Text Classification Using a Convolutional Neural Network on MXNet

This tutorial is based of Yoon Kim’s paper on using convolutional neural networks for sentence sentiment classification.

For this tutorial, we will train a convolutional deep network model on movie review sentences from Rotten Tomatoes labeled with their sentiment. The result will be a model that can classify a sentence based on its sentiment (with 1 being a purely positive sentiment, 0 being a purely negative sentiment and 0.5 being neutral).

Our first step will be to fetch the labeled training data of positive and negative sentiment sentences and process it into sets of vectors that are then randomly split into train and test sets.

import urllib2
import numpy as np
import re
import itertools
from collections import Counter

def clean_str(string):
    """
    Tokenization/string cleaning for all datasets except for SST.
    Original taken from https://github.com/yoonkim/CNN_sentence/blob/master/process_data.py
    """
    string = re.sub(r"[^A-Za-z0-9(),!?\'\`]", " ", string)
    string = re.sub(r"\'s", " \'s", string)
    string = re.sub(r"\'ve", " \'ve", string)
    string = re.sub(r"n\'t", " n\'t", string)
    string = re.sub(r"\'re", " \'re", string)
    string = re.sub(r"\'d", " \'d", string)
    string = re.sub(r"\'ll", " \'ll", string)
    string = re.sub(r",", " , ", string)
    string = re.sub(r"!", " ! ", string)
    string = re.sub(r"\(", " \( ", string)
    string = re.sub(r"\)", " \) ", string)
    string = re.sub(r"\?", " \? ", string)
    string = re.sub(r"\s{2,}", " ", string)
    return string.strip().lower()

def load_data_and_labels():
    """
    Loads MR polarity data from files, splits the data into words and generates labels.
    Returns split sentences and labels.
    """
    # Pull sentences with positive sentiment
    pos_file = urllib2.urlopen('https://raw.githubusercontent.com/yoonkim/CNN_sentence/master/rt-polarity.pos')

    # Pull sentences with negative sentiment
    neg_file = urllib2.urlopen('https://raw.githubusercontent.com/yoonkim/CNN_sentence/master/rt-polarity.neg')

    # Load data from files
    positive_examples = list(pos_file.readlines())
    positive_examples = [s.strip() for s in positive_examples]
    negative_examples = list(neg_file.readlines())
    negative_examples = [s.strip() for s in negative_examples]
    # Split by words
    x_text = positive_examples + negative_examples
    x_text = [clean_str(sent) for sent in x_text]
    x_text = [s.split(" ") for s in x_text]
    # Generate labels
    positive_labels = [1 for _ in positive_examples]
    negative_labels = [0 for _ in negative_examples]
    y = np.concatenate([positive_labels, negative_labels], 0)
    return [x_text, y]


def pad_sentences(sentences, padding_word="</s>"):
    """
    Pads all sentences to the same length. The length is defined by the longest sentence.
    Returns padded sentences.
    """
    sequence_length = max(len(x) for x in sentences)
    padded_sentences = []
    for i in range(len(sentences)):
        sentence = sentences[i]
        num_padding = sequence_length - len(sentence)
        new_sentence = sentence + [padding_word] * num_padding
        padded_sentences.append(new_sentence)
    return padded_sentences


def build_vocab(sentences):
    """
    Builds a vocabulary mapping from word to index based on the sentences.
    Returns vocabulary mapping and inverse vocabulary mapping.
    """
    # Build vocabulary
    word_counts = Counter(itertools.chain(*sentences))
    # Mapping from index to word
    vocabulary_inv = [x[0] for x in word_counts.most_common()]
    # Mapping from word to index
    vocabulary = {x: i for i, x in enumerate(vocabulary_inv)}
    return [vocabulary, vocabulary_inv]


def build_input_data(sentences, labels, vocabulary):
    """
    Maps sentences and labels to vectors based on a vocabulary.
    """
    x = np.array([[vocabulary[word] for word in sentence] for sentence in sentences])
    y = np.array(labels)
    return [x, y]

"""
Loads and preprocessed data for the MR dataset.
Returns input vectors, labels, vocabulary, and inverse vocabulary.
"""
# Load and preprocess data
sentences, labels = load_data_and_labels()
sentences_padded = pad_sentences(sentences)
vocabulary, vocabulary_inv = build_vocab(sentences_padded)
x, y = build_input_data(sentences_padded, labels, vocabulary)

vocab_size = len(vocabulary)

# randomly shuffle data
np.random.seed(10)
shuffle_indices = np.random.permutation(np.arange(len(y)))
x_shuffled = x[shuffle_indices]
y_shuffled = y[shuffle_indices]

# split train/dev set
# there are a total of 10662 labeled examples to train on
x_train, x_dev = x_shuffled[:-1000], x_shuffled[-1000:]
y_train, y_dev = y_shuffled[:-1000], y_shuffled[-1000:]

sentence_size = x_train.shape[1]

print 'Train/Dev split: %d/%d' % (len(y_train), len(y_dev))
print 'train shape:', x_train.shape
print 'dev shape:', x_dev.shape
print 'vocab_size', vocab_size
print 'sentence max words', sentence_size
Train/Dev split: 9662/1000
train shape: (9662, 56)
dev shape: (1000, 56)
vocab_size 18766
sentence max words 56

Now that we prepared the training and test data by loading, vectorizing and shuffling it we can go on to defining the network architecture we want to train with the data.

