radixor — Fastest Stemming for Python

radixor is a Python extension for the Radixor stemmer library, built on a Rust core via PyO3. It provides sub-microsecond per-word stemming with a batch API that amortises the Python↔Rust bridge overhead across thousands of words at once.

Why radixor?

Library	Approach	Batch API
radixor	Compiled patch-command trie in Rust	✅ stem_batch()
PyStemmer (Snowball)	C extension (libstemmer)	✅ stemWords()
snowballstemmer	Pure-Python Snowball	✅ (Python loop)
NLTK Porter / CISTEM	Pure Python	❌

Performance. On the shared UniMorph gold-standard corpus, measuring runtime stemming only (construction excluded) and with a fair, cache-disabled, same-input methodology, radixor won all 18 / 18 direct comparisons with PyStemmer 3.1.0 (Snowball's C libstemmer) in the published 2026-08-08 run. At batch size 100, the geometric-mean speedup was 1.67×. The complete machine metadata and current results are in the Python performance documentation; benchmark implementation and fairness notes are in benchmarks/.

Installation

From PyPI, once publication is enabled:

python -m pip install --only-binary=:all: radixor

The GitHub Releases-backed index is the independent alternative:

python -m pip install --only-binary=:all: \
  --index-url https://leogalambos.github.io/Radixor/python/simple/ radixor

The GitHub command becomes usable after the first model and native releases populate that index. See the installation guide for current availability and source-checkout builds.

Wheels are provided for Linux, macOS, and Windows (Python 3.9+). The install also resolves the mandatory pure radixor-models-standard dependency with 20 precompiled standard models. Building the native source distribution requires Rust ≥ 1.75 and maturin.

Quick start

from radixor import Stemmer

s = Stemmer("en")          # English (us-uk-default model)
s.stem("running")          # → "run"
s.stem("cats")             # → "cat"
s.stem("unknown_word")     # → None

Batch API — the fast path

words = ["running", "cats", "stemming", "quickly"]

# Amortises the Python→Rust bridge cost across all words at once
stems = s.stem_batch(words)
# → ["run", "cat", "stem", "quick"]

For large corpora (tens of thousands of words) the batch call is the recommended interface. It avoids per-call Python frame overhead and keeps the hot loop entirely inside Rust.

Migrating from PyStemmer

Radixor provides PyStemmer's stemWord and stemWords method names. These compatibility methods also follow PyStemmer's fallback behavior: when the trie has no patch command, they return the original word instead of None.

# PyStemmer: import Stemmer
import radixor as Stemmer

stemmer = Stemmer.Stemmer("english")
stemmer.stemWord("running")                 # → "run"
stemmer.stemWord("unknown_word")            # → "unknown_word"
stemmer.stemWords(["running", "unknown"])  # → ["run", "unknown"]

The original Radixor methods remain unchanged: stem and stem_batch return None for words without a matching patch command. Radixor accepts PyStemmer's full language names for the languages represented by its bundled models, as well as its existing two-letter codes and model IDs.

Supported languages

Code	Language	Model ID
cs	Czech	cs-cz-default
da	Danish	da-dk-default
de	German	de-de-default
en	English	us-uk-default
es	Spanish	es-es-default
fa	Persian	fa-ir-default
fi	Finnish	fi-fi-default
fr	French	fr-fr-default
he	Hebrew	he-il-default
hu	Hungarian	hu-hu-default
it	Italian	it-it-default
nb	Norwegian Bokmål	nb-no-default
nl	Dutch	nl-nl-default
nn	Norwegian Nynorsk	nn-no-default
pl	Polish	pl-pl-unimorph
pt	Portuguese	pt-pt-default
ru	Russian	ru-ru-default
sv	Swedish	sv-se-default
uk	Ukrainian	uk-ua-default
yi	Yiddish	yi-default

API reference

Stemmer(language=None, *, path=None, compiled=None, backward=None, store_original=True, lowercase=True, cache_size=10_000)

Create a stemmer for the given language code, model ID, custom textual dictionary, or previously compiled version 7 trie. Textual dictionaries are compiled in Rust; compiled= loads a prepared binary directly.

