Source code for nltk.stem.cistem

# -*- coding: utf-8 -*-
# Natural Language Toolkit: CISTEM Stemmer for German
# Copyright (C) 2001-2019 NLTK Project
# Author: Leonie Weissweiler <l.weissweiler@outlook.de>
# Algorithm: Leonie Weissweiler <l.weissweiler@outlook.de>
#            Alexander Fraser <fraser@cis.lmu.de>
# URL: <http://nltk.org/>
# For license information, see LICENSE.TXT

from __future__ import unicode_literals
import re
from nltk.stem.api import StemmerI
from nltk.compat import python_2_unicode_compatible

[docs]@python_2_unicode_compatible class Cistem(StemmerI): """ CISTEM Stemmer for German This is the official Python implementation of the CISTEM stemmer. It is based on the paper Leonie Weissweiler, Alexander Fraser (2017). Developing a Stemmer for German Based on a Comparative Analysis of Publicly Available Stemmers. In Proceedings of the German Society for Computational Linguistics and Language Technology (GSCL) which can be read here: http://www.cis.lmu.de/~weissweiler/cistem/ In the paper, we conducted an analysis of publicly available stemmers, developed two gold standards for German stemming and evaluated the stemmers based on the two gold standards. We then proposed the stemmer implemented here and show that it achieves slightly better f-measure than the other stemmers and is thrice as fast as the Snowball stemmer for German while being about as fast as most other stemmers. case_insensitive is a a boolean specifiying if case-insensitive stemming should be used. Case insensitivity improves performance only if words in the text may be incorrectly upper case. For all-lowercase and correctly cased text, best performance is achieved by setting case_insensitive for false. :param case_insensitive: if True, the stemming is case insensitive. False by default. :type case_insensitive: bool """ strip_ge = re.compile(r"^ge(.{4,})") repl_xx = re.compile(r"(.)\1") strip_emr = re.compile(r"e[mr]$") strip_nd = re.compile(r"nd$") strip_t = re.compile(r"t$") strip_esn = re.compile(r"[esn]$") repl_xx_back = re.compile(r"(.)\*") def __init__(self, case_insensitive=False): self._case_insensitive = case_insensitive
[docs] @staticmethod def replace_to(word): word = word.replace("sch", "$") word = word.replace("ei", "%") word = word.replace("ie", "&") word = Cistem.repl_xx.sub(r"\1*", word) return word
[docs] @staticmethod def replace_back(word): word = Cistem.repl_xx_back.sub(r"\1\1", word) word = word.replace("%", "ei") word = word.replace("&", "ie") word = word.replace("$", "sch") return word
[docs] def stem(self, word): """ This method takes the word to be stemmed and returns the stemmed word. :param word: the word that is to be stemmed :type word: unicode :return word: the stemmed word :rtype: unicode >>> from nltk.stem.cistem import Cistem >>> stemmer = Cistem() >>> s1 = "Speicherbehältern" >>> stemmer.stem(s1) 'speicherbehalt' >>> s2 = "Grenzpostens" >>> stemmer.stem(s2) 'grenzpost' >>> s3 = "Ausgefeiltere" >>> stemmer.stem(s3) 'ausgefeilt' >>> stemmer = Cistem(True) >>> stemmer.stem(s1) 'speicherbehal' >>> stemmer.stem(s2) 'grenzpo' >>> stemmer.stem(s3) 'ausgefeil' """ if len(word) == 0: return word upper = word[0].isupper() word = word.lower() word = word.replace("ü", "u") word = word.replace("ö", "o") word = word.replace("ä", "a") word = word.replace("ß", "ss") word = Cistem.strip_ge.sub(r"\1", word) word = Cistem.replace_to(word) while len(word) > 3: if len(word) > 5: (word, success) = Cistem.strip_emr.subn("", word) if success != 0: continue (word, success) = Cistem.strip_nd.subn("", word) if success != 0: continue if not upper or self._case_insensitive: (word, success) = Cistem.strip_t.subn("", word) if success != 0: continue (word, success) = Cistem.strip_esn.subn("", word) if success != 0: continue else: break word = Cistem.replace_back(word) return word
[docs] def segment(self, word): """ This method works very similarly to stem (:func:'cistem.stem'). The difference is that in addition to returning the stem, it also returns the rest that was removed at the end. To be able to return the stem unchanged so the stem and the rest can be concatenated to form the original word, all subsitutions that altered the stem in any other way than by removing letters at the end were left out. :param word: the word that is to be stemmed :type word: unicode :return word: the stemmed word :rtype: unicode :return word: the removed suffix :rtype: unicode >>> from nltk.stem.cistem import Cistem >>> stemmer = Cistem() >>> s1 = "Speicherbehältern" >>> print("('" + stemmer.segment(s1)[0] + "', '" + stemmer.segment(s1)[1] + "')") ('speicherbehält', 'ern') >>> s2 = "Grenzpostens" >>> stemmer.segment(s2) ('grenzpost', 'ens') >>> s3 = "Ausgefeiltere" >>> stemmer.segment(s3) ('ausgefeilt', 'ere') >>> stemmer = Cistem(True) >>> print("('" + stemmer.segment(s1)[0] + "', '" + stemmer.segment(s1)[1] + "')") ('speicherbehäl', 'tern') >>> stemmer.segment(s2) ('grenzpo', 'stens') >>> stemmer.segment(s3) ('ausgefeil', 'tere') """ rest_length = 0 if len(word) == 0: return ("", "") upper = word[0].isupper() word = word.lower() original = word[:] word = Cistem.replace_to(word) while len(word) > 3: if len(word) > 5: (word, success) = Cistem.strip_emr.subn("", word) if success != 0: rest_length += 2 continue (word, success) = Cistem.strip_nd.subn("", word) if success != 0: rest_length += 2 continue if not upper or self._case_insensitive: (word, success) = Cistem.strip_t.subn("", word) if success != 0: rest_length += 1 continue (word, success) = Cistem.strip_esn.subn("", word) if success != 0: rest_length += 1 continue else: break word = Cistem.replace_back(word) if rest_length: rest = original[-rest_length:] else: rest = "" return (word, rest)