######################################################
#Multinomial Naive Bayes Classifier
print '*************************\nNaive Bayes\n*************************'
from sklearn.naive_bayes import MultinomialNB
from sklearn import metrics
newsgroups_test = fetch_20newsgroups(subset = 'test',
                                     categories = categories);
fea_test = vectorizer.fit_transform(newsgroups_test.data);
#create the Multinomial Naive Bayesian Classifier
clf = MultinomialNB(alpha = 0.01) 
clf.fit(fea_train,newsgroup_train.target);
pred = clf.predict(fea_test);
calculate_result(newsgroups_test.target,pred);
#notice here we can see that f1_score is not equal to 2*precision*recall/(precision+recall)
#because the m_precision and m_recall we get is averaged, however, metrics.f1_score() calculates
#weithed average, i.e., takes into the number of each class into consideration.