list iteration question

[David R. Favor]

In the attached code, would someone point out why the function
chomp4() throws an exception?

Thanks.
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#!/usr/local/bin/python

import sys

def slurp(filename):

   "Slurp all the lines of a file into an list"

   try:
      f = open(filename)
   except IOError:
      print "IOError: ", sys.exc_value
      sys.exit

   return f.readlines()

def chomp1(lines):
   "remove the end of line markers from list"
   i = 0
   while i < len(lines):
      if lines[i][-1] == '\n':
         lines[i] = lines[i][:-1]
      i = i + 1

def chop(str):
   "remove the end of line marker from string"
   if str[-1] == '\n':
      str = str[:-1]
   return str

def chomp2(lines):
   "remove the end of line markers from list"
   i = 0
   while i < len(lines):
      lines[i] = chop(lines[i])
      i = i + 1

def chomp3(lines):
   "remove the end of line markers from list"
   lines = map(chop,lines)
   return lines

def chomp4(lines):
   "remove the end of line markers from list"
   lines = map(string.rstrip,lines)
   return lines

if __name__ == '__main__':
    for file in sys.argv[1:]:
       lines = slurp(file)
       if lines:
          lines = chomp4(lines)
          for line in lines:
             print line