sentiment prediction with Naive Bayes

Thu 01 May 2014

Filed under python

Tags python

This project consists of two sets of labelled text reviews (positive and negative) for movies.

These are used as the training set; the words are split into unigrams, and the words are classified as either positive or negative given their frequencies.

Once the model is built, the test set is used to help determine the accuracy of the model.

This is an early project I did some time ago, please forgive some of the less graceful elements.

The algorithm uses these steps:

  1. For each iteration
    (a) Randomly divide your data in training and testing using 1/3 for testing and 2/3 for training.
    (b) Use the training set to estimate the parameters $P(w|c)$ and $P(c)$ as per the naive bayes statement
    (c) For every document in the training set use the equation $$c^{*}=argmax_{c}log(P(c)) + \sum_{i=1}^{M}n_{i}(d) * log(P(w_{i}|c))$$ to compute $P(c|d)$ and predict the class c.
    (d) Compute the accuracy of the testing set.

  2. Do at least 3 iterations to compute the average accuracy as your performance metric.

The output will print results for each iteration, giving key metrics like this:

iteration 1:
num pos test_docs:333
num pos training docs:667
num pos correct_docs:267
num neg test_docs:331
num neg training_docs:669
num neg correct_docs:261
iteration 2:
iteration 3:

Make sure to include the command line parameters python -d my_directory where my_directory is the path to the directory that holds the positive and negative reviews along with the python script.

#!/usr/bin/env python
import argparse
import os
import random
import math
import collections

def parseArgument():
    ### Code for parsing arguments
    parser = argparse.ArgumentParser(description='Parsing a file.')
    parser.add_argument('-d', nargs=1, required=True)
    args = vars(parser.parse_args())
    return args 
###  This function takes the directory path and divides the file names found in the pos / neg directories
###  into the proper proportions, stored into a dict named 'D'.
def CreateD(directory):
    pos_train = []
    pos_test = []
    neg_train = []
    neg_test = []
    pos = directory + '/pos'
    neg = directory + '/neg'
    pos_names = os.listdir(pos)
    neg_names = os.listdir(neg)
    while pos_names:
        if pos_names:
        if pos_names:
    while neg_names:
        if neg_names:
        if neg_names:
    fileDict = {}
    fileDict["pos_train"] = pos_train
    fileDict["pos_test"] = pos_test
    fileDict["neg_train"] = neg_train
    fileDict["neg_test"] = neg_test
    return fileDict

### This function used a counter object to count the number of documents in the class 'pos' or 'neg'.  This is used
### for printing summary statistics at the end.
def CountDocsInClass(directory,subdir,file_list):
    cntr = collections.Counter()
    for file in file_list:
        f = open(directory+'/'+subdir+'/'+file)#open each File
        cntr = cntr + collections.Counter([t for t in ( if not t in stopWords])#add word counts to counter object
        f.close()#Close each File
    return cntr

def main():
    args = parseArgument()
    directory = args['d'][0]
    # variable for calculating average accuracy
    tot_acc = 0
    # Iterations
    for step in range(1,4):

### Here is the training code:
### First we create the dictionary of filenames using the CreateD function.
        file_dict = CreateD(directory)

### Here we count the number of documents in the 'pos' and 'neg' class using the CountDocsInClass function.
        positive_counter = CountDocsInClass(directory,'pos',file_dict['pos_train'])
        negative_counter = CountDocsInClass(directory,'neg',file_dict['neg_train'])
### These 2 lines find a total count in the counters for each class.
### We create V, as specified in the Vocabulary list
        V = set(positive_counter.keys()+ negative_counter.keys())
### The final_dict will hold the conditional probabilities for each t in V
        final_dict = {}
        for t in V:
            final_dict[t] = {'pos':(positive_counter[t]+1.0)/(tot_cnt_pos+len(V)+1),'neg':(negative_counter[t]+1.0)/(tot_cnt_neg+len(V)+1)}
### calculating prior probabilities for each class
        pos_prior = len(file_dict['pos_train'])/float(len(file_dict['pos_train']) + float(len(file_dict['neg_train'])))
        neg_prior = len(file_dict['neg_train'])/float(len(file_dict['pos_train']) + float(len(file_dict['neg_train'])))
### This code is for the Testing part of the Naive Bayes algorithm
### We create a list of classes
        C = ['pos','neg']
### initializing filescore dictionary to keep track of test data - has the test file as key and four other associated
        # parameters namely:
        #                   orig    -  is file originally +ve or -ve
        #                   pos     -  sum of log probs for +ve class
        #                   neg     -  sum of log probs for +ve class
        #                   output  -  output file classification after comparing 'pos' and 'neg'
        filescore = {}
        # for all test files combined run loop
        for file in (file_dict['pos_test']+file_dict['neg_test']):
            # initialize  the file dictionary within filescore dictionary
            filescore[file] = {}
            # populate 'orig' attribute for each file i.e. filescore[file]['orig']
            if file in file_dict['pos_test']:
                f = open(directory+'/pos/'+file)#open each File
                filescore[file]['orig'] = 'pos'
                f = open(directory+'/neg/'+file)#open each File
                filescore[file]['orig'] = 'neg'
            # extract all words for the given file into t_list after removing stopwords
            t_list = [t for t in ( if not t in stopWords]
            # within a file run the below loop for each class to sum the log probabilities for each class
            for clss in C:
                # initializing score variable with prior probbabilities
                if (clss == 'pos'):
                    score = math.log(pos_prior)
                    score = math.log(neg_prior)
### The code below takes logs as per pseudocode
                for t in t_list:
                    if final_dict.has_key(t):
                        score = score + math.log(final_dict[t][clss])
                    # handling unknown words in test file
                        if clss=='pos':
                            score = score + (1/(tot_cnt_pos+len(V)+1))
                        if clss=='neg':
                            score = score + (1/(tot_cnt_neg+len(V)+1))
                filescore[file][clss] = score
###   Counting the results
            if (filescore[file]['pos'] > filescore[file]['neg']):
                filescore[file]['output'] = 'pos'
                filescore[file]['output'] = 'neg'

###   Create some counters to track the numbers of successes for each class
        count_positive_success = 0
        count_negative_success = 0
        for fl in filescore:
            if ((filescore[fl]['orig'] == 'pos') and (filescore[fl]['output'] == filescore[fl]['orig'])):
                count_positive_success = count_positive_success + 1
            if ((filescore[fl]['orig'] == 'pos') and (filescore[fl]['output'] == filescore[fl]['orig'])):
                count_negative_success = count_negative_success + 1
### Print results to screen
        print "iteration ",step,":"
        print "num_pos_test_docs:",len(file_dict['pos_test'])
        print "num_pos_training_docs:",len(file_dict['pos_train'])
        print "num_pos_correct_docs:",count_positive_success
        print "num_neg_test_docs:",len(file_dict['neg_test'])
        print "num_neg_training_docs:",len(file_dict['neg_train'])
        print "num_neg_correct_docs:",count_negative_success
        acc = 100 * (count_positive_success+count_negative_success)/(len(file_dict['pos_test'])+len(file_dict['neg_test']))
        print "accuracy:",acc,'%'
        tot_acc = tot_acc + acc
    print "ave_accuracy:",tot_acc/3.0



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