python - new line from a text file -

i have file example.txt in format:

a,b,c,d 

and want convert to:

a b c d 

i wrote not work:

with open('newline.txt', 'w') f:      f.write (file("example.txt", "r").read().replace(",", "\n")) 

you well:

with open('newline.txt', 'w') f:       f.writelines (w + '\n' w in open("example.txt").read().split(","))