Preface
Topic modeling is a type of statistical modeling for discovering the abstract “topics” that occur in a collection of documents. Latent Dirichlet Allocation (LDA) is an example of topic model and is used to classify text in a document to a particular topic. It builds a topic per document model and words per topic model, modeled as Dirichlet distributions.
Here we are going to apply LDA to a set of documents and split them into topics. Let’s get started!
The Data
The data set we’ll use is a list of over one million news headlines published over a period of 15 years and can be downloaded from Kaggle: (abcnews-date-text.csv.zip)
- import pandas as pd
- data = pd.read_csv('abcnews-date-text.csv', error_bad_lines=False);
- data_text = data[['headline_text']]
- data_text['index'] = data_text.index
- documents = data_text
- print(len(documents))
- documents.head()
Data Pre-processing
We will perform the following steps:
Loading gensim and nltk libraries:
- import gensim
- from gensim.utils import simple_preprocess
- from gensim.parsing.preprocessing import STOPWORDS
- from nltk.stem import WordNetLemmatizer, SnowballStemmer
- from nltk.stem.porter import *
- import numpy as np
- np.random.seed(2018)
- import nltk
- nltk.download('wordnet')
- stemmer = PorterStemmer()
- def lemmatize_stemming(text):
- return stemmer.stem(WordNetLemmatizer().lemmatize(text, pos='v'))
- def preprocess(text):
- result = []
- for token in gensim.utils.simple_preprocess(text):
- if token not in gensim.parsing.preprocessing.STOPWORDS and len(token) > 3:
- result.append(lemmatize_stemming(token))
- return result
- doc_sample = documents[documents['index'] == 4310]['headline_text'].iloc[0]
- print('original document: ')
- words = []
- for word in doc_sample.split(' '):
- words.append(word)
- print(words)
- print('\n\n tokenized and lemmatized document: ')
- print(preprocess(doc_sample))
Preprocess the headline text, saving the results as ‘processed_docs’:
- processed_docs = documents['headline_text'].map(preprocess)
- processed_docs[:10]
Bag of Words on the Data set
Create a dictionary from ‘processed_docs’ containing the number of times a word appears in the training set:
- dictionary = gensim.corpora.Dictionary(processed_docs)
- count = 0
- for k, v in dictionary.iteritems():
- print(k, v)
- count += 1
- if count > 10:
- break
Filter out tokens that appear in:
- dictionary.filter_extremes(no_below=15, no_above=0.5, keep_n=100000)
For each document we create a dictionary reporting how many words and how many times those words appear. Save this to ‘bow_corpus’, then check our selected document earlier:
- bow_corpus = [dictionary.doc2bow(doc) for doc in processed_docs]
- bow_corpus[4310]
Preview Bag Of Words for our sample preprocessed document:
- bow_doc_4310 = bow_corpus[4310]
- for i in range(len(bow_doc_4310)):
- print("Word {} (\"{}\") appears {} time.".format(bow_doc_4310[i][0],
- dictionary[bow_doc_4310[i][0]],
- bow_doc_4310[i][1]))
TF-IDF
Create tf-idf model object using models.TfidfModel on ‘bow_corpus’ and save it to ‘tfidf’, then apply transformation to the entire corpus and call it ‘corpus_tfidf’. Finally we preview TF-IDF scores for our first document:
- from gensim import corpora, models
- tfidf = models.TfidfModel(bow_corpus)
- corpus_tfidf = tfidf[bow_corpus]
- from pprint import pprint
- for doc in corpus_tfidf:
- pprint(doc)
- break
Train our lda model using gensim.models.LdaMulticore and save it to ‘lda_model’:
- lda_model = gensim.models.LdaMulticore(bow_corpus, num_topics=10, id2word=dictionary, passes=2, workers=2)
- for idx, topic in lda_model.print_topics(-1):
- print('Topic: {} \nWords: {}'.format(idx, topic))
Can you distinguish different topics using the words in each topic and their corresponding weights?
Running LDA using TF-IDF
- lda_model_tfidf = gensim.models.LdaMulticore(corpus_tfidf, num_topics=10, id2word=dictionary, passes=2, workers=4)
- for idx, topic in lda_model_tfidf.print_topics(-1):
- print('Topic: {} Word: {}'.format(idx, topic))
Again, can you distinguish different topics using the words in each topic and their corresponding weights?
Performance evaluation by classifying sample document using LDA Bag of Words model
We will check where our test document would be classified:
- processed_docs[4310]
Show out the top 5 topics according to score decreasingly:
- for index, score in sorted(lda_model[bow_corpus[4310]], key=lambda tup: -1*tup[1])[:5]:
- print("\nScore: {}\t \nTopic: {}".format(score, lda_model.print_topic(index, 10)))
Our test document has the highest probability to be part of the topic that our model assigned, which is the accurate classification.
Performance evaluation by classifying sample document using LDA TF-IDF model.
- for index, score in sorted(lda_model_tfidf[bow_corpus[4310]], key=lambda tup: -1*tup[1])[:5]:
- print("\nScore: {}\t \nTopic: {}".format(score, lda_model_tfidf.print_topic(index, 10)))
Our test document has the highest probability to be part of the topic that our model assigned, which is the accurate classification.
Testing model on unseen document
- unseen_document = 'How a Pentagon deal became an identity crisis for Google'
- bow_vector = dictionary.doc2bow(preprocess(unseen_document))
- for index, score in sorted(lda_model[bow_vector], key=lambda tup: -1*tup[1]):
- print("Score: {}\t Topic: {}".format(score, lda_model.print_topic(index, 4)))
Source code can be found on Github (Or lda_example.ipynb). I look forward to hearing any feedback or questions.
Supplement
* Youtube - (ML 7.7.A1) Dirichlet distribution
