ex20.py

#!/usr/bin/python2

from sys import argv

if (len(argv) != 2):
    print "You must provide one filename as an argument."
else:
    script, input_file = argv

    def print_all(f):
        print f.read()

    def rewind(f):
        f.seek(0)

    def print_a_line(line_count, f):
        print line_count, f.readline()

    current_file = open(input_file)

    print "First let's print the while file:\n"

    print_all(current_file)

    print "Now let's rewind, kind of like a tape."

    rewind(current_file)

    print "Let's print three lines:"

    current_line = 1
    print_a_line(current_line, current_file)

    current_line = current_line + 1
    print_a_line(current_line, current_file)

    current_line = current_line + 1
    print_a_line(current_line, current_file)