append.py

import csv  # The csv module

with open('tmp.csv', 'w') as f:  # Opening a file in write mode
    csvwriter = csv.writer(f)
    csvwriter.writerow(['Header'])

with open('tmp.csv', 'a') as f:  # Opening a file in append mode
    csvwriter = csv.writer(f)
    for k in range(100):  # Looping over first 99 integers
        csvwriter.writerow([k])