Help in rotate 13 program

Thu Dec 29 06:48:28 EST 2011

please help me, this code doesn't work,
the output file comes out to be empty. How do i fix it? and where is it
going wrong?

==============rot13.py==============
#!/usr/bin/env python

import sys
import string

CHAR_MAP=dict(zip(string.ascii_lowercase, string.ascii_lowercase[13:26] +
string.ascii_lowercase[0:13]))
def rotate13_letter(letter) :
  """
  Return the 13-char rotation of a letter
  """
  do_upper = False
  if letter.isupper() :
    do_upper = True
  letter = letter.lower()
  if letter not in CHAR_MAP :
    return letter
  else :
    letter = CHAR_MAP[letter]
    if do_upper :
      letter=letter.upper()
    return letter
  if __name__ == '__main__' :
    for line in sys.stdin :
      for char in line :
sys.stdout.write(rotate13_letter(char))

===========sample.html===========
<html>
   <head>
      <title> Hello, World! </title>
   </head>
   <body>
      <p>
      Hi there, all of you earthlings
      </p>
      <p>
      Take us to your leader.
      </p>
      </body>
</html>

===============TERMINAL============
this is what i run in the terminal,
cat sample.html | python rot13.py rot13.html
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