Lemmatizing words with WordNet

Lemmatization is very similar to stemming, but is more akin to synonym replacement. A lemma is a root word, as opposed to the root stem. So unlike stemming, you are always left with a valid word that means the same thing. However, the word you end up with can be completely different. A few examples will explain this.

Getting ready

Make sure that you have unzipped the wordnet corpus in nltk_data/corpora/wordnet. This will allow the WordNetLemmatizer class to access WordNet. You should also be familiar with the part-of-speech tags covered in the Looking up Synsets for a word in WordNet recipe of Chapter 1, Tokenizing Text and WordNet Basics.

How to do it...

We will use the WordNetLemmatizer class to find lemmas:

>>> from nltk.stem import WordNetLemmatizer
>>> lemmatizer = WordNetLemmatizer()
>>> lemmatizer.lemmatize('cooking')
'cooking'
>>> lemmatizer.lemmatize('cooking', pos='v')
'cook'
>>> lemmatizer.lemmatize('cookbooks...