Text classification is a core problem to many applications, like spam detection, sentiment analysis or smart replies. In this tutorial, we describe how to build a text classifier with the fastText tool.
What is text classification?
The goal of text classification is to assign documents (such as emails, posts, text messages, product reviews, etc...) to one or multiple categories. Such categories can be review scores, spam v.s. non-spam, or the language in which the document was typed. Nowadays, the dominant approach to build such classifiers is machine learning, that is learning classification rules from examples. In order to build such classifiers, we need labeled data, which consists of documents and their corresponding categories (or tags, or labels).
As an example, we build a classifier which automatically classifies stackexchange questions about cooking into one of several possible tags, such as
The first step of this tutorial is to install and build fastText. It only requires a c++ compiler with good support of c++11.
Let us start by downloading the most recent release:
$ wget https://github.com/facebookresearch/fastText/archive/v0.2.0.zip $ unzip v0.2.0.zip
Move to the fastText directory and build it:
$ cd fastText-0.2.0 $ make
Running the binary without any argument will print the high level documentation, showing the different use cases supported by fastText:
>> ./fasttext usage: fasttext <command> <args> The commands supported by fasttext are: supervised train a supervised classifier quantize quantize a model to reduce the memory usage test evaluate a supervised classifier predict predict most likely labels predict-prob predict most likely labels with probabilities skipgram train a skipgram model cbow train a cbow model print-word-vectors print word vectors given a trained model print-sentence-vectors print sentence vectors given a trained model nn query for nearest neighbors analogies query for analogies
In this tutorial, we mainly use the
predict subcommands, which corresponds to learning (and using) text classifier. For an introduction to the other functionalities of fastText, please see the tutorial about learning word vectors.
Getting and preparing the data
As mentioned in the introduction, we need labeled data to train our supervised classifier. In this tutorial, we are interested in building a classifier to automatically recognize the topic of a stackexchange question about cooking. Let's download examples of questions from the cooking section of Stackexchange, and their associated tags:
>> wget https://dl.fbaipublicfiles.com/fasttext/data/cooking.stackexchange.tar.gz && tar xvzf cooking.stackexchange.tar.gz >> head cooking.stackexchange.txt
Each line of the text file contains a list of labels, followed by the corresponding document. All the labels start by the
__label__ prefix, which is how fastText recognize what is a label or what is a word. The model is then trained to predict the labels given the word in the document.
Before training our first classifier, we need to split the data into train and validation. We will use the validation set to evaluate how good the learned classifier is on new data.
>> wc cooking.stackexchange.txt 15404 169582 1401900 cooking.stackexchange.txt
Our full dataset contains 15404 examples. Let's split it into a training set of 12404 examples and a validation set of 3000 examples:
>> head -n 12404 cooking.stackexchange.txt > cooking.train >> tail -n 3000 cooking.stackexchange.txt > cooking.valid
Our first classifier
We are now ready to train our first classifier:
>> ./fasttext supervised -input cooking.train -output model_cooking Read 0M words Number of words: 14598 Number of labels: 734 Progress: 100.0% words/sec/thread: 75109 lr: 0.000000 loss: 5.708354 eta: 0h0m
-input command line option indicates the file containing the training examples, while the
-output option indicates where to save the model. At the end of training, a file
model_cooking.bin, containing the trained classifier, is created in the current directory.
It is possible to directly test our classifier interactively, by running the command:
>> ./fasttext predict model_cooking.bin -
and then typing a sentence. Let's first try the sentence:
Which baking dish is best to bake a banana bread ?
The predicted tag is
baking which fits well to this question. Let us now try a second example:
Why not put knives in the dishwasher?
The label predicted by the model is
food-safety, which is not relevant. Somehow, the model seems to fail on simple examples. To get a better sense of its quality, let's test it on the validation data by running:
>> ./fasttext test model_cooking.bin cooking.valid N 3000 [email protected] 0.124 [email protected] 0.0541 Number of examples: 3000
>> ./fasttext test model_cooking.bin cooking.valid 5 N 3000 [email protected] 0.0668 [email protected] 0.146 Number of examples: 3000
Advanced readers: precision and recall
The precision is the number of correct labels among the labels predicted by fastText. The recall is the number of labels that successfully were predicted, among all the real labels. Let's take an example to make this more clear:
Why not put knives in the dishwasher?
