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%matplotlib inline
import matplotlib.pyplot as plt
import numpy as np

# Some nice default configuration for plots
plt.rcParams['figure.figsize'] = 10, 7.5
plt.rcParams['axes.grid'] = True

Large Scale Text Classification for Sentiment Analysis

Scalability Issues

The sklearn.feature_extraction.text.CountVectorizer and sklearn.feature_extraction.text.TfidfVectorizer classes suffer from a number of scalability issues that all stem from the internal usage of the vocabulary_ attribute (a Python dictionary) used to map the unicode string feature names to the integer feature indices.

The main scalability issues are:

Memory usage of the text vectorizer: the all the string representations of the features are loaded in memory
Parallelization problems for text feature extraction: the vocabulary_ would be a shared state: complex synchronization and overhead
Impossibility to do online or out-of-core / streaming learning: the vocabulary_ needs to be learned from the data: its size cannot be known before making one pass over the full dataset

To better understand the issue let's have a look at how the vocabulary_ attribute work. At fit time the tokens of the corpus are uniquely indentified by a integer index and this mapping stored in the vocabulary:



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from sklearn.feature_extraction.text import CountVectorizer

vectorizer = CountVectorizer(min_df=1)

vectorizer.fit([
    "The cat sat on the mat.",
])
vectorizer.vocabulary_

The vocabulary is used at transform time to build the occurrence matrix:



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X = vectorizer.transform([
    "The cat sat on the mat.",
    "This cat is a nice cat.",
]).toarray()

print(len(vectorizer.vocabulary_))
print(vectorizer.get_feature_names())
print(X)

Let's refit with a slightly larger corpus:



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vectorizer = CountVectorizer(min_df=1)

vectorizer.fit([
    "The cat sat on the mat.",
    "The quick brown fox jumps over the lazy dog.",
])
vectorizer.vocabulary_

The vocabulary_ is the (logarithmically) growing with the size of the training corpus. Note that we could not have built the vocabularies in parallel on the 2 text documents as they share some words hence would require some kind of shared datastructure or synchronization barrier which is complicated to setup, especially if we want to distribute the processing on a cluster.

With this new vocabulary, the dimensionality of the output space is now larger:



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X = vectorizer.transform([
    "The cat sat on the mat.",
    "This cat is a nice cat.",
]).toarray()

print(len(vectorizer.vocabulary_))
print(vectorizer.get_feature_names())
print(X)

The Sentiment 140 Dataset

To illustrate the scalability issues of the vocabulary-based vectorizers, let's load a more realistic dataset for a classical text classification task: sentiment analysis on tweets. The goal is to tell apart negative from positive tweets on a variety of topics.

Assuming that the ../fetch_data.py script was run successfully the following files should be available:



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import os

sentiment140_folder = os.path.join('datasets', 'sentiment140')
training_csv_file = os.path.join(sentiment140_folder, 'training.1600000.processed.noemoticon.csv')
testing_csv_file = os.path.join(sentiment140_folder, 'testdata.manual.2009.06.14.csv')

Those files were downloaded from the research archive of the http://www.sentiment140.com/ project. The first file was gathered using the twitter streaming API by running stream queries for the positive ":)" and negative ":(" emoticons to collect tweets that are explicitly positive or negative. To make the classification problem non-trivial, the emoticons were stripped out of the text in the CSV files.

Let's parse the CSV files and load everything in memory. As loading everything can take up to 2GB, let's limit the collection to 100K tweets of each (positive and negative) out of the total of 1.6M tweets.



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from helpers import read_sentiment_csv
%time text_train_all, target_train_all = read_sentiment_csv(training_csv_file, max_count=200000)



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len(text_train_all), len(target_train_all)

Let's display the first samples:



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for text in text_train_all[:3]:
    print(text + "\n")



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print(target_train_all[:3])

A polarity of "-1" means negative while a polarity of "1" means positive. All the positive tweets are at the end of the file:



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for text in text_train_all[-3:]:
    print(text + "\n")



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print(target_train_all[-3:])

Let's split the training CSV file into a smaller training set and a validation set with 100k random tweets each:



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from sklearn.cross_validation import train_test_split

text_train_small, text_validation, target_train_small, target_validation = train_test_split(
    text_train_all, np.array(target_train_all), test_size=.5, random_state=42)



