python parse xml file

#!/usr/bin/python2.6


#su xinmin 2014/2/17


import xml.etree.ElementTree as ET
import sys


def usage():
  sys.stderr.write("""Usage: %(progName)s xmlfilename casename --d=true --t=false
Options:
   --d=true
       set dislplay true or false.
   --t=false
       set testable true or false.
""" % {
      "progName": os.path.split(sys.argv[0])[1],
    })
  sys.exit(1)


def main(argv):
  filename = argv[1]
  casename = argv[2]
  tree = ET.parse(filename)
  root = tree.getroot()
  
  i=3
  while i<len(argv) and len(argv[i])>2 and argv[i][0:2] == "--":
    arg = argv[i]
    if arg.startswith("--d="):
      try:
        display = arg[len("--d="):]
      except ValueError:
        usage()
    elif arg.startswith("--t="):
      try:
        test = arg[len("--t="):]
      except ValueError:
        usage()
    else:
      usage()
    i += 1


  for item in root.findall('item'):
    if casename == item.find('CaseName').text:
      item.set('Displayable',display)
      item.set('Testable',test)
      break


  tree.write('output.xml')


if __name__ == "__main__":
  main(sys.argv)