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A Complete Guide to Naive Bayes Algorithm in Python

Naive Bayes is a classification algorithm for binary class and multiclass classification problems. Naive Bayes is applied on each row and column. It is used for text classification.

What is Probability?

The chance of something happening is called ‘probability.’

Probability = No. of outcome / Total No. of outcome

  • Flipping a Coin(H,T) = ½ — Independent Event
    Rolling a Dice, P(5) = ⅙ — Independent Event
    Let’s take both events
    P(H,5)= P(H) * P(5) —Joint Event
  • Event A = Taking a blue marble
Event B= Taking Second blue marble
P(2B)=2/4 = ½ 

P(getting 2nd blue marble / when already 1st blue marble has been taken out)=½

Event A and B are dependent events, hence we will use Conditional Probability.

General Formula

Let’s look at an example :
Task: Based on weather conditions predict if the player will play or not.

Step 1: Make a Frequency table

                     FREQUENCY TABLE    

Step 2 : Create a Likelihood table

                                LIKELIHOOD TABLE                

The problem statement is whether the players would play if the weather is sunny.

P(Yes/Sunny) = P(Sunny/Yes) * P(Yes) / P(Sunny)
= 3/9 * 9/14 / 5/14
P ( No / Overcast ) = P ( Overcast / No ) * P ( No ) / P (Overcast)
=0/5 * 5/14 / 0/14
= 0

Naive Bayes Algorithm Explained

Feature Vector Creation

The feature vector is the method which converts text data into feature vectors. Feature vector creation can be done by
Bag of words


Bag of words

Step1: Tokenization ( Removing Stopwords)

Sentence 1: He is a good boy.                  — good boy
Sentence 2: She is a good girl.                 — good girl
Sentence 3: Both are good boy and girl    — good boy girl

Step2: Feature vector creation ( unique tokens are getting features )

f1    f2      f3
  Good     boy    girl

Sent1 1 1 0

Sent2 1 0 1

Sent3 1 1 1


TF-IDF is short for Term Frequency Inverse Document Frequency

TF = ( No. of times the word repeated in sentence /
       Total No. of words in a sentence)

IDF = log ( Total No. of sentence / 
        No.of sentences containing that word )

Step1: Tokenization ( Removing Stopwords )

         Sentence 1:    — good boy
Sentence 2:    — good girl
         Sentence 3:    — good boy good girl

Step2: Creating TF table

        Sent1   Sent2   Sent3 

    Good         1/2             1/2               1/3

     Boy            1/2               0                1/3

     Girl              0               1/2               1/3

Step3: Creating IDF table

Good log( 3 / 3 ) = log(1) =0

Boy log ( 3 / 2 )

Girl log ( 3 / 2 )

Step4: Feature vector creation

      Good            Boy               Girl

Sent1 1/2 * 0 1/2 * log( 3 / 2 ) 0

Sent2 1/2 * 0 0 1/2 * log( 3 / 2 )

Sent3 1/3 * 0 1/3 * log( 3 / 2 ) 1/3 * log( 3 / 2 )

3 Types of Naive Bayes in Scikit Learn


  • It is used for classification problems and it assumes that features have a normal distribution.


  • It is used for discrete counts.


  • It is used for binary counts(ie., Zeroes and One).

Pros of Naive Bayes

  • Naive Bayes is a fast and highly scalable algorithm.
  • Naive Bayes can be classified into both binary classification and multi-class classification. It has different types of Naive Bayes Algorithms like GaussianNB, MultinominalNB, and BernoulliNB.
  • The algorithm is simple and depends on doing a bunch of counts.
  • Best choice for text classification problems. Widely used in spam mail classification.
  • Training on small datasets is easy.

Cons of Naive Bayes

  • It cannot learn the relationship between features because it considers all the features to be unrelated.

Application of Naive Bayes

  • Naive Bayes is broadly utilized for text classification.
  • News article classification SPORTS, TECHNOLOGY etc.
  • Spam or Ham: Naive Bayes is the most popularly used for mail filtering.

Python Implementation of Naive Bayes

  • Importing the required libraries
  • import pandas as pd
  • from sklearn.model_selection import train_test_split
  • from sklearn.feature_extraction.text import CountVectorizer,TfidfVectorizer
  • from sklearn.naive_bayes import MultinomialNB
  • from sklearn.metrics import accuracy_score
  • import string
  • import matplotlib.pyplot as plt

Refer to the following articles:

Why is Python an Interpreted Programming Language?

Scala vs Python for Apache Spark

What is Python Programming?

Loading the dataset

data = pd.read_csv(“spam.tsv”,sep=’\t’,names=[‘Class’,’Message’])
data.head(8) # View the first 8 records of our dataset


The mails are categorized into 2 classes ie., spam and ham.

#Let’s see the count of each class



Text PreProcessing

#Lets assign ham as 1

data.loc[data[‘Class’]==”ham”,”Class”] = 1

#Lets assign spam as 0

data.loc[data[‘Class’]==”spam”,”Class”] = 0

Removing punctuations

#the default list of punctuations

import string


#Let’s remove the punctuation

def remove_punct(text1):
text1 = “”.join([c for c in text if c not in string.punctuation])
return text1
data[‘text_clean’] = data[‘Message’].apply(lambda x: remove_punct(x))



#Countvectorizer is used to convert text to numerical data.

#Initialize the object for countvectorizer

CV = CountVectorizer(stop_words=”english”)

Splitting x and y
xSet = data[‘text_clean’].values
ySet = data[‘Class’].values

Splitting Train and Test Data
xSet_train,xSet_test,ySet_train,ySet_test = train_test_split(xSet,ySet,test_size=0.2, random_state=10)

xSet_train_CV = CV.fit_transform(xSet_train)

Feature vectorizer

#text preprocessing and feature vectorizer

#To extract features from a document of words, we import TfidfVectorizer

from sklearn.feature_extraction.text import TfidfVectorizer
tf=TfidfVectorizer() ## object creation
X=tf.fit_transform(X) ## fitting and transforming the data into vectors

#Training the model

#Creating training and testing

from sklearn.model_selection import train_test_split

#Model creation

from sklearn.naive_bayes import MultinomialNB

#model object creation


#fitting the model



#getting the prediction



#Evaluating the model

from sklearn.metrics import classification_report,confusion_matrix

msg = input(“Enter Message: “) # to get the input message
msgInput = CV.transform([msg]) #
predict = NB.predict(msgInput)


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