FA-47841 / Delimited text / Open access

A BOM hides the delimiter preamble · case 01

A structured table violates the declared record or column contract.

Verified by executionVariant 1 · 9 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Preamble recognition runs on text still carrying its file signature.

THE FAILURE

Preamble recognition runs on text still carrying its file signature.

Unsuccessful approach: Removing only a stand-alone signature does not handle a signature before sep=.

Case contract

Read a table with optional exact first line sep=<single nonquote, nonnewline character>. A first-line BOM is removed before recognizing this directive. Without directive use comma. The directive is not data. Later sep= lines are ordinary records. Return delimiter and split records; empty input has no records.

Why this case matters

Delimited interchange needs explicit framing, schema and field semantics at ingestion and emission boundaries.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
def _vary(value):
    if value == '@END': return 3 + 4*N
    if isinstance(value, str): return value.replace('@', 'cell' * N)
    if isinstance(value, list): return [_vary(x) for x in value]
    if isinstance(value, dict): return {_vary(k): _vary(v) for k,v in value.items()}
    return value
N = 1
observations = []
def solve(data):
    lines = list(data)
    if not lines: return [',', []]
    if False: lines[0] = lines[0][1:]
    delimiter = ','
    if lines[0].startswith('sep='):
        value = lines.pop(0)[4:]
        if len(value) != 1 or value in '\r\n"': return None
        delimiter = value
    return [delimiter, [line.split(delimiter) for line in lines]]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('custom', solve(_vary(['sep=;', '@;x'])), _vary([';', [['@', 'x']]]))
check('BOM directive', solve(_vary(['\ufeffsep=;', '@;x'])), _vary([';', [['@', 'x']]]))
check('default', solve(_vary(['@,x'])), _vary([',', [['@', 'x']]]))
check('later directive', solve(_vary(['a,b', 'sep=;'])), _vary([',', [['a', 'b'], ['sep=;']]]))
check('bad multi', solve(_vary(['sep=;;', 'x'])), _vary(None))
check('bad quote', solve(_vary(['sep="', 'x'])), _vary(None))
check('empty marker', solve(_vary(['sep=', 'x'])), _vary(None))
check('directive only', solve(_vary(['sep=;'])), _vary([';', []]))
check('zero', solve(_vary([])), _vary([',', []]))
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
custom	`[';', [['cell', 'x']]]`	`[';', [['cell', 'x']]]`	Passed
BOM directive	`[',', [['\ufeffsep=;'], ['cell;x']]]`	`[';', [['cell', 'x']]]`	Failed
default	`[',', [['cell', 'x']]]`	`[',', [['cell', 'x']]]`	Passed
later directive	`[',', [['a', 'b'], ['sep=;']]]`	`[',', [['a', 'b'], ['sep=;']]]`	Passed
bad multi	`None`	`None`	Passed
bad quote	`None`	`None`	Passed
empty marker	`None`	`None`	Passed
directive only	`[';', []]`	`[';', []]`	Passed
zero	`[',', []]`	`[',', []]`	Passed

SHA-256 / 82f2f0ceb26e8b0b929be1d95084889a69f17ef73bce97e2c77dfa101a3104de

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
def _vary(value):
    if value == '@END': return 3 + 4*N
    if isinstance(value, str): return value.replace('@', 'cell' * N)
    if isinstance(value, list): return [_vary(x) for x in value]
    if isinstance(value, dict): return {_vary(k): _vary(v) for k,v in value.items()}
    return value
N = 1
observations = []
def solve(data):
    lines = list(data)
    if not lines: return [',', []]
    if lines[0] == '\ufeff': lines[0] = ''
    delimiter = ','
    if lines[0].startswith('sep='):
        value = lines.pop(0)[4:]
        if len(value) != 1 or value in '\r\n"': return None
        delimiter = value
    return [delimiter, [line.split(delimiter) for line in lines]]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('custom', solve(_vary(['sep=;', '@;x'])), _vary([';', [['@', 'x']]]))
check('BOM directive', solve(_vary(['\ufeffsep=;', '@;x'])), _vary([';', [['@', 'x']]]))
check('default', solve(_vary(['@,x'])), _vary([',', [['@', 'x']]]))
check('later directive', solve(_vary(['a,b', 'sep=;'])), _vary([',', [['a', 'b'], ['sep=;']]]))
check('bad multi', solve(_vary(['sep=;;', 'x'])), _vary(None))
check('bad quote', solve(_vary(['sep="', 'x'])), _vary(None))
check('empty marker', solve(_vary(['sep=', 'x'])), _vary(None))
check('directive only', solve(_vary(['sep=;'])), _vary([';', []]))
check('zero', solve(_vary([])), _vary([',', []]))
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
custom	`[';', [['cell', 'x']]]`	`[';', [['cell', 'x']]]`	Passed
BOM directive	`[',', [['\ufeffsep=;'], ['cell;x']]]`	`[';', [['cell', 'x']]]`	Failed
default	`[',', [['cell', 'x']]]`	`[',', [['cell', 'x']]]`	Passed
later directive	`[',', [['a', 'b'], ['sep=;']]]`	`[',', [['a', 'b'], ['sep=;']]]`	Passed
bad multi	`None`	`None`	Passed
bad quote	`None`	`None`	Passed
empty marker	`None`	`None`	Passed
directive only	`[';', []]`	`[';', []]`	Passed
zero	`[',', []]`	`[',', []]`	Passed

SHA-256 / 77cc43296e5973814dd0b81a44a04faf5de73c527ac53b1f728b6fd06229486c

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 9 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

Verification & scope

Deterministic bounded in-memory model. No claim of complete CSV or external format conformance. 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 2026-09-29T14:44:45.478995+00:00.

Case digest / 38762df5a875fc57a5de96ffdae89be6ec18a902ed3bded88eb0932b2a92faaf