51 lines
2.0 KiB
Markdown
51 lines
2.0 KiB
Markdown
# difflib - Helpers for computing deltas (Python docs)
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## Metadata
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| Field | Value |
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| Institute | Molodetz Institute — Research & Retrieval Division |
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| Subject | fuzzy-matching-selectors |
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| Document type | Source Dossier |
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| Tier | 1 |
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| Source title | difflib - Helpers for computing deltas (Python docs) |
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| Source URL | https://docs.python.org/3/library/difflib.html |
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| Source publisher / author | Python Software Foundation |
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| Source type | vendor docs |
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| Source date (published) | Unknown — not stated |
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| Date researched | 2026-10-25 |
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| Researched by | tai (agent), model: Claude Sonnet 5, on behalf of retoor |
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| Research method | direct page fetch |
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| Confidence | High |
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| Status | Active |
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## Summary
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Official documentation of SequenceMatcher (Ratcliff/Obershelp variant) and get_close_matches.
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## Key findings
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- 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.
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- autojunk: for a second sequence of 200+ items, items above 1% frequency are junk; asymmetric.
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- get_close_matches(word, possibilities, n=3, cutoff=0.6) returns the best matches sorted by score, most similar first.
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- 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.
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- Examples: get_close_matches('appel', ['ape','apple','peach','puppy']) = ['apple','ape'].
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## Pros / strengths
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Canonical stdlib reference with a published cutoff default (0.6).
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## Cons / limitations / caveats
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Gestalt matching has no boundary or camelCase awareness and is asymmetric in its junk heuristic. The result is a heuristic, not a minimal edit.
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## Relevance to tai
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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).
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## Changelog
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| Date | Change |
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| 2026-10-25 | Initial dossier created. |
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