Stemming 可以抽取词的词干或词根形式，NLTK中提供了三种最常用的词干提取器接口

'''基于Porter词干提取算法'''

from nltk.stem.porter import PorterStemmer
porter_stemmer = PorterStemmer()

porter_stemmer.stem(‘multiply’)  #  u’multipli’

''' 基于Lancaster 词干提取算法  '''
from nltk.stem.lancaster import LancasterStemmer

lancaster_stemmer = LancasterStemmer()
lancaster_stemmer.stem(‘multiply’)      #  ‘multiply’ 

'''基于Snowball 词干提取算法   ''''
from nltk.stem import SnowballStemmer

snowball_stemmer = SnowballStemmer(“english”)
snowball_stemmer.stem(‘multiply’)   #  u’multipli’