NLP Gensim Tutorial – Complete Guide For Beginners

This tutorial is going to provide you with a walk-through of the Gensim library.
Gensim : It is an open source library in python written by Radim Rehurek which is used in unsupervised topic modelling and natural language processing. It is designed to extract semantic topics from documents. It can handle large text collections. Hence it makes it different from other machine learning software packages which target memory processing. Gensim also provides efficient multicore implementations for various algorithms to increase processing speed. It provides more convenient  facilities for text processing than other packages like Scikit-learn, R etc.
This tutorial will cover these concepts: 

1. Create a Corpus from a given Dataset
2. Create a TFIDF matrix in Gensim
3. Create Bigrams and Trigrams with Gensim
4. Create Word2Vec model using Gensim
5. Create Doc2Vec model using Gensim
6. Create Topic Model with LDA
7. Create Topic Model with LSI
8. Compute Similarity Matrices
9. Summarize text documents

Let us understand what some of the below mentioned terms mean before moving forward.

• Corpus: A collection of text documents.
• Vector: Form of representing text.
• Model: Algorithm used to generate representation of data.
• Topic Modelling: It is an information mining tool which is used to extract semantic topics from documents.
• Topic: A repeating group of words frequently occurring together.

For example:
    You have a document which consists of words like - 
    bat, car, racquet, score, glass, drive, cup, keys, water, game, steering, liquid
These can be grouped into different topics as-

Topic 1	Topic 2	Topic 3
glass	bat	car
cup	racquet	drive
water	score	keys
liquid	game	steering

Some of the Topic Modelling Techniques are – 

• Latent Semantic Indexing(LSI)
• Latent Dirichlet Allocation(LDA)

Now that we have the basic idea of the terminologies let’s start with the use of Gensim package.
First Install the library using the commands- 

#for linux


#for anaconda prompt

Step 1: Create a Corpus from a given Dataset
You need to follow these steps to create your corpus: 

1. Load your Dataset
2. Preprocess the Dataset
3. Create a Dictionary
4. Create Bag of Words Corpus

1.1 Load your Dataset:
You can have a .txt file as your dataset or you can also load datasets using the Gensim Downloader API.
Code: 

python3

import os

# open the text file as an object
doc = open('sample_data.txt', encoding ='utf-8')

• Gensim Downloader API: This is a module available in the Gensim library which is an API for downloading, getting information and loading datasets/models.

Code: 

python3

import gensim.downloader as api

# check available models and datasets
info_datasets = api.info()
print(info_datasets)
#>{'corpora':
#>    {'semeval-2016-2017-task3-subtaskBC':
#>        {'num_records': -1, 'record_format': 'dict', 'file_size': 6344358, ....}

# information of a particular dataset
dataset_info = api.info("text8")

# load the "text8" dataset
dataset = api.load("text8")

# load a pre-trained model
word2vec_model = api.load('word2vec-google-news-300')

Here we are going to consider a text file as raw dataset which consist of data from a wikipedia page. 
1.2 Preprocess the Dataset
Text preprocessing: In natural language preprocessing, text preprocessing is the practice of cleaning and preparing text data. For this purpose we will use the simple_preprocess( ) function.This function returns a list of tokens after tokenizing and normalizing them. 
Code: 

python3

import gensim
import os
from gensim.utils import simple_preprocess

# open the text file as an object
doc = open('sample_data.txt', encoding ='utf-8')

# preprocess the file to get a list of tokens
tokenized =[]
for sentence in doc.read().split('.'):
  # the simple_preprocess function returns a list of each sentence
  tokenized.append(simple_preprocess(sentence, deacc = True))

print(tokenized)

Output:

Output: tokenized

1.3 Create a Dictionary
Now we have our preprocessed data which can be converted into a dictionary by using the corpora.Dictionary( ) function. This dictionary is a map for unique tokens.
Code: 

python3

from gensim import corpora
# storing the extracted tokens into the dictionary
my_dictionary = corpora.Dictionary(tokenized)
print(my_dictionary)

Output: 

my_dictionary

1.3.1 Saving Dictionary on Disk or as Text File
 You can save/load your dictionary on the disk as well as a text file as mentioned below: 
Code: 

python3

# save your dictionary to disk
my_dictionary.save('my_dictionary.dict')

# load back
load_dict = corpora.Dictionary.load(my_dictionary.dict')

# save your dictionary as text file
from gensim.test.utils import get_tmpfile
tmp_fname = get_tmpfile("dictionary")
my_dictionary.save_as_text(tmp_fname)

