CBSE CS and IP

CBSE Class 11 & 12 Computer Science and Informatics Practices Python Materials, Video Lecture

CSV (Comma Separated Values) Handling in Python

CSV File Handling in Python


What is CSV File

  • A Comma Separated Values (CSV) file is a plain text file that contains the comma-separated data.
  • These files are often used for exchanging data between different applications.
  • CSV files are usually created by programs that handle huge amounts of data. They are used to export data from spreadsheets (ex:- excel file) and databases (Ex:- Oracle, MySQL). It can be used to import data into a spreadsheet or a database.


CSV File Structure


## sample.csv file structure

Name, DOB, City
Ram, 12-Jul-2001, Delhi
Mohan, 23-Jan-2005, Delhi
Suraj, 17-Dec-2002, Kolkata


Python CSV module

  • CSV Module is available in Python Standard Library.
  • The CSV module contains classes that are used to read and write tabular form of data into CSV format.
  • To work with CSV Files, programmer have to import CSV Module.

import csv

Methods of CSV Module :
  • writer( ) 
  • reader( )
Both the methods return an Object of writer or reader class. Writer Object again have two methods  – writerow( ) , writerows( ).

writer( ) Methods

This function returns a writer object which is used for converting the data given by the user into delimited strings on the file object.
W_obj = csv.writer(csvfile, quotechar='"', delimiter=',',
lineterminator='\r\n', quoting=csv.QUOTE_MINIMAL )



writer( ) Object Methods –

  • w_obj . writerow( <Sequence> ) : Write a Single Line
  • w_obj . writerows ( <Nested Sequence> ) : Write Multiple Lines
    Example:-
    ## writerow()
    import csv
    row=['Nikhil', 'CEO', '2', '9.0']
    f=open("myfile.csv", 'w')
    w_obj = csv.writer(f)
    w_obj.writerow(row)
    f.close()
    
    
    ## writerows()
    import csv
    rows = [['Nikhil','CEO','2','9.0'],
            ['Sanchit','CEO','2','9.1']]
    f=open("myfile.csv",'w')
    w_obj = csv.writer(f)
    w_obj.writerows(rows)
    f.close()
    
    

reader( ) Methods

This function returns a reader object which will be used to iterate over lines of a given CSV file.
r_obj = csv.reader( csvfile_obj )

To access each row, we have to iterate over this Object.
for i in r_obj:
    print(i)

Example:-
import csv
f=open("myfile.csv",'r')
r_obj = csv.reader(f)
for data in r_obj:
    print(data)
f.close()


If we consider the sample.csv file given above in the CSV file structure the output of the above code will be:
## OUTPUT

['Name', 'DOB', 'City']
['Ram', '12-Jul-2001', 'Delhi']
['Mohan', '23-Jan-2005', 'Delhi']
['Suraj', '17-Dec-2002', 'Kolkata']


Problem with writer( ) and Solution

  • Extra Newline after each line in CSV File.
  • To remove this we have to Open the CSV File with newline Parameter as Empty '' string:
    f=open("myfile.csv", 'w', newline = '')
  • New Code:
  • import csv
    row=['Nikhil', 'CEO', '2', '9.0']
    f=open("myfile.csv", 'w', newline = '')
    w_obj = csv.writer(f)
    w_obj.writerow(row)
    f.close()
    
    

Why Problem with the writer( )


Universal Newline
'\r' Carriage Return Mac OS
'\n' Line Feed Unix
'\r\n' Carriage Return + New Line MS-DOS, Windows

  • since the CSV module does its own (universal) newline handling. writer() has default line terminator parameter is '\r\n'. When we do not Open the file with newline=''(Empty) python replaces '\r' to '\n'.
  • So the newline end will become '\n\n'. That’s why there is an extra newline that will come in CSV.
Ref:-

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