Convert the first line of all csv files in the current directory to lowercase

lowecase.py

import glob
for filename in glob.iglob("*.csv"):
    firstline = None
    rest = None
    with open(filename, 'r') as f:
        firstline = f.readline().lower()
        rest = f.read()
    with open(filename,'w') as f:
        f.writelines([firstline])
        f.write(rest)