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len(text_train_small)



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(target_train_small == -1).sum(), (target_train_small == 1).sum()



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len(text_validation)



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(target_validation == -1).sum(), (target_validation == 1).sum()

Let's open the manually annotated tweet files. The evaluation set also has neutral tweets with a polarity of "2" which we ignore. We can build the final evaluation set with only the positive and negative tweets of the evaluation CSV file:



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text_test_all, target_test_all = read_sentiment_csv(testing_csv_file)



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len(text_test_all), len(target_test_all)

The Hashing Trick

Remember the bag of word representation using a vocabulary based vectorizer:

To workaround the limitations of the vocabulary-based vectorizers, one can use the hashing trick. Instead of building and storing an explicit mapping from the feature names to the feature indices in a Python dict, we can just use a hash function and a modulus operation:



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from sklearn.utils.murmurhash import murmurhash3_bytes_u32

# encode for python 3 compatibility
for word in "the cat sat on the mat".encode("utf-8").split():
    print("{0} => {1}".format(
        word, murmurhash3_bytes_u32(word, 0) % 2 ** 20))

This mapping is completely stateless and the dimensionality of the output space is explicitly fixed in advance (here we use a modulo 2 ** 20 which means roughly 1M dimensions). The makes it possible to workaround the limitations of the vocabulary based vectorizer both for parallelizability and online / out-of-core learning.

The HashingVectorizer class is an alternative to the TfidfVectorizer class with use_idf=False that internally uses the murmurhash hash function:



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from sklearn.feature_extraction.text import HashingVectorizer

h_vectorizer = HashingVectorizer(encoding='latin-1')
h_vectorizer

It shares the same "preprocessor", "tokenizer" and "analyzer" infrastructure:



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analyzer = h_vectorizer.build_analyzer()
analyzer('This is a test sentence.')

We can vectorize our datasets into a scipy sparse matrix exactly as we would have done with the CountVectorizer or TfidfVectorizer, except that we can directly call the transform method: there is no need to fit as HashingVectorizer is a stateless transformer:



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%time X_train_small = h_vectorizer.transform(text_train_small)

The dimension of the output is fixed ahead of time to n_features=2 ** 20 by default (nearly 1M features) to minimize the rate of collision on most classification problem while having reasonably sized linear models (1M weights in the coef_ attribute):



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X_train_small

As only the non-zero elements are stored, n_features has little impact on the actual size of the data in memory. We can combine the hashing vectorizer with a Passive-Aggressive linear model in a pipeline:



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from sklearn.linear_model import PassiveAggressiveClassifier
from sklearn.pipeline import Pipeline

h_pipeline = Pipeline((
    ('vec', HashingVectorizer(encoding='latin-1')),
    ('clf', PassiveAggressiveClassifier(C=1, n_iter=1)),
))

%time h_pipeline.fit(text_train_small, target_train_small).score(text_validation, target_validation)

Let's check that the score on the validation set is reasonably in line with the set of manually annotated tweets:



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h_pipeline.score(text_test_all, target_test_all)

As the text_train_small dataset is not that big we can still use a vocabulary based vectorizer to check that the hashing collisions are not causing any significant performance drop on the validation set (WARNING this is twice as slow as the hashing vectorizer version, skip this cell if your computer is too slow):



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from sklearn.feature_extraction.text import TfidfVectorizer

vocabulary_vec = TfidfVectorizer(encoding='latin-1', use_idf=False)
vocabulary_pipeline = Pipeline((
    ('vec', vocabulary_vec),
    ('clf', PassiveAggressiveClassifier(C=1, n_iter=1)),
))

%time vocabulary_pipeline.fit(text_train_small, target_train_small).score(text_validation, target_validation)

We get almost the same score but almost twice as slower with also a big, slow to (un)pickle datastructure in memory:



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len(vocabulary_vec.vocabulary_)

More info and reference for the original papers on the Hashing Trick in the following site as well as a description specific to language here.

Out-of-Core learning

Out-of-Core learning is the task of training a machine learning model on a dataset that does not fit in the main memory. This requires the following conditions:

a feature extraction layer with fixed output dimensionality
knowing the list of all classes in advance (in this case we only have positive and negative tweets)
a machine learning algorithm that supports incremental learning (the partial_fit method in scikit-learn).

