Snowball stemming algorithms, for information retrieval

python, information, retrieval, language, processing, morphological, analysis, stemming, algorithms, stemmers
pip install PyStemmer==2.0.1



What is PyStemmer?

PyStemmer is a Python interface to the stemming algorithms from the Snowball project ( A stemming algorithm (or stemmer) is a process for removing the commoner morphological and inflexional endings from words in English. Its main use is as part of a term normalisation process that is usually done when setting up Information Retrieval systems. A stemmer aims to conflate words with the same linguistic base form, in order that the resulting "stem" may be considered to represent all words with that base form.

Stemmers can be used to make searches more comprehensive. For example, stemming can ensure that a search for 'cars' will also find all documents that contain only 'car'.

Snowball is a small string processing language designed for creating stemming algorithms for use in Information Retrieval. It is also the name of a project to develop a good base set of stemming algorithms.

PyStemmer uses the "libstemmer_c" C interface to the snowball algorithms, provided by the snowball project itself. This library is unmodified, but contained within the PyStemmer distribution. If you wish to upgrade PyStemmer to a more recent version of libstemmer_c (or the snowball algorithms), it should suffice to download a new copy of libstemmer_c from the snowball project, and replace the contents of the libstemmer_c subdirectory with the contents of the download.


Python header files should be installed.

This version of PyStemmer has been tested using Python series 2.6, 2.7, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9 and pypy. Builds are checked with travis:


PyStemmer uses distutils, so all that is necessary to build and install PyStemmer is the usual distutils invocation:

python install

You can also install using pip:

  • from PyPI: pip install pystemmer
  • from a local copy of the code: pip install .
  • from git: pip install git+git://


PyStemmer's API is documented by documentation comments.

A brief overview can be found in docs/quickstart.txt


PyStemmer is copyright (c) 2006, Richard Boulton, and is licensed under the MIT license: see the file "LICENSE" for the full text of this. It is was inspired by an earlier implementation (which was copyright (c) 2001, Andreas Jung, and also licensed under the MIT license, but no portions of which remain in this package, and had a different API).

The snowball algorithms, and the snowball library, are copyright (c) 2001-2006, Dr Martin Porter and Richard Boulton, and are licensed under the BSD license.