ex20.py

from sys import argv

script, input_file = argv

def print_all(f):
    # read and print all the content in the file
    print f.read()

def rewind(f):
    # set the current postiion at the offset
    print f.seek(0)

def print_a_line(line_count,f):
    # print the line count and the content of each line
    print line_count, f.readline()

current_file = open(input_file)

print "First let's print the whole 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 += 1
print_a_line(current_line, current_file)

current_line += 1
print_a_line(current_line, current_file)