Episode 2: Preprocessing

Last updated on 2024-07-09 | Edit this page

Overview

Questions

  • What different types of preprocessing steps are there?
  • Why we need preprocessing?
  • What are the consequences of applying data preprocessing on our text?

Objectives

After following this lesson, learners will be able to:

  • Explain what preprocessing means.
  • Perform lowercasing, handling new lines, tokenization, stop words removal, part-of-speech tagging, stemmatization/lemmatization.
  • Apply and use a spacy pretrained model.

Preprocessing

NLP models work by learning the statistical regularities within the constituent parts of the language (i.e, letters, digits, words and sentences) in a text. Before applying these models, the input text must often be modified to make it into a format that is better interpretable by the model. This operation is known as data preprocessing and its goal is to make the text ready to be processed by the model. Applying these preprocessing steps will give better results in the end.

Examples of preprocessing steps are:

  • tokenization: this means splitting up the text into individual tokens. You can for example create sentence tokens or words tokens, or any others.
  • lowercasing
  • stop words removal, where you remove common words such as the or a that you would note need in some further analysis steps.
  • lemmatization: with this step you obtain the lemma of each word. You would get the form in which you find the word in the dictionary, such as the singular version of a plural of a noun, or the first person present tense of a verb instead of the past principle. You are making sure that you do not get different versions of the same word: you would convert words into word and talking into talk
  • part of speech tagging: This means that you identify what type of word each is; such as nouns and verbs.

The above examples of techniques of data preprocessing modify the input text to make it interpretable and analyzable by the NLP model of our choice. Here we will go through all these steps to be aware of which steps can be performed and what are their consequences. However, It is important to realize that you do not always need to do all the preprocessing steps, and which ones you should do depends on what you want to do. For example, if you want to extract entities from the text using named entity recognition, you explicitly do not want to lowercase the text, as captials are one component in the odentification. Another important thing is that NLP tasks and the preprocessing steps can be very diffent for different languages. This is even more so if you are comparing steps for alphabetical languages such as English to those for non-alphabetical languages such as Chinese.

Loading the corpus

In order to start the preprocessing we first load in the data. For that we need a number of python packages.

PYTHON 

# import packages
import spacy
import io
import matplotlib.pyplot as plt

We can then open the text file that contains the text and save it in a variable called corpus_full.

PYTHON 

# Load the book The case-book of Sherlock Holmes by Arthur Conan Doyle
path = "../pg69700.txt"
f = io.open(path, mode="r", encoding="utf-8-sig")
corpus_full = f.read()

Let’s check out the start of the corpus

PYTHON 

# Print the text
print(corpus_full[0:1000])

This shows that the corpus contains a lot of text before the actual first story starts. Let’s therefore select the part of the corpus_full that contains the first story. We determined beforehand which part of the string corpus_full catches the first story, and we can save it in the parameter corpus:

PYTHON 

# Select the first story
corpus = corpus_full[5048:200000]

Let’s again have a look at what the text looks like:

PYTHON 

print(corpus)

The print statement automatically formats the text. We can also have a look at what the unformatted text looks like:

PYTHON 

corpus

This shows that there are things in there such as \n which defines new lines. This is one of the things we want to eliminate from the text in the preprocessing steps so that we have a more analyzable text to work with.

Tokenization

We will now start by splitting the text up into individual sentences and words. This process is referred to as tokenizing; instead of having one long text we will create individual tokens.

Tokens can be defined in different ways: here we will first split text up into sentence tokens, so that each token represents one sentence in the text. Then we will extract the word tokens, where each token is one word.

Individual sentences and words

Sentences are separated by points, and words are separated by spaces, we can use this information to split the text. However, we saw that when we printed the corpus, that the text is not so ‘clean’. If we were to split the text now using points, there would be a lot of redundant symbols that we do not want to include in the individual sentences and words, such as the \n symbols, but also we do not want to include punctuation symbols in our sentences and words. So let’s remove these from the text before splitting it up based on.

Sentences

The text can be split into sentences based on points. From the corpus as we have it, we do not want to include the end of line symbols, backslashes before apostrophes, and any double spaces that might occur from new lines or new pages.

