Reading and Writing CSV Files in Python

Last Updated : 22 Jun, 2020

CSV (Comma Separated Values) format is the most common import and export format for spreadsheets and databases. It is one of the most common methods for exchanging data between applications and popular data format used in Data Science. It is supported by a wide range of applications. A CSV file stores tabular data in which each data field is separated by a delimiter(comma in most cases). To represent a CSV file, it must be saved with the .csv file extension.

Reading from CSV file

Python contains a module called csv for the handling of CSV files. The reader class from the module is used for reading data from a CSV file. At first, the CSV file is opened using the open() method in ‘r’ mode(specifies read mode while opening a file) which returns the file object then it is read by using the reader() method of CSV module that returns the reader object that iterates throughout the lines in the specified CSV document.

Syntax:

csv.reader(csvfile, dialect='excel', **fmtparams

Note: The ‘with‘ keyword is used along with the open() method as it simplifies exception handling and automatically closes the CSV file.

Example:

Consider the below CSV file –

import csv  
    
# opening the CSV file  
with open('Giants.csv', mode ='r')as file:  
      
  # reading the CSV file  
  csvFile = csv.reader(file)  
    
  # displaying the contents of the CSV file  
  for lines in csvFile:  
        print(lines)  

Output:

[['Steve', 13, 'A'],
['John', 14, 'F'],
['Nancy', 14, 'C'],
['Ravi', 13, 'B']]

Writing to CSV file

csv.writer class is used to insert data to the CSV file. This class returns a writer object which is responsible for converting the user’s data into a delimited string. A CSV file object should be opened with newline=” otherwise, newline characters inside the quoted fields will not be interpreted correctly.

Syntax:

csv.writer(csvfile, dialect='excel', **fmtparams)

csv.writer class provides two methods for writing to CSV. They are writerow() and writerows().

writerow(): This method writes a single row at a time. Field row can be written using this method.
Syntax:
```
writerow(fields)
```
writerows(): This method is used to write multiple rows at a time. This can be used to write rows list.
Syntax:
```
writerows(rows)
```

Example:

# Python program to demonstrate 
# writing to CSV 
  
  
import csv  
    
# field names  
fields = ['Name', 'Branch', 'Year', 'CGPA']  
    
# data rows of csv file  
rows = [ ['Nikhil', 'COE', '2', '9.0'],  
         ['Sanchit', 'COE', '2', '9.1'],  
         ['Aditya', 'IT', '2', '9.3'],  
         ['Sagar', 'SE', '1', '9.5'],  
         ['Prateek', 'MCE', '3', '7.8'],  
         ['Sahil', 'EP', '2', '9.1']]  
    
# name of csv file  
filename = "university_records.csv"
    
# writing to csv file  
with open(filename, 'w') as csvfile:  
    # creating a csv writer object  
    csvwriter = csv.writer(csvfile)  
        
    # writing the fields  
    csvwriter.writerow(fields)  
        
    # writing the data rows  
    csvwriter.writerows(rows) 

Output:

python-write-to-csv

We can also write dictionary to the CSV file. For this the CSV module provides the csv.DictWriter class. This class returns a writer object which maps dictionaries onto output rows.

Syntax:

csv.DictWriter(csvfile, fieldnames, restval=”, extrasaction=’raise’, dialect=’excel’, *args, **kwds)

csv.DictWriter provides two methods for writing to CSV. They are:

writeheader(): writeheader() method simply writes the first row of your csv file using the pre-specified fieldnames.
Syntax:
```
writeheader()
```
writerows(): writerows method simply writes all the rows but in each row, it writes only the values(not keys).
Syntax:
```
writerows(mydict)
```

Example:

# importing the csv module  
import csv  
    
# my data rows as dictionary objects  
mydict =[{'branch': 'COE', 'cgpa': '9.0', 'name': 'Nikhil', 'year': '2'},  
         {'branch': 'COE', 'cgpa': '9.1', 'name': 'Sanchit', 'year': '2'},  
         {'branch': 'IT', 'cgpa': '9.3', 'name': 'Aditya', 'year': '2'},  
         {'branch': 'SE', 'cgpa': '9.5', 'name': 'Sagar', 'year': '1'},  
         {'branch': 'MCE', 'cgpa': '7.8', 'name': 'Prateek', 'year': '3'},  
         {'branch': 'EP', 'cgpa': '9.1', 'name': 'Sahil', 'year': '2'}]  
    
# field names  
fields = ['name', 'branch', 'year', 'cgpa']  
    
# name of csv file  
filename = "university_records.csv"
    
# writing to csv file  
with open(filename, 'w') as csvfile:  
    # creating a csv dict writer object  
    writer = csv.DictWriter(csvfile, fieldnames = fields)  
        
    # writing headers (field names)  
    writer.writeheader()  
        
    # writing data rows  
    writer.writerows(mydict)  