Example – text classification workflow

The preceding process is fairly generic. What would it look like for one of the most common natural language applications – text classification?

The following flow diagram was built by Microsoft Azure, and is used here to explain how their own technology fits directly into our workflow template. There are several new words that they have introduced to feature engineering, such as unigrams, TF-IDF, TF, n-grams, and so on:

The main steps in their flow diagram are as follows:

Step 1: Data preparation
Step 2: Text pre-processing
Step 3: Feature engineering:
- Unigrams TF-IDF extraction
- N-grams TF extraction
Step 4: Train and evaluate models
Step 5: Deploy trained models as web services

This means that it's time to stop talking and start programming. Let's quickly set up the environment first and then we will work on building our first text classification system in 30 lines of code or less.

Launchpad – programming environment setup

We will use the fast.ai machine learning setup for this exercise. Their setup environment is great for personal experimentation and industry-grade proof-of-concept projects. I have used the fast.ai environment on both Linux and Windows. We will use Python 3.6 here since our code will not run for other Python versions.

A quick search on their forums will also take you to the latest instructions on how to set up the same on most cloud computing solutions including AWS, Google Cloud Platform, and Paperspace.

This environment covers the tools that we will use across most of the major tasks that we will perform: text processing (including cleaning), feature extraction, machine learning and deep learning models, model evaluation, and deployment.

It includes spaCy out of the box. spaCy is an open source tool that was made for an industry-grade NLP toolkit. If someone recommends that you use NLTK for a task, use spaCy instead. The demo ahead works out of the box in their environment.

There are a few more packages that we will need for later tasks. We will install and set them up as and when required. We don't want to bloat your installation with unnecessary packages that you might not even use.

Text classification in 30 lines of code

Let's divide the classification problem into the following steps:

Getting the data
Text to numbers
Running ML algorithms with sklearn

Getting the data

The 20 newsgroups dataset is a fairly well-known dataset among the NLP community. It is near-ideal for demonstration purposes. This dataset has a near-uniform distribution across 20 classes. This uniform distribution makes iterating rapidly on classification and clustering techniques easy.

We will use the famous 20 newsgroups dataset for our demonstrations as well:

from sklearn.datasets import fetch_20newsgroups  # import packages which help us download dataset 
twenty_train = fetch_20newsgroups(subset='train', shuffle=True, download_if_missing=True)
twenty_test = fetch_20newsgroups(subset='test', shuffle=True, download_if_missing=True)

Most modern NLP methods rely heavily on machine learning methods. These methods need words that are written as strings of text to be converted into a numerical representation. This numerical representation can be as simple as assigning a unique integer ID to slightly more comprehensive vector of float values. In the case of the latter, this is sometimes referred to as vectorization.

Text to numbers

We will be using a bag of words model for our example. We simply convert the number of times every word occurs per document. Therefore, each document is a bag and we count the frequency of each word in that bag. This also means that we lose any ordering information that's present in the text. Next, we assign each unique word an integer ID. All of these unique words become our vocabulary. Each word in our vocabulary is treated as a machine learning feature. Let's make our vocabulary first.

Scikit-learn has a high-level component that will create feature vectors for us. This is called CountVectorizer. We recommend reading more about it from the scikit-learn docs:

# Extracting features from text files
from sklearn.feature_extraction.text import CountVectorizer

count_vect = CountVectorizer()
X_train_counts = count_vect.fit_transform(twenty_train.data)

print(f'Shape of Term Frequency Matrix: {X_train_counts.shape}')

By using count_vect.fit_transform(twenty_train.data), we are learning the vocabulary dictionary, which returns a Document-Term matrix of shape [n_samples, n_features]. This means that we have n_samples documents or bags with n_features unique words across them.

We will now be able to extract a meaningful relationship between these words and the tags or classes they belong to. One of the simplest ways to do this is to count the number of times a word occurs in each class.

We have a small issue with this – long documents then tend to influence the result a lot more. We can normalize this effect by dividing the word frequency by the total words in that document. We call this Term Frequency, or simply TF.

Words like the, a, and of are common across all documents and don't really help us distinguish between document classes or separate them. We want to emphasize rarer words, such as Manmohan and Modi, over common words. One way to do this is to use inverse document frequency, or IDF. Inverse document frequency is a measure of whether the term is common or rare in all documents.