# load your dictionary text file
load_dict = corpora.Dictionary.load_from_text(tmp_fname)

1.4 Create Bag of Words Corpus
Once we have the dictionary we can create a Bag of Word corpus using the doc2bow( ) function. This function counts the number of occurrences of each distinct word, convert the word to its integer word id and then the result is returned as a sparse vector.
Code: 

python3

# converting to a bag of word corpus
BoW_corpus =[my_dictionary.doc2bow(doc, allow_update = True) for doc in tokenized]
print(BoW_corpus)

Output: 

BoW_corpus

1.4.1 Saving Corpus on Disk: 
Code: To save/load your corpus

python3

from gensim.corpora import MmCorpus
from gensim.test.utils import get_tmpfile

output_fname = get_tmpfile("BoW_corpus.mm")

# save corpus to disk
MmCorpus.serialize(output_fname, BoW_corpus)

# load corpus
load_corpus = MmCorpus(output_fname)

Step 2: Create a TFIDF matrix in Gensim
TFIDF: Stands for Term Frequency – Inverse Document Frequency. It is a commonly used natural language processing model that helps you determine the most important words in each document in a corpus. This was designed for a modest-size corpora.
Some words might not be stopwords but may occur more often in the documents and may be of less importance. Hence these words need to be removed or down-weighted in importance. The TFIDF model takes the text that share a common language and ensures that most common words across the entire corpus don’t show as keywords. You can build a TFIDF model using Gensim and the corpus you developed previously as:
Code: 

python3

from gensim import models
import numpy as np

# Word weight in Bag of Words corpus
word_weight =[]
for doc in BoW_corpus:
  for id, freq in doc:
    word_weight.append([my_dictionary[id], freq])
print(word_weight)

Output:

Word weight before applying TFIDF Model

Code: applying TFIDF model

python3

# create TF-IDF model
tfIdf = models.TfidfModel(BoW_corpus, smartirs ='ntc')

# TF-IDF Word Weight
weight_tfidf =[]
for doc in tfIdf[BoW_corpus]:
  for id, freq in doc:
    weight_tfidf.append([my_dictionary[id], np.around(freq, decimals = 3)])
print(weight_tfidf)

Output: 

word weights after applying TFIDF model

You can see that the words occurring frequently across the documents now have lower weights assigned.
Step 3: Creating Bigrams and Trigrams with Gensim
Many words tend to occur in the content together. The words when occur together have a different meaning than as individuals. 

for example:
    Beatboxing --> the word beat and boxing individually have meanings of their own 
            but these together have a different meaning. 

Bigrams: Group of two words 
Trigrams: Group of three words
We will be using the text8 dataset here which can be downloaded using the Gensim downloader API
Code: Building bigrams and trigrams

python3

import gensim.downloader as api
from gensim.models.phrases import Phrases

# load the text8 dataset
dataset = api.load("text8")

# extract a list of words from the dataset
data =[]
for word in dataset:
  data.append(word)

# Bigram using Phraser Model
bigram_model = Phrases(data, min_count = 3, threshold = 10)

print(bigram_model[data[0]])

Bigram model

To create a Trigram we simply pass the above obtained bigram model to the same function. 
Code: 

python3

# Trigram using Phraser Model
trigram_model = Phrases(bigram_model[data], threshold = 10)

# trigram
print(trigram_model[bigram_model[data[0]]])

Output: 

Trigram 

Step 4: Create Word2Vec model using Gensim
The ML/DL algorithms cannot access text directly which is why we need some numerical representation so that these algorithms can process the data. In simple Machine Learning applications CountVectorizer and TFIDF are used which do not preserve the relationship between the words.
Word2Vec: Method to represent text to generate Word Embeddings which map all the words present in a language into a vector space of a given dimension. We can perform mathematical operations on these vectors which help preserve the relationship between the words. 

Pre-built word embedding models like word2vec, GloVe, fasttext etc. can be downloaded using the Gensim downloader API. Sometimes you may not find word embeddings for certain words in your document. So you can train your model.
4.1) Train the model
Code: 

python3

import gensim.downloader as api
from multiprocessing import cpu_count
from gensim.models.word2vec import Word2Vec

# load the text8 dataset
dataset = api.load("text8")

# extract a list of words from the dataset
data =[]
for word in dataset:
  data.append(word)

# We will split the data into two parts
data_1 = data[:1200]   # this is used to train the model
data_2 = data[1200:]   # this part will be used to update the model

# Training the Word2Vec model
w2v_model = Word2Vec(data_1, min_count = 0, workers = cpu_count())

# word vector for the word "time"
print(w2v_model['time'])

Output: 

word vector for the word time

You can also use the most_similar( ) function to find similar words to a given word.
Code: 

python3

# similar words to the word "time"
print(w2v_model.most_similar('time'))

# save your model
w2v_model.save('Word2VecModel')

# load your model
model = Word2Vec.load('Word2VecModel')

Output: 

most similar words to ‘time’

