# difflib - Helpers for computing deltas (Python docs)

## Metadata

| Field | Value |
|---|---|
| Institute | Molodetz Institute — Research & Retrieval Division |
| Subject | fuzzy-matching-selectors |
| Document type | Source Dossier |
| Tier | 1 |
| Source title | difflib - Helpers for computing deltas (Python docs) |
| Source URL | https://docs.python.org/3/library/difflib.html |
| Source publisher / author | Python Software Foundation |
| Source type | vendor docs |
| 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

Official documentation of SequenceMatcher (Ratcliff/Obershelp variant) and get_close_matches.

## Key findings

- ratio() = 2.0*M/T; over 0.6 is a rule of thumb for close matches; quick_ratio and real_quick_ratio are upper bounds.
- autojunk: for a second sequence of 200+ items, items above 1% frequency are junk; asymmetric.
- get_close_matches(word, possibilities, n=3, cutoff=0.6) returns the best matches sorted by score, most similar first.
- The algorithm finds the longest contiguous common block then recurses left and right; the docs say the base algorithm is cubic worst case, quadratic expected, and difflib's is quadratic worst case.
- Examples: get_close_matches('appel', ['ape','apple','peach','puppy']) = ['apple','ape'].

## Pros / strengths

Canonical stdlib reference with a published cutoff default (0.6).

## Cons / limitations / caveats

Gestalt matching has no boundary or camelCase awareness and is asymmetric in its junk heuristic. The result is a heuristic, not a minimal edit.

## Relevance to tai

tai already contains `sequencematcher.cpp`; it was evaluated and not used for ids because it lacks boundary bonuses. The 0.6 cutoff default informed choosing a documented floor (tai: 0.3, lower on purpose, see the conclusion).

## Changelog

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