On Stack Exchange, this sentence is labeled with three tags:
knives. The top five labels predicted by the model can be obtained with:
>> ./fasttext predict model_cooking.bin - 5
Thus, one out of five labels predicted by the model is correct, giving a precision of 0.20. Out of the three real labels, only one is predicted by the model, giving a recall of 0.33.
For more details, see the related Wikipedia page.
Making the model better
The model obtained by running fastText with the default arguments is pretty bad at classifying new questions. Let's try to improve the performance, by changing the default parameters.
preprocessing the data
Looking at the data, we observe that some words contain uppercase letter or punctuation. One of the first step to improve the performance of our model is to apply some simple pre-processing. A crude normalization can be obtained using command line tools such as
>> cat cooking.stackexchange.txt | sed -e "s/\([.\!?,'/()]\)/ \1 /g" | tr "[:upper:]" "[:lower:]" > cooking.preprocessed.txt >> head -n 12404 cooking.preprocessed.txt > cooking.train >> tail -n 3000 cooking.preprocessed.txt > cooking.valid
Let's train a new model on the pre-processed data:
>> ./fasttext supervised -input cooking.train -output model_cooking Read 0M words Number of words: 9012 Number of labels: 734 Progress: 100.0% words/sec/thread: 82041 lr: 0.000000 loss: 5.671649 eta: 0h0m h-14m >> ./fasttext test model_cooking.bin cooking.valid N 3000 [email protected] 0.164 [email protected] 0.0717 Number of examples: 3000
We observe that thanks to the pre-processing, the vocabulary is smaller (from 14k words to 9k). The precision is also starting to go up by 4%!
more epochs and larger learning rate
By default, fastText sees each training example only five times during training, which is pretty small, given that our training set only have 12k training examples. The number of times each examples is seen (also known as the number of epochs), can be increased using the
>> ./fasttext supervised -input cooking.train -output model_cooking -epoch 25 Read 0M words Number of words: 9012 Number of labels: 734 Progress: 100.0% words/sec/thread: 77633 lr: 0.000000 loss: 7.147976 eta: 0h0m
Let's test the new model:
>> ./fasttext test model_cooking.bin cooking.valid N 3000 [email protected] 0.501 [email protected] 0.218 Number of examples: 3000
This is much better! Another way to change the learning speed of our model is to increase (or decrease) the learning rate of the algorithm. This corresponds to how much the model changes after processing each example. A learning rate of 0 would mean that the model does not change at all, and thus, does not learn anything. Good values of the learning rate are in the range
0.1 - 1.0.
>> ./fasttext supervised -input cooking.train -output model_cooking -lr 1.0 Read 0M words Number of words: 9012 Number of labels: 734 Progress: 100.0% words/sec/thread: 81469 lr: 0.000000 loss: 6.405640 eta: 0h0m >> ./fasttext test model_cooking.bin cooking.valid N 3000 [email protected] 0.563 [email protected] 0.245 Number of examples: 3000
Even better! Let's try both together:
>> ./fasttext supervised -input cooking.train -output model_cooking -lr 1.0 -epoch 25 Read 0M words Number of words: 9012 Number of labels: 734 Progress: 100.0% words/sec/thread: 76394 lr: 0.000000 loss: 4.350277 eta: 0h0m >> ./fasttext test model_cooking.bin cooking.valid N 3000 [email protected] 0.585 [email protected] 0.255 Number of examples: 3000
Let us now add a few more features to improve even further our performance!
Finally, we can improve the performance of a model by using word bigrams, instead of just unigrams. This is especially important for classification problems where word order is important, such as sentiment analysis.
>> ./fasttext supervised -input cooking.train -output model_cooking -lr 1.0 -epoch 25 -wordNgrams 2 Read 0M words Number of words: 9012 Number of labels: 734 Progress: 100.0% words/sec/thread: 75366 lr: 0.000000 loss: 3.226064 eta: 0h0m >> ./fasttext test model_cooking.bin cooking.valid N 3000 [email protected] 0.599 [email protected] 0.261 Number of examples: 3000
With a few steps, we were able to go from a precision at one of 12.4% to 59.9%. Important steps included:
- preprocessing the data ;
- changing the number of epochs (using the option
-epoch, standard range
[5 - 50]) ;
- changing the learning rate (using the option
-lr, standard range
[0.1 - 1.0]) ;
- using word n-grams (using the option
-wordNgrams, standard range
[1 - 5]).