We multiply TF with IDF to get our TF-IDF metric, which is always greater than zero. TF-IDF is calculated for a triplet of term t, document d, and vocab dictionary D.

We can directly calculate TF-IDF using the following lines of code:

from sklearn.feature_extraction.text import TfidfTransformer

tfidf_transformer = TfidfTransformer()
X_train_tfidf = tfidf_transformer.fit_transform(X_train_counts)

print(f'Shape of TFIDF Matrix: {X_train_tfidf.shape}')

The last line will output the dimension of the Document-Term matrix, which is (11314, 130107).

Please note that in the preceding example we used each word as a feature, so the TF-IDF was calculated for each word. When we use a single word as a feature, we call it a unigram. If we were to use two consecutive words as a feature instead, we'd call it a bigram. In general, for n-words, we would call it an n-gram.

Machine learning

Various algorithms can be used for text classification. You can build a classifier in scikit using the following code:

from sklearn.linear_model import LogisticRegression as LR
from sklearn.pipeline import Pipeline

Let's dissect the preceding code, line by line.

The initial two lines are simple imports. We import the fairly well-known Logistic Regression model and rename the import LR. The next is a pipeline import:

"Sequentially apply a list of transforms and a final estimator. Intermediate steps of the pipeline must be "transforms", that is, they must implement fit and transform methods. The final estimator only needs to implement fit."

- from sklearn docs

Scikit-learn pipelines are, logistically, lists of operations that are applied, one after another. First, we applied the two operations we have already seen: CountVectorizer() and TfidfTransformer(). This was followed by LR(). The pipeline was created with Pipeline(...), but hasn't been executed. It is only executed when we call the fit() function from the Pipeline object:

text_lr_clf = Pipeline([('vect', CountVectorizer()), ('tfidf', TfidfTransformer()), ('clf',LR())])
text_lr_clf = text_lr_clf.fit(twenty_train.data, twenty_train.target)

When this is called, it calls the transform function of all but the last object. For the last object – our Logistic Regression classifier – its fit() function is called. These transforms and classifiers are also referred to as estimators:

"All estimators in a pipeline, except the last one, must be transformers (that is, they must have a transform method). The last estimator may be any type (transformer, classifier, and so on)."

- from sklearn pipeline docs

Let's calculate the accuracy of this model on the test data. For calculating the means on a large number of values, we will be using a scientific library called numpy:

import numpy as np
lr_predicted = text_lr_clf.predict(twenty_test.data)
lr_clf_accuracy = np.mean(lr_predicted == twenty_test.target) * 100.

print(f'Test Accuracy is {lr_clf_accuracy}')

This prints out the following output:

Test Accuracy is 82.79341476367499

We used the LR default parameters here. We can later optimize these using GridSearch or RandomSearch to improve the accuracy even more.

If you're going to remember only one thing from this section, remember to try a linear model such as logistic regression. They are often quite good for sparse high-dimensional data such as text, bag-of-words, or TF-IDF.

In addition to accuracy, it is useful to understand which categories of text are being confused for which other categories. We will call this a confusion matrix.

The following code uses the same variables we used to calculate the test accuracy for finding out the confusion matrix:

from sklearn.metrics import confusion_matrix
cf = confusion_matrix(y_true=twenty_test.target, y_pred=lr_predicted)
print(cf)

This prints a giant list of numbers which is not very interpretable. Let's try pretty printing this by using the print-json hack:

import json
print(json.dumps(cf.tolist(), indent=2))

This returns the following code:

This is slightly better. We now understand that this is a 20 × 20 grid of numbers. However, interpreting these numbers is a tedious task unless we can bring some visualization into this game. Let's do that next:

# this line ensures that the plot is rendered inside the Jupyter we used for testing this code
%matplotlib inline 

import seaborn as sns
import matplotlib.pyplot as plt

plt.figure(figsize=(20,10))
ax = sns.heatmap(cf, annot=True, fmt="d",linewidths=.5, center = 90, vmax = 200)
# plt.show() # optional, un-comment if the plot does not show

This gives us the following amazing plot:

This plot highlights information of interest to us in different color schemes. For instance, the light diagonal from the lupper-left corner to the lower-right corner shows everything we got right. The other grids are darker-colored if we confused those more. For instance, 97 samples of one class got wrongly tagged, which is quickly visible by the dark black color in row 18 and column 16.

We will dive deeper into both parts of this section – model interpretation and data visualization – in slightly more detail later in this book.

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