Python Add string to each line in a file

Question

I need to open a text file and then add a string to the end of each line.

So far:

appendlist = open(sys.argv[1], \"r\").read()

Answer 1

s = '123'
with open('out', 'w') as out_file:
    with open('in', 'r') as in_file:
        for line in in_file:
            out_file.write(line.rstrip('\n') + s + '\n')