replacing string in xml file--revisited

[saif.shakeel at gmail.com]

Hi,
     I need to replace a string in xml file with something else.Ex

- <SERVICEPARAMETER id="_775" Semantics="subfunction" DDORef="_54">
  <SHORTNAME>rate</SHORTNAME>
  <LONGNAME>rate</LONGNAME>
  <VALUE role="constant" DataType="unsigned" value="1" />
  <BYTEPOSITION role="position" BytePos="1" />
  </SERVICEPARAMETER>
- <SERVICEPARAMETER id="_776" Semantics="localId" DDORef="_54">


                                     Here i have opened an xml
file(small part is pasted here).I want to replace the word 'localId'
with 'dataPackageID' wherever it comes in xml file.I have asked this
before and got a code:
input_file = open(filename)
xmlcontents = input_file.read()
input_file.close()
xmlcontents = xmlcontents.replace("spam", "eggs")
output_file = open(filename,"w")
output_file.write(xmlcontents)
output_file.close()

                                 Although this works alone it is nto
working when i handle multiple file I/O.Is there a alternative to do
this.(maybe without read() operation)
                                 Thanks