This will not sink in. Have to keep looking at it over and over again. I'm just not clever enough, I think. The only way to make it stick is to look at the previous variables etc.

from sys import argv

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 whole file:\n"

print_all(current_file)

print "\nNow let's rewind. Just like a tape."

rewind(current_file)

print "Let's print three lines:\n"

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)

print "\n"