Python write csv file line by line

Summary: in this tutorial, you’ll learn how to write data into a CSV file using the built-in csv module.

Steps for writing a CSV file

To write data into a CSV file, you follow these steps:

• First, open the CSV file for writing [w mode] by using the open[] function.
• Second, create a CSV writer object by calling the writer[] function of the csv module.
• Third, write data to CSV file by calling the writerow[] or writerows[] method of the CSV writer object.
• Finally, close the file once you complete writing data to it.

The following code illustrates the above steps:

import csv

# open the file in the write mode
f = open['path/to/csv_file', 'w']

# create the csv writer
writer = csv.writer[f]

# write a row to the csv file
writer.writerow[row]

# close the file
f.close[]
Code language: Python [python]

It’ll be shorter if you use the with statement so that you don’t need to call the close[] method to explicitly close the file:

import csv

# open the file in the write mode
with open['path/to/csv_file', 'w'] as f:
    # create the csv writer
    writer = csv.writer[f]

    # write a row to the csv file
    writer.writerow[row]
Code language: PHP [php]

If you’re dealing with non-ASCII characters, you need to specify the character encoding in the open[] function.

The following illustrates how to write UTF-8 characters to a CSV file:

import csv

# open the file in the write mode
with open['path/to/csv_file', 'w', encoding='UTF8'] as f:
    # create the csv writer
    writer = csv.writer[f]

    # write a row to the csv file
    writer.writerow[row]

  

Code language: PHP [php]

Writing to CSV files example

The following example shows how to write data to the CSV file:

import csv  

header = ['name', 'area', 'country_code2', 'country_code3']
data = ['Afghanistan', 652090, 'AF', 'AFG']

with open['countries.csv', 'w', encoding='UTF8'] as f:
    writer = csv.writer[f]

    # write the header
    writer.writerow[header]

    # write the data
    writer.writerow[data]
Code language: PHP [php]

If you open the countries.csv, you’ll see one issue that the file contents have an additional blank line between two subsequent rows:

To remove the blank line, you pass the keyword argument newline='' to the open[] function as follows:

import csv

header = ['name', 'area', 'country_code2', 'country_code3']
data = ['Afghanistan', 652090, 'AF', 'AFG']


with open['countries.csv', 'w', encoding='UTF8', newline=''] as f:
    writer = csv.writer[f]

    # write the header
    writer.writerow[header]

    # write the data
    writer.writerow[data]
Code language: PHP [php]

Output:

Writing multiple rows to CSV files

To write multiple rows to a CSV file at once, you use the writerows[] method of the CSV writer object.

The following uses the writerows[] method to write multiple rows into the countries.csv file:

import csv

header = ['name', 'area', 'country_code2', 'country_code3']
data = [
    ['Albania', 28748, 'AL', 'ALB'],
    ['Algeria', 2381741, 'DZ', 'DZA'],
    ['American Samoa', 199, 'AS', 'ASM'],
    ['Andorra', 468, 'AD', 'AND'],
    ['Angola', 1246700, 'AO', 'AGO']
]

with open['countries.csv', 'w', encoding='UTF8', newline=''] as f:
    writer = csv.writer[f]

    # write the header
    writer.writerow[header]

    # write multiple rows
    writer.writerows[data]
Code language: PHP [php]

Writing to CSV files using the DictWriter class

If each row of the CSV file is a dictionary, you can use the DictWriter class of the csv module to write the dictionary to the CSV file.

The example illustrates how to use the DictWriter class to write data to a CSV file:

import csv

# csv header
fieldnames = ['name', 'area', 'country_code2', 'country_code3']

# csv data
rows = [
    {'name': 'Albania',
    'area': 28748,
    'country_code2': 'AL',
    'country_code3': 'ALB'},
    {'name': 'Algeria',
    'area': 2381741,
    'country_code2': 'DZ',
    'country_code3': 'DZA'},
    {'name': 'American Samoa',
    'area': 199,
    'country_code2': 'AS',
    'country_code3': 'ASM'}
]

with open['countries.csv', 'w', encoding='UTF8', newline=''] as f:
    writer = csv.DictWriter[f, fieldnames=fieldnames]
    writer.writeheader[]
    writer.writerows[rows]
Code language: PHP [php]

How it works.

• First, define variables that hold the field names and data rows of the CSV file.
• Next, open the CSV file for writing by calling the open[] function.
• Then, create a new instance of the DictWriter class by passing the file object [f] and fieldnames argument to it.
• After that, write the header for the CSV file by calling the writeheader[] method.
• Finally, write data rows to the CSV file using the writerows[] method.

Summary

• Use the CSV Writer or the DictWriter class to write data to a CSV file.

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