Let us simulate an infinite tweeter stream that can generate batches of annotated tweet texts and there polarity. We can do this by recombining randomly pairs of positive or negative tweets from our fixed dataset:



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from random import Random


class InfiniteStreamGenerator(object):
    """Simulate random polarity queries on the twitter streaming API"""
    
    def __init__(self, texts, targets, seed=0, batchsize=100):
        self.texts_pos = [text for text, target in zip(texts, targets)
                               if target > 0]
        self.texts_neg = [text for text, target in zip(texts, targets)
                               if target <= 0]
        self.rng = Random(seed)
        self.batchsize = batchsize

    def next_batch(self, batchsize=None):
        batchsize = self.batchsize if batchsize is None else batchsize
        texts, targets = [], []
        for i in range(batchsize):
            # Select the polarity randomly
            target = self.rng.choice((-1, 1))
            targets.append(target)
            
            # Combine 2 random texts of the right polarity
            pool = self.texts_pos if target > 0 else self.texts_neg
            text = self.rng.choice(pool) + " " + self.rng.choice(pool)
            texts.append(text)
        return texts, targets

infinite_stream = InfiniteStreamGenerator(text_train_small, target_train_small)



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texts_in_batch, targets_in_batch = infinite_stream.next_batch(batchsize=3)



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for t in texts_in_batch:
    print(t + "\n")



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targets_in_batch

We can now use our infinte tweet source to train an online machine learning algorithm using the hashing vectorizer. Note the use of the partial_fit method of the PassiveAggressiveClassifier instance in place of the traditional call to the fit method that needs access to the full training set.

From time to time, we evaluate the current predictive performance of the model on our validation set that is guaranteed to not overlap with the infinite training set source:



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n_batches = 1000
validation_scores = []
training_set_size = []

# Build the vectorizer and the classifier
h_vectorizer = HashingVectorizer(encoding='latin-1')
clf = PassiveAggressiveClassifier(C=1)

# Extract the features for the validation once and for all
X_validation = h_vectorizer.transform(text_validation)
classes = np.array([-1, 1])

n_samples = 0
for i in range(n_batches):
    
    texts_in_batch, targets_in_batch = infinite_stream.next_batch()    
    n_samples += len(texts_in_batch)

    # Vectorize the text documents in the batch
    X_batch = h_vectorizer.transform(texts_in_batch)
    
    # Incrementally train the model on the new batch
    clf.partial_fit(X_batch, targets_in_batch, classes=classes)
    
    if n_samples % 100 == 0:
        # Compute the validation score of the current state of the model
        score = clf.score(X_validation, target_validation)
        validation_scores.append(score)
        training_set_size.append(n_samples)

    if i % 100 == 0:
        print("n_samples: {0}, score: {1:.4f}".format(n_samples, score))

We can now plot the collected validation score values, versus the number of samples generated by the infinite source and feed to the model:



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plt.plot(training_set_size, validation_scores)
plt.ylim(0.5, 1)
plt.xlabel("Number of samples")
plt.ylabel("Validation score")

Limitations of the Hashing Vectorizer

Using the Hashing Vectorizer makes it possible to implement streaming and parallel text classification but can also introduce some issues:

The collisions can introduce too much noise in the data and degrade prediction quality,
The HashingVectorizer does not provide "Inverse Document Frequency" reweighting (lack of a use_idf=True option).
There is no easy way to inverse the mapping and find the feature names from the feature index.

The collision issues can be controlled by increasing the n_features parameters.

The IDF weighting might be reintroduced by appending a TfidfTransformer instance on the output of the vectorizer. However computing the idf_ statistic used for the feature reweighting will require to do at least one additional pass over the training set before being able to start training the classifier: this breaks the online learning scheme.

The lack of inverse mapping (the get_feature_names() method of TfidfVectorizer) is even harder to workaround. That would require extending the HashingVectorizer class to add a "trace" mode to record the mapping of the most important features to provide statistical debugging information.

In the mean time to debug feature extraction issues, it is recommended to use TfidfVectorizer(use_idf=False) on a small-ish subset of the dataset to simulate a HashingVectorizer() instance that have the get_feature_names() method and no collision issues.