# CPython Lib/traceback.py (did-you-mean suggestions)

## Metadata

| Field | Value |
|---|---|
| Institute | Molodetz Institute — Research & Retrieval Division |
| Subject | fuzzy-matching-selectors |
| Document type | Source Dossier |
| Tier | 1 |
| Source title | CPython Lib/traceback.py (did-you-mean suggestions) |
| Source URL | https://raw.githubusercontent.com/python/cpython/main/Lib/traceback.py |
| Source publisher / author | Python core developers |
| Source type | repository |
| Source date (published) | Unknown — not stated |
| Date researched | 2026-10-25 |
| Researched by | tai (agent), model: Claude Sonnet 5, on behalf of retoor |
| Research method | direct page fetch |
| Confidence | High |
| Status | Active |

## Summary

The pure-Python fallback of CPython's NameError/AttributeError suggestion engine, a widely shipped 'closest name' resolver.

## Key findings

- Constants: _MOVE_COST 2, _CASE_COST 1 (a substitution that differs only in case), _MAX_CANDIDATE_ITEMS 750, _MAX_STRING_SIZE 40.
- Maximum accepted distance: (len(candidate) + len(wrong_name) + 3) * _MOVE_COST // 6, additionally capped below the best distance so far.
- Exact matches are skipped; a candidate replaces the current suggestion only on a strictly smaller distance, so ties go to the first candidate in iteration order.
- Early exits trim common prefix/suffix, bail when the length difference alone exceeds the budget, and refuse strings over 40 characters or lists over 750 candidates.

## Pros / strengths

Shows that a production resolver caps candidate count and string length and resolves ties by first-seen, matching tai's static-ceiling and deterministic-tie rules.

## Cons / limitations / caveats

Distance-threshold design rejects far matches by construction, the opposite of 'always pick the closest'. Names only, not ids with version numbers.

## Relevance to tai

Confirmed ceilings (tai: 4096 candidates, 64/256 bytes) and first-wins ties. Its reject-by-threshold policy was deliberately not adopted for explicit `@agent:model`.

## Changelog

| Date | Change |
|---|---|
| 2026-10-25 | Initial dossier created. |