We will define corpus_sentences to do all preprocessing steps we need to split the text into individual sentences. First we replace the end of lines and backslashes:

PYTHON 

# Replace newlines with spaces:
corpus_sentences = corpus.replace("\n", " ")

# Replace backslashes
corpus_sentences = corpus_sentences.replace("\"", "")

Then we can replace the double spaces with single spaces. However, there might be multiple double spaces in the text after one another. To catch these, we can repeat the action of replacing double spaces a couple of times, using a loop:

PYTHON 

# Replace double spaces with single spaces
for i in range(10):
      corpus_sentences = corpus_sentences.replace("  ", " ")
      i = i + 1

PYTHON 

# Check that there a no more double spaces
"  " in corpus_sentences

Indeed there a no more double spaces.

Now we are ready to split the text into sentences based on points:

PYTHON 

sentences = corpus_sentences.split(". ")

What this does it that the corpus_sentences is split up every time a .is found, and the results are stored in a python list.

If we print the first 20 items in the resulting list, we can see that indeed the data is split up into sentences, but there are some mistakes, where for example a new sentence is defined because the word ‘mister’ was abbreviated which also resulted in a new sentence definition. This shows that these kind of steps will never be fully perfect, but it good enough to proceed.

PYTHON 

sentences[0:20]

Words

We can now procede from corpus_sentences to split the corpus into individual words based on spaces. To get ‘clean words’ we need to replace some more punctuation marks, so that these are not included in the list of words.

Let’s first define the punctuation marks we want to remove:

PYTHON 

# Punctuation symbols
punctuation = (".", ",", ":", ";", "(", ")", "!", "?", "\"")

Then we go over all these punctuation symbols one by one using a loop to replace them:

PYTHON 

# Loop over the punctuation symbols to remove them
corpus_words = corpus_sentences

for punct in punctuation:
      corpus_words = corpus_words.replace(punct, "")

# Again replace double spaces with single spaces
for i in range(10):
      corpus_words = corpus_words.replace("  ", " ")
      i = i + 1

Next, we should lowercase all text so that we don’t get a word in two forms in the list, once with capital, once without, and have a consistent list of words:

PYTHON 

# Lowercase the text
corpus_words = corpus_words.lower()

Now we can split the text into individual words based by splitting them up on every space:

PYTHON 

words = corpus_words.split(" ")
words

Using a spacy pipeline to analyse texts

Before the break we did a number of preprocessing steps to get the sentence tokens and word tokens. We took the following steps:

  • We loaded the corpus into one long string and selected the part of the string that we wanted to analyse, which is the first story
  • We replaced new lines with spaces and removed all double spaces.
  • We split the string into sentences based on points To continue getting the individual words:
  • we removed punctuation marks
  • removed double spaces
  • we lowercased the text
  • We split the text into a list of words based on spaces.
  • We selected all individual words by converting the list into a set.

We did all these steps by hand, to get an understanding of what is needed to create the tokens. However these steps can also be done with a Python package, where these things happen behind the scenes. We will now start using this package to look at the results of the preprocessing steps of stop word removal, stemming and part-of-speech tagging.

Spacy NLP pipeline

There are multiple python packages that can be used to for NLP, such as Spacy, NLTK, Gensim and PyTorch. Here we will be using the Spacy package.

Let’s first load a few packages that we will be using:

PYTHON 

import spacy
from wordcloud import WordCloud, STOPWORDS, ImageColorGenerator

The pipeline that we are going to

The pipeline that we are going to use is called en_core_web_md. This a pipeline from Spacy that is pretrained to do a number of NLP tasks for English texts. We first have to download this model model from the Spacy library:

PYTHON 

!python -m spacy download en_core_web_sm

It is important to realize that in many cases you can use a pretrained model such as this one. You then do not have to do any training of your data. This is very nice, because training a model requires a whole lot of data, that would have to be analyzed by hand before you can start. It also requires a lot of specific understanding of NLP and a lot of time, and often it is simply not neccesary. These available models are trained on a lot of data, and have very good accuracy.

Free available pretrained models can be found on Hugging Face, along with instructions on how to use them. This website contains a lot of models trained for specific tasks and use cases. It also contains data sets that can be used to train new models.