4.2) Update the model
Code: 

python3

# build model vocabulary from a sequence of sentences
w2v_model.build_vocab(data_2, update = True)

# train word vectors
w2v_model.train(data_2, total_examples = w2v_model.corpus_count, epochs = w2v_model.iter)

print(w2v_model['time'])

Output: 

Step 5: Create Doc2Vec model using Gensim
In contrast to the Word2Vec model, the Doc2Vec model gives the vector representation for an entire document or group of words. With the help of this model, we can find the relationship among different documents such as- 

If we train the model for literature such as "Through the Looking Glass".We can say that-  

5.1) Train the model
Code: 

python3

import gensim
import gensim.downloader as api
from gensim.models import doc2vec

# get dataset
dataset = api.load("text8")
data =[]
for w in dataset:
  data.append(w)

# To train the model we need a list of tagged documents
def tagged_document(list_of_ListOfWords):
  for x, ListOfWords in enumerate(list_of_ListOfWords):
    yield doc2vec.TaggedDocument(ListOfWords, [x])

# training data
data_train = list(tagged_document(data))

# print trained dataset
print(data_train[:1])

Output: 

OUTPUT – trained dataset

5.2) Update the modelCode: 

python3

# Initialize the model
d2v_model = doc2vec.Doc2Vec(vector_size = 40, min_count = 2, epochs = 30)

# build the vocabulary
d2v_model.build_vocab(data_train)

# Train Doc2Vec model
d2v_model.train(data_train, total_examples = d2v_model.corpus_count, epochs = d2v_model.epochs)

# Analyzing the output
Analyze = d2v_model.infer_vector(['violent', 'means', 'to', 'destroy'])
print(Analyze)

Output: 

Output of updated model

Step 6: Create Topic model with LDA
LDA is a popular method for topic modelling which considers each document as a collection of topics in a certain proportion. We need to take out the good quality of topics such as how segregated and meaningful they are. The good quality topics depend on- 

1. the quality of text processing
2. finding the optimal number of topics
3. Tuning parameters of the algorithm

NOTE: If you run this code on python3.7 version you might get a StopIteration Error.
      It is advisable to use python3.6 version for this.

Follow the below steps to create the model: 
6.1 Prepare the Data
This is done by removing the stopwords and then lemmatizing it. In order to lemmatize using Gensim, we need to first download the pattern package and the stopwords. 

#download pattern package
pip install pattern

#run in python console
>> import nltk
>> nltk.download('stopwords')

Code: 

python3

import gensim
from gensim import corpora
from gensim.models import LdaModel, LdaMulticore
import gensim.downloader as api
from gensim.utils import simple_preprocess, lemmatize
# from pattern.en import lemma
import nltk
# nltk.download('stopwords')
from nltk.corpus import stopwords
import re
import logging

logging.basicConfig(format ='%(asctime)s : %(levelname)s : %(message)s')
logging.root.setLevel(level = logging.INFO)

# import stopwords
stop_words = stopwords.words('english')
# add stopwords
stop_words = stop_words + ['subject', 'com', 'are', 'edu', 'would', 'could']

# import the dataset
dataset = api.load("text8")
data = [w for w in dataset]

# Preparing the data
processed_data = []

for x, doc in enumerate(data[:100]):
    doc_out = []
    for word in doc:
        if word not in stop_words:  # to remove stopwords
            Lemmatized_Word = lemmatize(word, allowed_tags = re.compile('(NN|JJ|RB)'))  # lemmatize
            if Lemmatized_Word:
                doc_out.append(Lemmatized_Word[0].split(b'/')[0].decode('utf-8'))
        else:
            continue
    processed_data.append(doc_out) # processed_data is a list of list of words

# Print sample
print(processed_data[0][:10])

Output: 

OUTPUT – processed_data

6.2 Create Dictionary and Corpus
The processed data will now be used to create the dictionary and corpus. 
Code: 

python3

# create dictionary and corpus
dict = corpora.Dictionary(processed_data)
Corpus = [dict.doc2bow(l) for l in processed_data]

6.3 Train LDA model
We will be training the LDA model with 5 topics using the dictionary and corpus created previously.Here the LdaModel( ) function is used but you can also use the LdaMulticore( ) function as it allows parallel processing. 
Code: 

python3

# Training
LDA_model = LdaModel(corpus = LDA_corpus, num_topics = 5)
# save model
LDA_model.save('LDA_model.model')

# show topics
print(LDA_model.print_topics(-1))

Output: 

OUTPUT – topics

The words which can be seen in more than one topic and are of less relevance can be added to the stopwords list.
6.4 Interpret the Output
The LDA model majorly gives us information regarding 3 things: 

