[Tut] How to Append a New Row to a CSV File in Python?

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How to Append a New Row to a CSV File in Python?

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Python Append Row to CSV

To append a row (=dictionary) to an existing CSV, open the file object in append mode using

Code:

open('my_file.csv', 'a', newline='')

. Then create a

Code:

csv.DictWriter()

to append a dict row using

Code:

DictWriter.writerow(my_dict)

.

Given the following file

Code:

'my_file.csv'

:



You can append a row (dict) to the CSV file via this code snippet:

Code:

import csv # Create the dictionary (=row) row = {'A':'Y1', 'B':'Y2', 'C':'Y3'} # Open the CSV file in "append" mode with open('my_file.csv', 'a', newline='') as f: # Create a dictionary writer with the dict keys as column fieldnames writer = csv.DictWriter(f, fieldnames=row.keys()) # Append single row to CSV writer.writerow(row)

After running the code in the same folder as your original

Code:

'my_file.csv'

, you’ll see the following result:



Append Multiple Rows to CSV

Given the following CSV file:



To add multiple rows (i.e., dicts) to an old existing CSV file, iterate over the rows and write each

Code:

row

by calling

Code:

csv.DictWriter.writerow(row)

on the initially created

Code:

DictWriter

object.

Here’s an example (major changes highlighted):

Code:

import csv # Create the dictionary (=row) rows = [{'A':'Z1', 'B':'Z2', 'C':'Z3'}, {'A':'ZZ1', 'B':'ZZ2', 'C':'ZZ3'}, {'A':'ZZZ1', 'B':'ZZZ2', 'C':'ZZZ3'}] # Open the CSV file in "append" mode with open('my_file.csv', 'a', newline='') as f: # Create a dictionary writer with the dict keys as column fieldnames writer = csv.DictWriter(f, fieldnames=rows[0].keys()) # Append multiple rows to CSV for row in rows: writer.writerow(row)

The resulting CSV file has all three rows added to the first row:


Python Add Row to CSV Pandas

To add a row to an existing CSV using Pandas, you can set the write

Code:

mode

argument to append

Code:

'a'

in the pandas DataFrame

Code:

to_csv()

method like so:

Code:

df.to_csv('my_csv.csv', mode='a', header=False)

.

Code:

df.to_csv('my_csv.csv', mode='a', header=False)

For a full example, check out this code snippet:

Code:

import pandas as pd # Create the initial CSV data rows = [{'A':'Z1', 'B':'Z2', 'C':'Z3'}, {'A':'ZZ1', 'B':'ZZ2', 'C':'ZZ3'}, {'A':'ZZZ1', 'B':'ZZZ2', 'C':'ZZZ3'}] # Create a DataFrame and write to CSV df = pd.DataFrame(rows) df.to_csv('my_file.csv', header=False, index=False) # Create another row and append row (as df) to existing CSV row = [{'A':'X1', 'B':'X2', 'C':'X3'}] df = pd.DataFrame(row) df.to_csv('my_file.csv', mode='a', header=False, index=False)

The output file looks like this (new row highlighted):



Alternatively, you can open the file in append mode using normal

Code:

open()

function with the append

Code:

'a'

argument and pass it into the pandas DataFrame

Code:

to_csv()

method.

Here’s an example snippet for copy&paste:

Code:

with open('my_csv.csv', 'a') as f: df.to_csv(f, header=False)

Learn More: If you want to learn about 7 Best Ways to Convert Dict to CSV in Python, check out this Finxter tutorial.