We will first set up some placeholders for the input and output of the network then define the first layer, an embedding layer, which learns to map word vectors into a lower dimensional vector space where distances between words correspond to how related they are (with respect to sentiment they convey).

import mxnet as mx
import sys,os

'''
Define batch size and the place holders for network inputs and outputs
'''

batch_size = 50 # the size of batches to train network with
print 'batch size', batch_size

input_x = mx.sym.Variable('data') # placeholder for input data
input_y = mx.sym.Variable('softmax_label') # placeholder for output label


'''
Define the first network layer (embedding)
'''

# create embedding layer to learn representation of words in a lower dimensional subspace (much like word2vec)
num_embed = 300 # dimensions to embed words into
print 'embedding dimensions', num_embed

embed_layer = mx.sym.Embedding(data=input_x, input_dim=vocab_size, output_dim=num_embed, name='vocab_embed')

# reshape embedded data for next layer
conv_input = mx.sym.Reshape(data=embed_layer, target_shape=(batch_size, 1, sentence_size, num_embed))
batch size 50
embedding dimensions 300

The next layer in the network performs convolutions over the ordered embedded word vectors in a sentence using multiple filter sizes, sliding over 3, 4 or 5 words at a time. This is the equivalent of looking at all 3-grams, 4-grams and 5-grams in a sentence and will allow us to understand how words contribute to sentiment in the context of those around them.

After each convolution, we add a max-pool layer to extract the most significant elements in each convolution and turn them into a feature vector.

Because each convolution+pool filter produces tensors of different shapes we need to create a layer for each of them, and then concatenate the results of these layers into one big feature vector.

# create convolution + (max) pooling layer for each filter operation
filter_list=[3, 4, 5] # the size of filters to use
print 'convolution filters', filter_list

num_filter=100
pooled_outputs = []
for i, filter_size in enumerate(filter_list):
    convi = mx.sym.Convolution(data=conv_input, kernel=(filter_size, num_embed), num_filter=num_filter)
    relui = mx.sym.Activation(data=convi, act_type='relu')
    pooli = mx.sym.Pooling(data=relui, pool_type='max', kernel=(sentence_size - filter_size + 1, 1), stride=(1,1))
    pooled_outputs.append(pooli)

# combine all pooled outputs
total_filters = num_filter * len(filter_list)
concat = mx.sym.Concat(*pooled_outputs, dim=1)

# reshape for next layer
h_pool = mx.sym.Reshape(data=concat, target_shape=(batch_size, total_filters))
convolution filters [3, 4, 5]

Next, we add dropout regularization, which will randomly disable a fraction of neurons in the layer (set to 50% here) to ensure that that model does not overfit. This works by preventing neurons from co-adapting and forcing them to learn individually useful features.

This is necessary for our model because the dataset has a vocabulary of size around 20k and only around 10k examples so since this data set is pretty small we’re likely to overfit with a powerful model (like this neural net).

# dropout layer
dropout=0.5
print 'dropout probability', dropout

if dropout > 0.0:
    h_drop = mx.sym.Dropout(data=h_pool, p=dropout)
else:
    h_drop = h_pool
dropout probability 0.5

Finally, we add a fully connected layer to add non-linearity to the model. We then classify the resulting output of this layer using a softmax function, yielding a result between 0 (negative sentiment) and 1 (positive).

# fully connected layer
num_label=2

cls_weight = mx.sym.Variable('cls_weight')
cls_bias = mx.sym.Variable('cls_bias')

fc = mx.sym.FullyConnected(data=h_drop, weight=cls_weight, bias=cls_bias, num_hidden=num_label)

# softmax output
sm = mx.sym.SoftmaxOutput(data=fc, label=input_y, name='softmax')

# set CNN pointer to the "back" of the network
cnn = sm

Now that we have defined our CNN model we will define the device on our machine that we will train and execute this model on, as well as the datasets to train and test this model with.

If you are running this code be sure that you have a GPU on your machine if your ctx is set to mx.gpu(0) otherwise you can set your ctx to mx.cpu(0) which will run the training much slower

from collections import namedtuple
import time
import math

# Define the structure of our CNN Model (as a named tuple)
CNNModel = namedtuple("CNNModel", ['cnn_exec', 'symbol', 'data', 'label', 'param_blocks'])

