Class 12 Python CSV File Handling Important Notes : Reading and Writing Handout

Python CSV File Handling

• CSV (Comma-separated values) is a common data exchange format used by the applications to produce and consume data.  
• A CSV file is a simple text file where each line contains a list of values (or fields) delimited by commas (mostly), but you will encounter CSV files where data is delimited using tab (\t) or pipe (|) or any other character. 
• The first line of the CSV file represents the header containing a list of column names in the file. 
• CSV file is commonly used to represent tabular data 
• See the following table 

Name	Class	Section
Amit	X	A
Sumit	XI	B
Ashish	XII	C

The above table will be stored in CSV format as follows: 

If the values in the table contain comma(,) like below in column Address 

Then in CSV it will be stored like below (Values containing comma will be enclosed in double quotes) 

Python Provides CSV module to work with csv file: 

Main Functions are: 

1. reader() 
2. writer() 
3. DictReader() 
4. DictWriter() 

1. reader() function :  This function help us to read the csv file. This function takes a file object and returns a _csv.reader object that can be used to iterate over the contents of a CSV file. 

How to read entire data from data.csv file? 

Let we try to read the following file ie “data.csv” 

data.csv 

CODING	OUTPUT
import csv f = open(“data.csv”, ‘r’) row = csv.reader(f) for i in row : print(i)	[‘Name’ , ‘Class’ , ‘Subject’] [‘Amit’ , ‘XII’ , ‘ CS’] [‘Sumit’ , ‘X’ , ‘IP’]   Notice that each line in the CSV file is returned as a list of strings

Line-wise explanation of the code 

1. The first line is importing a csv module. 
2. This line is opening a file (data.csv) in reading mode and f is file object or file handle. 
3. This line is returning a csv.reader object that can be used to iterate over csv file contents 
4. We are using loop on the object which is returned by the previous line 

CODING	OUTPUT
import csv f = open(“data.csv”, ‘r’) row = csv.reader(f) print(row)	Returns the memory address where csv.reader object is stored

How to read specific column from data.csv? 

CODING	OUTPUT	Actual Data of File
import csv f = open(“data.csv”, ‘r’) row = csv.reader(f) for i in row : print(i[0] , i[2])   The above code is reading first and third column  from “data.csv” file	Name Subject Amit CS Sumit IP	[‘Name’ , ‘Class’ , ‘Subject’] [‘Amit’ , ‘XII’ , ‘ CS’] [‘Sumit’ , ‘X’ , ‘IP’]

If you want to skip the First Row (header of the table) 

CODING	OUTPUT	Actual Data of File
import csv f = open(“data.csv”, ‘r’) row = csv.reader(f) next(row) for i in row : print(i[0] , i[2])  next () function will jump to the next row.      import csv f = open(“data.csv”, ‘r’) row = csv.reader(f) next(row) next(row) for i in row : print(i[0] , i[2])  If we use the next() function two times then it will skip two lines from beginning	[ ‘Amit’ , ‘CS’] [‘Sumit’ , ‘IP’ ] [‘Sumit’ , ‘IP’ ]	[‘Name’ , ‘Class’ , ‘Subject’] [‘Amit’ , ‘XII’ , ‘ CS’] [‘Sumit’ , ‘X’ , ‘IP’]  [‘Name’ , ‘Class’ , ‘Subject’] [‘Amit’ , ‘XII’ , ‘CS’] [‘Sumit’ , ‘X’ , ‘IP’]

_______________________________________________________ 

If there is any other delimiter other than comma like in the following file 

CODING	OUTPUT
import csv f = open(“data.csv”, ‘r’) row = csv.reader(f, delimiter = ‘l’) for i in row : print(i)	[‘id’ , ‘name’ , ’email’ , ‘salary’] [‘1’ , ‘Amit’ , ‘amit@mail.com’ , ‘25000’] [‘2’ , ‘Sumit’ , ‘sumit@mail.com’ , ‘30000’] [‘3’ , ‘Ashu’ , ‘ashu@mail.com’ , ‘40000’]

2. writer() function : This function help us to write data in a csv file. It accepts the same argument as the reader() function but returns a writer object (i.e _csv.writer) 

There are two main methods used in writing in csv file 

1. writerow() 
2. writerows() 

Example using writerow() 

CODING	DATA.CSV
import csv f = open(“data.csv”, ‘w’) wr = csv.writer(f) wr.writerow([‘Name’ , ‘Class’]) wr.writerow([‘Amit’ , ‘XII’]) f.close( )	Name , Class ‘Amit’ , ‘XII’

Example using writerows()  

CODING	DATA.CSV
import csv fields = [‘Name’ , ‘Class’ , ‘Subject’] rows = [[“Amit” , ‘XII’ , ‘CS’] , [‘Sumit’ , ‘X’ , ‘IP’] , [‘Ashu’ , ‘XI’ , ‘CS’]] f = open(“data.csv” , ‘w’ , newline = ‘ ‘) wr = csv.writer(f) wr.writerow(fields) wr.writerows(rows) f.close	Name, Class, Subject Amit, XII, CS Sumit, X, IP Ashu, XI, CS

Reading a CSV file with DictReader: 

This function(DictReader) is working similar to reader(). This function return lines as a dictionary instead of list. 

CODING	OUTPUT	data.csv
import csv f = open(“data.csv” , ‘r’) row = csv.DictReader(f) for i in row: print(i)	{‘Name’ : ‘Amit, ‘Class’ : ‘XII’} {‘Name’ : ‘Sumit, ‘Class’ : ‘X’} {‘Name’ : ‘Ashu, ‘Class’ : ‘XI’}	Name, Class Amit, XII Sumit, X Ashu, XI

This CSV file has no header. So we have to provide field names via the fieldnames parameter 

CODING

import csv f = open(“data.csv” , ‘r’) row = csv.DictReader(f, fieldnames = [‘Name’ , ‘Class’ , ‘Subject’]) for i in row: print(i)

OUTPUT

{‘Name’ : ‘Amit, ‘Class’ : ‘XII’ , ‘Subject’ : ‘CS’} {‘Name’ : ‘Sumit, ‘Class’ : ‘X’ , ‘Subject’ : ‘IP’} {‘Name’ : ‘Ashu, ‘Class’ : ‘XI’ , ‘Subject’ : ‘CS’}

data.csv

Amit, XII, CS Sumit, X, IP Ashu, XI, CS

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