Writing to a file (python)

     if data.find('!add') != -1:
       f = open('masters.txt', 'w')
       f.writelines(args, '\n')
       sck.send('PRIVMSG ' + chan + ' :' + ' added' + " " + args + '\r\n')
       f.close()

When I use this code it replaces the old data with the new data, how can I make it so that the new data doesnt replaced the old data but ends at the end of the file.