IndexError: string index out of range

[michael]

#!/usr/bin/python

import sys

FILENAME = "./testfile"
EXCLUDE='#'   #Lines beginning with this should not be printed.

def get_line() :
  buf = f.readline()
  if (buf[0] == EXCLUDE):
    return 0
  else:
    return buf

f = open(FILENAME, "r")

while 1:
  buf = get_line()
  if (buf == ''):
    break
  if (buf == 0):   # If 0 is returned then the line read on this loop
begins with
    continue        #    the EXCLUDE char and is not to be printed
  else:
    sys.stdout.write(buf)

f.close()

#############   END OF PROGRAM   ##############

Im new to Python.This is my prgram. It reads the contents of testfile
line by line and prints it out unless the line begins with a #. I know
that it might be pointless to use a function for this but it is for
learning purposes. This program works fine until it gets to the last
line in the testfile and I get this error.

Traceback (innermost last):
  File "./test3.py", line 18, in ?
    buf = get_line()
  File "./test3.py", line 10, in get_line
    if (buf[0] == EXCLUDE):
IndexError: string index out of range

Can someone please tell me what I'm doing wrong? I've looked all over
the net and in several books for a solution.

  TIA ,    Michael