nl.py

import sys

args = len(sys.argv)
if args == 1:
    print("Enter a file name below")
    f = sys.stdin.readline().rstrip()
elif args == 2:
    f = sys.argv[1]
else:
    sys.exit("please enter the command this way: 'python nl.py [a file]'")

try:
    f = open(f, "rU")
except IOError:
    sys.exit("%s: No such a file or directory" % (f))

i = 1
for s in f:
    s = s.rstrip()
    if s:
        print "%5d\t%s" % (i, s)
        i += 1
    else:
        print
f.close()