Let’s load the model:

PYTHON 

nlp = spacy.load('en_core_web_md')

We can check out which components are available in this pipeline:

PYTHON 

# loaded components
print("components:", nlp.component_names)

Let’s now apply this pipeline on our data. Remember that we as a first step before the break we loaded the data in the variable called corpus. We now give this as an argument to the pipeline; this will apply the model to our specific data; such that we have all components of the pipeline available on our specific corpus:

PYTHON 

# apply model to our corpus
doc = nlp(corpus)

On of the things that the pipeline does, is tokenization as we did in the first part. We can now check out the sentence tokens like this:

PYTHON 

# Get sentences
for sentence in doc.sents:
    print(sentence)

and the word tokens like this:

PYTHON 

# Get word tokens
for token in doc[0:6]:
    print(token.text)

Stop word removal

If we want to get an idea of what the text is about we can visualize the word tokens in a word cloud to see which words are most common. To do this we can define a function:

PYTHON 

from wordcloud import WordCloud

# Define function that returns a word cloud
def plot_wordcloud(sw = (""), doc = doc):
      wc = WordCloud(stopwords=sw).generate(str(doc))
      plt.imshow(wc, interpolation='bilinear')
      plt.axis("off")
      return plt.show()

PYTHON 

plot_wordcloud(doc=doc)

From this we get no idea what the text is about because the most common words are word such as ‘the’, ‘a’, and ‘I’, which are referred to as stopwords. We could therefore want to remove these stopwords. The spacy package has a list of stopwords available. Let’s have a look at these:

PYTHON 

# Stop word removal
stopwords = nlp.Defaults.stop_words

print(len(stopwords))
print(stopwords)

The pipeline has 326 stopwords, and if we have a look at them you could indeed say that these words do not add much if we want to get an idea of what the text is about. So let’s create the word cloud again, but without the stopwords:

PYTHON 

plot_wordcloud(sw= stopwords, doc = doc)

This shows that Holmes is the most common word in the text, as one might expect. There are also words in this word cloud that would also consider as stop words in this case, such as said and know. If you would want to remove these as well you can add them to the list of stopwords that we used.

Lemmatization

Let’s now have a look at the lemmatization. From the wordcloud, we can see that one of the most common words in the text is the word said. This is past tense of the word say. If we want all the words referring to the word say we should look at the lemmatized text. We saw in the pipeline that this is also one of the components of the pipeline, so we already have all the lemmas available. We can check them out using:

PYTHON 

# Lemmas
for token in doc:
      print(token.text, token.lemma_)

Here we can for example see that even the n't is recognized as not.

Part-of-speech tagging

The last thing we want to look at right now is part-of-speech tagging. The loaded model can tell for each word token what type of word it is grammatically. We can access these as follows:

PYTHON 

# Part-of-speech tags
for token in doc:
    print(token.text, token.pos_)

It recognizes determiners, nouns, adpositions, and more. But we can also see that it is not perfect and mistakes are made. That is something important to remember; any model, pretrained or if you train it yourself: there are always mistakes in it.

Challenge

We have gone through various data preprocessing techniques in this episode. Now that you know how to apply them all, let’s see how they affect each other.

  • Above we removed the stopwords from the text before lemmatization. What happens if you use the lemmatized text? Create a word cloud of your results.
  • The word clouds that we created can give an idea on what the text is about. However, there are still some terms in the word cloud that are not so useful to do this aim. Which further words would you remove? Add them to the stop words to improve your word cloud so that it better represents to subject of the text.
  • Lemmatized word cloud

The doc can be created to consist only of lemma’s as follows:

PYTHON 

lemmas = ' '.join([token.lemma_ for token in doc])

Create the word cloud using the lemmatized text and the stopwords we defined earlier.

PYTHON 

plot_wordcloud(doc=lemmas, sw=stopwords)
  • Additional stop words

Add some more words to the stopwords set:

PYTHON 

add_stopwords = ['ask', 'tell', 'like', 'want', 'case', 'come']
new_stopwords = stopwords.update(set(add_stopwords))

Create the word cloud:

PYTHON 

plot_wordcloud(doc=lemmas, sw=new_stopwords)

Key Points

  • Preprocessing involves a number of steps that one can apply to their text to prepare it for further processing.
  • Preprocessing is important because it can improve your results
  • You do not always need to do all preprocessing steps. It depends on the task at hand which preprocessing steps are important.
  • A number of preprocessing steps are: lowercasing, tokenization, stop word removal, lemmatization, part-of-speech tagging.
  • Often you can use a pretrained model to process and analyse your data.