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Text Preprocessing in Python | Set – 1
  • Difficulty Level : Basic
  • Last Updated : 26 Apr, 2020

Prerequisites: Introduction to NLP

Whenever we have textual data, we need to apply several pre-processing steps to the data to transform words into numerical features that work with machine learning algorithms. The pre-processing steps for a problem depend mainly on the domain and the problem itself, hence, we don’t need to apply all steps to every problem.

In this article, we are going to see text preprocessing in Python. We will be using the NLTK (Natural Language Toolkit) library here.




# import the necessary libraries
import nltk
import string
import re

Text Lowercase:

We lowercase the text to reduce the size of the vocabulary of our text data.




def text_lowercase(text):
    return text.lower()
  
input_str = "Hey, did you know that the summer break is coming? Amazing right !! It's only 5 more days !!"
text_lowercase(input_str)

Example:



Input: “Hey, did you know that the summer break is coming? Amazing right!! It’s only 5 more days!!”
Output: “hey, did you know that the summer break is coming? amazing right!! it’s only 5 more days!!”

Remove numbers:

We can either remove numbers or convert the numbers into their textual representations.
We can use regular expressions to remove the numbers.




# Remove numbers
def remove_numbers(text):
    result = re.sub(r'\d+', '', text)
    return result
  
input_str = "There are 3 balls in this bag, and 12 in the other one."
remove_numbers(input_str)

Example:

Input: “There are 3 balls in this bag, and 12 in the other one.”
Output: ‘There are balls in this bag, and in the other one.’

We can also convert the numbers into words. This can be done by using the inflect library.




# import the inflect library
import inflect
p = inflect.engine()
  
# convert number into words
def convert_number(text):
    # split string into list of words
    temp_str = text.split()
    # initialise empty list
    new_string = []
  
    for word in temp_str:
        # if word is a digit, convert the digit
        # to numbers and append into the new_string list
        if word.isdigit():
            temp = p.number_to_words(word)
            new_string.append(temp)
  
        # append the word as it is
        else:
            new_string.append(word)
  
    # join the words of new_string to form a string
    temp_str = ' '.join(new_string)
    return temp_str
  
input_str = 'There are 3 balls in this bag, and 12 in the other one.'
convert_number(input_str)

Example:

Input: “There are 3 balls in this bag, and 12 in the other one.”
Output: “There are three balls in this bag, and twelve in the other one.”

Remove punctuation:

We remove punctuations so that we don’t have different forms of the same word. If we don’t remove the punctuation, then been. been, been! will be treated separately.






# remove punctuation
def remove_punctuation(text):
    translator = str.maketrans('', '', string.punctuation)
    return text.translate(translator)
  
input_str = "Hey, did you know that the summer break is coming? Amazing right !! It's only 5 more days !!"
remove_punctuation(input_str)

Example:

Input: “Hey, did you know that the summer break is coming? Amazing right!! It’s only 5 more days!!”
Output: “Hey did you know that the summer break is coming Amazing right Its only 5 more days”

Remove whitespaces:

We can use the join and split function to remove all the white spaces in a string.




# remove whitespace from text
def remove_whitespace(text):
    return  " ".join(text.split())
  
input_str = "   we don't need   the given questions"
remove_whitespace(input_str)

Example:

Input: "   we don't need   the given questions"
Output: "we don't need the given questions"

Remove default stopwords:

Stopwords are words that do not contribute to the meaning of a sentence. Hence, they can safely be removed without causing any change in the meaning of the sentence. The NLTK library has a set of stopwords and we can use these to remove stopwords from our text and return a list of word tokens.




from nltk.corpus import stopwords
from nltk.tokenize import word_tokenize
  
# remove stopwords function
def remove_stopwords(text):
    stop_words = set(stopwords.words("english"))
    word_tokens = word_tokenize(text)
    filtered_text = [word for word in word_tokens if word not in stop_words]
    return filtered_text
  
example_text = "This is a sample sentence and we are going to remove the stopwords from this."
remove_stopwords(example_text)

Example:

Input: “This is a sample sentence and we are going to remove the stopwords from this”
Output: [‘This’, ‘sample’, ‘sentence’, ‘going’, ‘remove’, ‘stopwords’]

Stemming:

Stemming is the process of getting the root form of a word. Stem or root is the part to which inflectional affixes (-ed, -ize, -de, -s, etc.) are added. The stem of a word is created by removing the prefix or suffix of a word. So, stemming a word may not result in actual words.

Example:

books      --->    book
looked     --->    look
denied     --->    deni
flies      --->    fli


If the text is not in tokens, then we need to convert it into tokens. After we have converted strings of text into tokens, we can convert the word tokens into their root form. There are mainly three algorithms for stemming. These are the Porter Stemmer, the Snowball Stemmer and the Lancaster Stemmer. Porter Stemmer is the most common among them.




from nltk.stem.porter import PorterStemmer
from nltk.tokenize import word_tokenize
stemmer = PorterStemmer()
  
# stem words in the list of tokenised words
def stem_words(text):
    word_tokens = word_tokenize(text)
    stems = [stemmer.stem(word) for word in word_tokens]
    return stems
  
text = 'data science uses scientific methods algorithms and many types of processes'
stem_words(text)

Example:

Input: ‘data science uses scientific methods algorithms and many types of processes’
Output: [‘data’, ‘scienc’, ‘use’, ‘scientif’, ‘method’, ‘algorithm’, ‘and’, ‘mani’, ‘type’, ‘of’, ‘process’]

Lemmatization:

Like stemming, lemmatization also converts a word to its root form. The only difference is that lemmatization ensures that the root word belongs to the language. We will get valid words if we use lemmatization. In NLTK, we use the WordNetLemmatizer to get the lemmas of words. We also need to provide a context for the lemmatization. So, we add the part-of-speech as a parameter.




from nltk.stem import WordNetLemmatizer
from nltk.tokenize import word_tokenize
lemmatizer = WordNetLemmatizer()
# lemmatize string
def lemmatize_word(text):
    word_tokens = word_tokenize(text)
    # provide context i.e. part-of-speech
    lemmas = [lemmatizer.lemmatize(word, pos ='v') for word in word_tokens]
    return lemmas
  
text = 'data science uses scientific methods algorithms and many types of processes'
lemmatize_word(text)

Example:

Input: ‘data science uses scientific methods algorithms and many types of processes’
Output: [‘data’, ‘science’, ‘use’, ‘scientific’, ‘methods’, ‘algorithms’, ‘and’, ‘many’, ‘type’, ‘of’, ‘process’]

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