[XML-SIG] can you tell me how to use stdin?

[ÒÊÃÉɽÈË]

In the PyXml module,there is an example like this:

     from xml.dom.ext.reader.Sax2 import FromXmlStream
     doc = FromXmlStream(sys.stdin)
>From the example the argument of FromXmlStream is sys.stdin,
but now i have a xml file namede "test.xml",how can i redirect the
stdin to the file?

For a long time,i have been confused about stdin , can you give me
an example ? Thanks a bounch.