s = Stemmer("de")                           # by language code
s = Stemmer("de-de-default")               # by model ID
s = Stemmer(path="/data/custom.gz")        # custom gzipped dictionary
s = Stemmer(compiled="/data/custom.rxc")   # prepared v7 binary

backward selects the traversal direction; when left as None it is derived from the language (BACKWARD, except right-to-left fa/he/yi which use FORWARD). store_original (default True) maps each canonical stem to a no-op patch so the stem itself is recognised. lowercase=False skips runtime lowercasing for already-normalized input, and cache_size enables the bounded result cache. The default holds up to 10,000 entries, matching PyStemmer; cache_size=0 disables it. One cache is shared by stem(), stemWord(), stem_batch(), and stemWords(); the stem_all*() methods are not cached.

stem(word: str) → str | None

Return the stem, or None when the compiled trie finds no applicable patch command. This does not mean that lookup is restricted to exact training words.

stem_batch(words: list[str]) → list[str | None]

Stem an entire list. Preferred for large inputs.

stemWord(word: str) → str

PyStemmer-compatible scalar method. Return the original word if it cannot be stemmed.

stemWords(words: list[str]) → list[str]

PyStemmer-compatible batch method. Return each unrecognized word unchanged.

stem_all(word: str) → list[str]

Return all stems ordered by descending corpus frequency. Useful when multiple valid stems exist.

stem_all_batch(words: list[str]) → list[list[str]]

Return all stems for each word in a batch.

Compiling a model (compile once, load instantly)

Compiling the trie from a textual dictionary takes time for large languages (seconds). You can compile it once to Radixor's binary format and then load it near-instantly — the same workflow Java users have:

import radixor
radixor.compile("stemmer.gz", "en.rxc", language="en")   # or backward=True/False
s = radixor.Stemmer(compiled="en.rxc")                    # instant load, no re-compile

The compiled file uses Radixor's v7 trie format and is byte-compatible with the Java StemmerPatchTrieBinaryIO (the inner stream is identical), so a file compiled by Java can be loaded by Python and vice versa. Stemmer(path=...) auto-detects whether it was given a compiled trie or a textual dictionary.

Using a custom model

Provide your own gzipped source dictionary (tab-separated stem<TAB>variant1<TAB>variant2… per line, # / // line remarks allowed) and load it directly:

s = Stemmer(path="my_dictionary.gz")            # BACKWARD by default
s = Stemmer(path="my_rtl_dictionary.gz", backward=False)   # right-to-left

The dictionary is compiled to a patch-command trie in Rust at construction time.

Building from source

cd Radixor/
pip install maturin build setuptools wheel pytest
./gradlew pythonBuildStandardModels
pip install --no-deps build/python/dist/standard/radixor_models_standard-0.0.0-py3-none-any.whl
cd python/
maturin develop --release    # editable install with release optimisations

From the repository root, Gradle builds the native wheel/sdist and pure standard-model wheel/sdist without installing them globally:

./gradlew pythonBuild

The convenience tasks pythonBuildLinux, pythonBuildWindows, and pythonBuildMacos use the host build when the requested platform matches the current system. Other platforms are cross-compiled with the corresponding Rust target and therefore require that target and its linker/SDK to be installed. Override a default target with, for example, -PpythonWindowsTarget=x86_64-pc-windows-gnu. Build artifacts are written below build/python/dist/.

The complete batch benchmark runs Radixor for all bundled languages and every available comparison engine for the languages it supports, using batch sizes 10, 20, 50, and 100:

Comparison engines are auto-detected in the environment of pythonExecutable. Install python/benchmarks/requirements-bench.txt there to enable the complete comparison set.

./gradlew pythonBenchmarkAllLanguagesBatch

Use pythonBenchmarkWords, pythonBenchmarkRepeats, and pythonBenchmarkWarmup Gradle properties to tune the run. CSV and JSON reports are written below build/reports/python-benchmarks/.

Neither runtime distribution contains textual dictionaries. The standard data sdist contains build-ready gzip v7 .rxc files, a checksummed provenance manifest, and per-model CC BY-SA 3.0 notices. They are generated below build/ from canonical models/*/src/modelInput/stemmer.gz inputs and are never stored in Git. ./gradlew regeneratePythonStandardModels performs this deterministic generation; repository topology selects the 20 defaults and excludes optional pl-pl-polimorf.

radixor requires radixor-models-standard>=1.0,<2.0. The Python distribution version is independent of its 2026.1 Java model-catalog identity and of the individual model versions recorded in the manifest.

License

The native/API package is BSD-3-Clause — see LICENSE. Model data is separately licensed under CC BY-SA 3.0 in its packaged notices.