CFE nvram modification tool that doesn't change order

#!/usr/bin/python
'extract and write nvram variables to CFE image'
import sys
NVRAM_OFFSET = 4096
NVRAM_SIZE = 4096
NVRAM_HEADER_SIZE = 20
NVRAM_DATA_OFFSET = NVRAM_OFFSET + NVRAM_HEADER_SIZE
def extract(filename):
 data = read(filename)
 nvram = data[NVRAM_OFFSET:NVRAM_OFFSET + NVRAM_SIZE]
 return filter(None, nvram[NVRAM_HEADER_SIZE:].split('\0'))
def modify(cfefile, changesfile):
 values = [line.split('=', 1) for line in extract(cfefile)]
 valuesdict = dict(values)
 changes = dict([line.split('=', 1) for line in readlines(changesfile)])
 valuesdict.update(changes)
 for index in range(len(values)):
  key, value = values[index]
  if valuesdict[key] != value:
   print >>sys.stderr, 'changing value of "%s" from "%s" to "%s"' % (
    key, value, valuesdict[key])
   values[index][1] = valuesdict[key]
  del valuesdict[key]
 if valuesdict:
  print >>sys.stderr, 'failed! remaining data: %s' % valuesdict
  sys.exit(1)
 nvram_data = '\0'.join(['='.join([k, v]) for [k, v] in values])
 data = read(cfefile)
 padding = '\0' * (NVRAM_SIZE - NVRAM_HEADER_SIZE)
 data = data[:NVRAM_DATA_OFFSET] + \
  (nvram_data + padding)[:len(padding)] + \
  data[NVRAM_OFFSET + NVRAM_SIZE:]
 write(cfefile, data)
def read(filename):
 input = open(filename)
 data = input.read()
 input.close()
 return data
def readlines(filename):
 input = open(filename)
 data = input.readlines()
 input.close()
 return map(str.rstrip, data)
def write(filename, data):
 output = open(filename, 'w')
 output.write(data)
 output.close()
if __name__ == '__main__':
 if len(sys.argv) > 1:
  if sys.argv[1] == 'extract':
   for line in extract(sys.argv[2]):
    print line
  elif sys.argv[1] == 'modify':
   modify(*sys.argv[2:])
  else:
   print >>sys.stderr, 'only "extract" or "modify" accepted'
 else:
  print >> sys.stderr, \
   'usage: %s extract binfile | modify binfile changesfile' % sys.argv[0]