#!/usr/bin/env python
    import libxml2
    import sys

    def grep(what, where):
        doc = libxml2.parseDoc(where)
        for found in doc.xpathEval(what):
            found.saveTo(sys.stdout, format=True)

    if __name__ == '__main__':
        try:
            what = sys.argv[1]
        except IndexError:
            sys.exit("Usage: %s pattern file ..." % sys.argv[0])
        else:
            for where in sys.argv[2:]:
                grep(what, file(where).read())

although you might want to be smarter with the errors...

I used this exercise as an opportunity to get something off my chest
about XML and Python - it's kind of a mess! More here:
  http://www.nelson.monkey.org/~nelson/weblog/tech/python/xpath.html

Here are my samples, in three libraries:

# PyXML

from xml.dom.ext.reader import Sax2
from xml import xpath
doc = Sax2.FromXmlFile('foo.opml').documentElement
for url in xpath.Evaluate('//@xmlUrl', doc):
  print url.value

# libxml2

import libxml2
doc = libxml2.parseFile('foo.opml')
for url in doc.xpathEval('//@xmlUrl'):
  print url.content

# ElementTree

from elementtree import ElementTree
tree = ElementTree.parse("foo.opml")
for outline in tree.findall("//outline"):
  print outline.get('xmlUrl')

http://www.oreillynet.com/pub/wlg/6224
http://www.oreillynet.com/pub/wlg/6225