verified repair

Exit 0

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

N = 1
observations = []
def solve(initial, commands):
    class D:
        def __get__(self, obj, owner):
            if obj is None: return 'class-descriptor'
            return obj.__dict__.get('_value',7)
        def __set__(self, obj, value):
            obj.__dict__['_value']=value
        def __delete__(self, obj):
            obj.__dict__.pop('_value',None)
    class C:
        field=D()
    c=C(); c.__dict__.update(initial); out=[]
    for op,value in commands:
        if op=='get': out.append(c.field)
        elif op=='class': out.append(C.field)
        elif op=='set': c.field=value
        elif op=='delete': del c.field
    return [out,dict(c.__dict__)]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('descriptor beats shadow', solve({'field': 99, '_value': 2}, [['get', None], ['class', None]]), [[2, 'class-descriptor'], {'field': 99, '_value': 2}])
check('set and delete dispatch', solve({}, [['set', 5], ['get', None], ['delete', None], ['get', None]]), [[5, 7], {}])
check('zero is stored', solve({'_value': 0}, [['get', None]]), [[0], {'_value': 0}])
check('shadow survives descriptor delete', solve({'field': 4, '_value': 8}, [['delete', None], ['get', None]]), [[7], {'field': 4}])
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
descriptor beats shadow	`[[2, 'class-descriptor'], {'_value': 2, 'field': 99}]`	`[[2, 'class-descriptor'], {'_value': 2, 'field': 99}]`	Passed
set and delete dispatch	`[[5, 7], {}]`	`[[5, 7], {}]`	Passed
zero is stored	`[[0], {'_value': 0}]`	`[[0], {'_value': 0}]`	Passed
shadow survives descriptor delete	`[[7], {'field': 4}]`	`[[7], {'field': 4}]`	Passed

SHA-256 / 27a2a5cc04c0bd5a04bb91f5efa124d7580b8de85d4df8668b76c112e7842118

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 2026-09-29T14:37:44.452064+00:00.

Case digest / 420ff62481a2ab8aa86d761bdb67f15ec881e5fc9be8f58d094c1ba9899409b7