Advanced readers: What is a Bigram?
A 'unigram' refers to a single undividing unit, or token, usually used as an input to a model. For example a unigram can be a word or a letter depending on the model. In fastText, we work at the word level and thus unigrams are words.
Similarly we denote by 'bigram' the concatenation of 2 consecutive tokens or words. Similarly we often talk about n-gram to refer to the concatenation any n consecutive tokens.
For example, in the sentence, 'Last donut of the night', the unigrams are 'last', 'donut', 'of', 'the' and 'night'. The bigrams are: 'Last donut', 'donut of', 'of the' and 'the night'.
Bigrams are particularly interesting because, for most sentences, you can reconstruct the order of the words just by looking at a bag of n-grams.
Let us illustrate this by a simple exercise, given the following bigrams, try to reconstruct the original sentence: 'all out', 'I am', 'of bubblegum', 'out of' and 'am all'. It is common to refer to a word as a unigram.
Scaling things up
Since we are training our model on a few thousands of examples, the training only takes a few seconds. But training models on larger datasets, with more labels can start to be too slow. A potential solution to make the training faster is to use the hierarchical softmax, instead of the regular softmax. This can be done with the option
>> ./fasttext supervised -input cooking.train -output model_cooking -lr 1.0 -epoch 25 -wordNgrams 2 -bucket 200000 -dim 50 -loss hs Read 0M words Number of words: 9012 Number of labels: 734 Progress: 100.0% words/sec/thread: 2199406 lr: 0.000000 loss: 1.718807 eta: 0h0m
Training should now take less than a second.
Advanced readers: hierarchical softmax
The hierarchical softmax is a loss function that approximates the softmax with a much faster computation.
The idea is to build a binary tree whose leaves correspond to the labels. Each intermediate node has a binary decision activation (e.g. sigmoid) that is trained, and predicts if we should go to the left or to the right. The probability of the output unit is then given by the product of the probabilities of intermediate nodes along the path from the root to the output unit leave.
For a detailed explanation, you can have a look on this video.
In fastText, we use a Huffman tree, so that the lookup time is faster for more frequent outputs and thus the average lookup time for the output is optimal.
When we want to assign a document to multiple labels, we can still use the softmax loss and play with the parameters for prediction, namely the number of labels to predict and the threshold for the predicted probability. However playing with these arguments can be tricky and unintuitive since the probabilities must sum to 1.
A convenient way to handle multiple labels is to use independent binary classifiers for each label. This can be done with
-loss one-vs-all or
>> ./fasttext supervised -input cooking.train -output model_cooking -lr 0.5 -epoch 25 -wordNgrams 2 -bucket 200000 -dim 50 -loss one-vs-all Read 0M words Number of words: 14543 Number of labels: 735 Progress: 100.0% words/sec/thread: 72104 lr: 0.000000 loss: 4.340807 ETA: 0h 0m
It is a good idea to decrease the learning rate compared to other loss functions.
Now let's have a look on our predictions, we want as many prediction as possible (argument
-1) and we want only labels with probability higher or equal to
>> ./fasttext predict-prob model_cooking.bin - -1 0.5
and then type the sentence:
Which baking dish is best to bake a banana bread ?
__label__baking 1.00000 __label__bananas 0.939923 __label__bread 0.592677
We can also evaluate our results with the
test command :
>> ./fasttext test model_cooking.bin cooking.valid -1 0.5 N 3000 [email protected] 0.702 [email protected] 0.2 Number of examples: 3000
and play with the threshold to obtain desired precision/recall metrics :
>> ./fasttext test model_cooking.bin cooking.valid -1 0.1 N 3000 [email protected] 0.591 [email protected] 0.272 Number of examples: 3000
In this tutorial, we gave a brief overview of how to use fastText to train powerful text classifiers. We had a light overview of some of the most important options to tune.