1. Topics in the document
2. What topic each word belongs to
3. Phi value

Phi value: It is the probability of a word to lie in a particular topic.For a given word, sum of the phi values give the number of times that word occurred in the document.
Code: 

python3

# probability of a word belonging to a topic
LDA_model.get_term_topics('fire')

bow_list =['time', 'space', 'car']
# convert to bag of words format first
bow = LDA_model.id2word.doc2bow(bow_list)

# interpreting the data
doc_topics, word_topics, phi_values = LDA_model.get_document_topics(bow, per_word_topics = True)

Step 7: Create Topic Model with LSI
To create the model with LSI just follow the steps same as with LDA. The only difference will be while training the model.
Use the LsiModel( ) function instead of the LdaMulticore( ) or LdaModel( ). 
Code: 

python3

# Training the model with LSI
LSI_model = LsiModel(corpus = Corpus, id2word = dct, num_topics = 7, decay = 0.5)

# Topics
print(LSI_model.print_topics(-1))

Step 8: Compute Similarity Matrices
Cosine Similarity: It is a measure of similarity between two non-zero vectors of an inner product space. It is defined to equal the cosine of the angle between them.
Soft Cosine Similarity: It is similar to cosine similarity but the difference is that cosine similarity considers the vector space model(VSM) features as independent whereas soft cosine proposes to consider the similarity of features in VSM.
We need to take a word embedding model to compute soft cosines.
Here we are using the pre-trained word2vec model. 

Note: If you run this code on python3.7 version you might get a StopIteration Error.
      It is advisable to use python3.6 version for this.

Code: 

python3

import gensim.downloader as api
from gensim.matutils import softcossim
from gensim import corpora

s1 = ' Afghanistan is an Asian country and capital is Kabul'.split()
s2 = 'India is an Asian country and capital is Delhi'.split()
s3 = 'Greece is an European country and capital is Athens'.split()

# load pre-trained model
word2vec_model = api.load('word2vec-google-news-300')

# Prepare the similarity matrix
similarity_matrix = word2vec_model.similarity_matrix(dictionary, tfidf = None, threshold = 0.0, exponent = 2.0, nonzero_limit = 100)

# Prepare a dictionary and a corpus.
docs = [s1, s2, s3]
dictionary = corpora.Dictionary(docs)

# Convert the sentences into bag-of-words vectors.
s1 = dictionary.doc2bow(s1)
s2 = dictionary.doc2bow(s2)
s3 = dictionary.doc2bow(s3)

# Compute soft cosine similarity
print(softcossim(s1, s2, similarity_matrix)) # similarity between s1 &s2

print(softcossim(s1, s3, similarity_matrix)) # similarity between s1 &s3

print(softcossim(s2, s3, similarity_matrix)) # similarity between s2 &s3

Some of the similarity and distance metrics which can be calculated for this word embedding model are mentioned below: 
Code: 

python3

# Find Odd one out
print(word2vec_model.doesnt_match(['india', 'bhutan', 'china', 'mango']))
#> mango

# cosine distance between two words.
word2vec_model.distance('man', 'woman')

# cosine distances from given word or vector to other words.
word2vec_model.distances('king', ['queen', 'man', 'woman'])

# Compute cosine similarities
word2vec_model.cosine_similarities(word2vec_model['queen'],
                                            vectors_all =(word2vec_model['king'],
                                                        word2vec_model['woman'],
                                                        word2vec_model['man'],
                                                        word2vec_model['king'] + word2vec_model['woman']))
# king + woman is very similar to queen.

# words closer to w1 than w2
word2vec_model.words_closer_than(w1 ='queen', w2 ='kingdom')

# top-N most similar words.
word2vec_model.most_similar(positive ='king', negative = None, topn = 5, restrict_vocab = None, indexer = None)

# top-N most similar words, using the multiplicative combination objective,
word2vec_model.most_similar_cosmul(positive ='queen', negative = None, topn = 5)

Step 9: Summarize Text Documents
The summarize( ) function implements the text rank summarization.
You do not have to generate a tokenized list by splitting the sentences as that is already handled by the gensim.summarization.textcleaner module. 
Code: 

python3

from gensim.summarization import summarize, keywords
import os

text = " ".join((l for l in open('sample_data.txt', encoding ='utf-8')))

# Summarize the paragraph
print(summarize(text, word_count = 25))

Output: 

OUTPUT – Summary

You can get the keywords by: 
Code: 

python3

# Important keywords from the paragraph
print(keywords(text))

OUTPUT – Keywords

Conclusion: 
These are some of the features of the Gensim library.This comes most handy while you are working on language processing.You can make use of these as per your need.
For any queries feel free to leave a comment down below.