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Snowball stemming library collection for Python
===============================================
Python 3 (>= 3.3) is supported. We no longer actively support Python 2 as
the Python developers stopped supporting it at the start of 2020. Snowball
2.1.0 was the last release to officially support Python 2.
What is Stemming?
-----------------
Stemming maps different forms of the same word to a common "stem" - for
example, the English stemmer maps *connection*, *connections*,
*connective*,
*connected*, and *connecting* to *connect*. So a searching for *connected*
would also find documents which only have the other forms.
This stem form is often a word itself, but this is not always the case as
this
is not a requirement for text search systems, which are the intended field
of
use. We also aim to conflate words with the same meaning, rather than all
words with a common linguistic root (so *awe* and *awful* don't have the
same
stem), and over-stemming is more problematic than under-stemming so we tend
not
to stem in cases that are hard to resolve. If you want to always reduce
words
to a root form and/or get a root form which is itself a word then
Snowball's
stemming algorithms likely aren't the right answer.
How to use library
------------------
The snowballstemmer module has two functions.
The ``snowballstemmer.algorithms`` function returns a list of available
algorithm names.
The ``snowballstemmer.stemmer function takes an algorithm name and returns
a
Stemmer`` object.
Stemmer objects have a ``Stemmer.stemWord(word) method and a
Stemmer.stemWords(word[])`` method.
.. code-block:: python
import snowballstemmer
stemmer = snowballstemmer.stemmer('english');
print(stemmer.stemWords("We are the world".split()));
Automatic Acceleration
----------------------
[PyStemmer] is a wrapper module for
Snowball's libstemmer_c and should provide results 100% compatible to
**snowballstemmer**.
**PyStemmer** is faster because it wraps generated C versions of the
stemmers;
**snowballstemmer** uses generate Python code and is slower but offers a
pure
Python solution.
If PyStemmer is installed, ``snowballstemmer.stemmer returns a PyStemmer
Stemmer object which provides the same Stemmer.stemWord() and
Stemmer.stemWords()`` methods.
Benchmark
~~~~~~~~~
This is a crude benchmark which measures the time for running each stemmer
on
every word in its sample vocabulary (10,787,583 words over 26 languages).
It's
not a realistic test of normal use as a real application would do much more
than just stemming. It's also skewed towards the stemmers which do more
work
per word and towards those with larger sample vocabularies.
* Python 2.7 + **snowballstemmer** : 13m00s (15.0 * PyStemmer)
* Python 3.7 + **snowballstemmer** : 12m19s (14.2 * PyStemmer)
* PyPy 7.1.1 (Python 2.7.13) + **snowballstemmer** : 2m14s (2.6 *
PyStemmer)
* PyPy 7.1.1 (Python 3.6.1) + **snowballstemmer** : 1m46s (2.0 * PyStemmer)
* Python 2.7 + **PyStemmer** : 52s
For reference the equivalent test for C runs in 9 seconds.
These results are for Snowball 2.0.0. They're likely to evolve over time
as
the code Snowball generates for both Python and C continues to improve (for
a much older test over a different set of stemmers using Python 2.7,
**snowballstemmer** was 30 times slower than **PyStemmer**, or 9 times
slower
with **PyPy**).
The message to take away is that if you're stemming a lot of words you
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