ex20.py

from sys import argv

script, input_file = argv

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

# seek(n) to read a file's content from byte-n
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 "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)

current_file.close()