Panda's Write CSV - Append vs. Write

Not sure there is a way in pandas but checking if the file exists would be a simple approach:

import os
# if file does not exist write header 
if not os.path.isfile('filename.csv'):
   df.to_csv('filename.csv', header='column_names')
else: # else it exists so append without writing the header
   df.to_csv('filename.csv', mode='a', header=False)

with open(filename, 'a') as f:
    df.to_csv(f, mode='a', header=f.tell()==0)

it will add header when writes to the file first time