# Define what device to train/test on
ctx=mx.gpu(0)
# If you have no GPU on your machine change this to
# ctx=mx.cpu(0)

arg_names = cnn.list_arguments()

input_shapes = {}
input_shapes['data'] = (batch_size, sentence_size)

arg_shape, out_shape, aux_shape = cnn.infer_shape(**input_shapes)
arg_arrays = [mx.nd.zeros(s, ctx) for s in arg_shape]
args_grad = {}
for shape, name in zip(arg_shape, arg_names):
    if name in ['softmax_label', 'data']: # input, output
        continue
    args_grad[name] = mx.nd.zeros(shape, ctx)

cnn_exec = cnn.bind(ctx=ctx, args=arg_arrays, args_grad=args_grad, grad_req='add')

param_blocks = []
arg_dict = dict(zip(arg_names, cnn_exec.arg_arrays))
initializer=mx.initializer.Uniform(0.1)
for i, name in enumerate(arg_names):
    if name in ['softmax_label', 'data']: # input, output
        continue
    initializer(name, arg_dict[name])

    param_blocks.append( (i, arg_dict[name], args_grad[name], name) )

out_dict = dict(zip(cnn.list_outputs(), cnn_exec.outputs))

data = cnn_exec.arg_dict['data']
label = cnn_exec.arg_dict['softmax_label']

cnn_model= CNNModel(cnn_exec=cnn_exec, symbol=cnn, data=data, label=label, param_blocks=param_blocks)

We can now execute the training and testing of our network, which in-part mxnet automatically does for us with its forward and backward propagation methods, along with its automatic gradient calculations.

'''
Train the cnn_model using back prop
'''

optimizer='rmsprop'
max_grad_norm=5.0
learning_rate=0.0005
epoch=50

print 'optimizer', optimizer
print 'maximum gradient', max_grad_norm
print 'learning rate (step size)', learning_rate
print 'epochs to train for', epoch

# create optimizer
opt = mx.optimizer.create(optimizer)
opt.lr = learning_rate

updater = mx.optimizer.get_updater(opt)

# create logging output
logs = sys.stderr

# For each training epoch
for iteration in range(epoch):
    tic = time.time()
    num_correct = 0
    num_total = 0

    # Over each batch of training data
    for begin in range(0, x_train.shape[0], batch_size):
        batchX = x_train[begin:begin+batch_size]
        batchY = y_train[begin:begin+batch_size]
        if batchX.shape[0] != batch_size:
            continue

        cnn_model.data[:] = batchX
        cnn_model.label[:] = batchY

        # forward
        cnn_model.cnn_exec.forward(is_train=True)

        # backward
        cnn_model.cnn_exec.backward()

        # eval on training data
        num_correct += sum(batchY == np.argmax(cnn_model.cnn_exec.outputs[0].asnumpy(), axis=1))
        num_total += len(batchY)

        # update weights
        norm = 0
        for idx, weight, grad, name in cnn_model.param_blocks:
            grad /= batch_size
            l2_norm = mx.nd.norm(grad).asscalar()
            norm += l2_norm * l2_norm

        norm = math.sqrt(norm)
        for idx, weight, grad, name in cnn_model.param_blocks:
            if norm > max_grad_norm:
                grad *= (max_grad_norm / norm)

            updater(idx, grad, weight)

            # reset gradient to zero
            grad[:] = 0.0

    # Decay learning rate for this epoch to ensure we are not "overshooting" optima
    if iteration % 50 == 0 and iteration > 0:
        opt.lr *= 0.5
        print >> logs, 'reset learning rate to %g' % opt.lr

    # End of training loop for this epoch
    toc = time.time()
    train_time = toc - tic
    train_acc = num_correct * 100 / float(num_total)

    # Saving checkpoint to disk
    if (iteration + 1) % 10 == 0:
        prefix = 'cnn'
        cnn_model.symbol.save('./%s-symbol.json' % prefix)
        save_dict = {('arg:%s' % k) :v  for k, v in cnn_model.cnn_exec.arg_dict.items()}
        save_dict.update({('aux:%s' % k) : v for k, v in cnn_model.cnn_exec.aux_dict.items()})
        param_name = './%s-%04d.params' % (prefix, iteration)
        mx.nd.save(param_name, save_dict)
        print >> logs, 'Saved checkpoint to %s' % param_name


    # Evaluate model after this epoch on dev (test) set
    num_correct = 0
    num_total = 0

    # For each test batch
    for begin in range(0, x_dev.shape[0], batch_size):
        batchX = x_dev[begin:begin+batch_size]
        batchY = y_dev[begin:begin+batch_size]

        if batchX.shape[0] != batch_size:
            continue

        cnn_model.data[:] = batchX
        cnn_model.cnn_exec.forward(is_train=False)

        num_correct += sum(batchY == np.argmax(cnn_model.cnn_exec.outputs[0].asnumpy(), axis=1))
        num_total += len(batchY)

    dev_acc = num_correct * 100 / float(num_total)
    print >> logs, 'Iter [%d] Train: Time: %.3fs, Training Accuracy: %.3f \
            --- Dev Accuracy thus far: %.3f' % (iteration, train_time, train_acc, dev_acc)

Now that we have gone through the trouble of training the model, we have stored the learned parameters in the .params file in our local directory. We can now load this file whenever we want and predict the sentiment of new sentences by running them through a forward pass of the trained model.