Welcome to Natural Language Processing in Python (Part 5)

If you have not seen Part 4 of this tutorial, please refer to the following link:

  1. NLP in Python Part 4 (Blog Post):

  2. NLP in Python Part 4 (YouTube Video):

The companion video to this post on NLP can be viewed here:

The primary goal of this post will be to:

  1. Make use of NLTK’s stemming functionality.

  2. Make use of NLTK’s lemmatization functionality.

Table of Contents of this tutorial:


Let us first focus on the notion of stemming according to Wikipedia:

“Stemming is the process of reducing inflected (or sometimes derived) words to their word stem, base, or root form–generally a written word form.”

That definition is a bit hard to follow, so let us considered an example.

Take the word “fishing”. This word is based on the so-called stem, that is, the word “fish”. Likewise, the stem of “fished”, “fisher”, etc. has the stem “fish”.

Writing your own function to determine the stem of a word is possible, although there are many potential edge cases. Many of these edge cases are automatically accounted for via the stemming tools provided by NLTK.

Applications of Stemming: According to the previously mentioned Wikipeda article on stemming:

“Stemming is used as an approximate method for grouping words with a similar basic meaning together. For example, a text mentioning “daffodils” is probably closely related to a text mentioning “daffodil” (without the “s”). But in some cases, words with the same stem have idiomatic meanings which are not closely related: a user searching for “marketing” will not be satisfied by most documents mentioning “markets” but not “marketing””.

One well-known application of stemming is used when you search in Google. For instance, searching for the term “fish” will also yields results for the term “fishing” as well, since “fish” is the stem of “fishing” and is most likely related to the stem in this case.

One of the stemming algorithms used via NLTK is the so-called Porter Stemmer:

from nltk.stem import PorterStemmer

Let us attempt to determine the stem for the following words in this word list:

porter = PorterStemmer()
word_list = ["connected", "connecting", "connection", "connections"]

for word in word_list:


The Porter Stemmer identifies “connect” as the stem for each of the words in the list above.

Let us take another example list of words:

word_list = ["argue", "argued", "argues", "arguing", "argus"]

for word in word_list:


Note that the term “stem” and “root” are independent. The word “argue” is the root word of the above word list, but according to the definition of “stem”, the term “argu” is the stem.

NLTK also provides access to a number of other stemmer algorithms.

from nltk.stem import LancasterStemmer
from nltk.stem import SnowballStemmer

lancaster = LancasterStemmer()
snowball = SnowballStemmer(language='english')

Using the Lancaster Stemmer on the “argue” word list:

for word in word_list:


Using the Snowball Stemmer on the “argue” word list:

for word in word_list:


Notice that each stemming algorithm provides a different output. Delving into how each of these stemming algorithms work along with what the pros and cons of each are is beyond the scope of this video. However, if you would like a high level overview of when to use a particular stemming algorithm for your purposes, the following StackOverflow answer by Slater Tyranus provides a very well-written and concise summary of each.


According to Wikipedia, the definition of lemmatization is:

“The process of grouping together the inflected forms of a word so they can be analyzed as a single item, identified by the word’s lemma, or dictionary form.

Lemmatization and stemming are related, but different. The difference is that a stemmer operates on a single word without knowledge of the context, and therefore cannot discriminate between words which have different meaning depending on part of speech.

Let us consider some examples of lemmatization and also of stemming to consider the contrast between the two ideas.

from nltk.stem import WordNetLemmatizer

lemmatizer = WordNetLemmatizer()

The WordNetLemmatizer class has a method called lemmatize which takes as arguments a word to lemmatize as well as what part of speech the word happens to be, i.e. noun, verb, adverb, etc.

Let us attempt to determine the lemma for the word “bats”:



By default, the part of speech is noun (unless specified otherwise). Note that the lemmatizer is able to ascertain the lemma of the plural “bats” by the word “bat”.

Note that “bats” can be considered a noun, as in the plural for the type of animal for instance, but it may also be considered a verb, as in to “hit at” something.

We can specify the part of speech to consider the word as by the optional pos argument, standing for “part-of-speech”:

print(lemmatizer.lemmatize("bats", pos="v"))


Let us now consider lemmatizing the word “better”. In fact, let us lemmatize this word when the term better is an adjective, adverb, noun, and verb, respectively.

# Adjective:
print(lemmatizer.lemmatize("better", pos="a"))
# Adverb:
print(lemmatizer.lemmatize("better", pos="r"))

# Noun:
print(lemmatizer.lemmatize("better", pos="n"))

# Verb:
print(lemmatizer.lemmatize("better", pos="v"))

Notice that the lemmatization of “better” when considered to be a noun or verb stays as “better”. Whereas when it is considered as an adjective it lemmatizes to “good” and when the part of speech is an adverb it lemmatizes to “well”.

If you consult Google’s dictionary tool, you will notice this coincides with this categorization as well.


That wraps up this tutorial on natural language processing in Python.

Part 6 of Natural Language Processing in Python

Table of Contents of this tutorial: