libxml2 has a number of advantages:

1. Compliance to the spec
2. Active development and a community participation
3. Speed. This is really a python wrapper around a C implementation.
4. Ubiquity. The libxml2 library is pervasive and thus well tested.

Downsides include:

1. Compliance to the spec. It's strict. Things like default namespace handling are easier in other libraries.
2. Use of native code. This can be a pain depending on your how your application is distributed / deployed. RPMs are available that ease some of this pain.
3. Manual resource handling. Note in the sample below the calls to freeDoc() and xpathFreeContext(). This is not very Pythonic.

If you are doing simple path selection, stick with ElementTree ( which is included in Python 2.5 ). If you need full spec compliance or raw speed and can cope with the distribution of native code, go with libxml2.

Sample of libxml2 XPath Use

import libxml2

doc = libxml2.parseFile("tst.xml")
ctxt = doc.xpathNewContext()
res = ctxt.xpathEval("//*")
if len(res) != 2:
    print "xpath query: wrong node set size"
    sys.exit(1)
if res[0].name != "doc" or res[1].name != "foo":
    print "xpath query: wrong node set value"
    sys.exit(1)
doc.freeDoc()
ctxt.xpathFreeContext()

Sample of ElementTree XPath Use

from elementtree.ElementTree import ElementTree
mydoc = ElementTree(file='tst.xml')
for e in mydoc.findall('/foo/bar'):
    print e.get('title').text

http://docs.python.org/library/xml.etree.elementtree.html

etree supports XPath queries, just like lxml.

etree is included in the standard library, but lxml is faster.

You could use getelementpath, which always returns the elements in Clark notation, and replace the namespaces manually:

x = """
<root 
xmlns="http://www.w3.org/TR/html4/"
xmlns:h="http://www.w3schools.com/furniture">

<table>
  <tr>
    <h:td>Apples</h:td>
    <h:td>Bananas</h:td>
  </tr>
</table>
</root>
"""

from lxml import etree 
root = etree.fromstring(x).getroottree()
ns = {'df': 'http://www.w3.org/TR/html4/', 'types': 'http://www.w3schools.com/furniture'}
for e in root.iter():
    path = root.getelementpath(e)
    root_path = '/' + root.getroot().tag
    if path == '.':
        path = root_path
    else:
        path = root_path + '/' + path
    for ns_key in ns:
        path = path.replace('{' + ns[ns_key] + '}', ns_key + ':')
    print(path)
    r = root.xpath(path, namespaces=ns)
    print(r)

Obviously, this example shows that getelementpath returns paths relative to the root node, like . and dt:table instead of /df:root and /df:root/df:table, so we use the tag of the root element to manually construct the full path.

Output:

/df:root
[<Element {http://www.w3.org/TR/html4/}root at 0x37f5348>]
/df:root/df:table
[<Element {http://www.w3.org/TR/html4/}table at 0x44bdb88>]
/df:root/df:table/df:tr
[<Element {http://www.w3.org/TR/html4/}tr at 0x37fa7c8>]
/df:root/df:table/df:tr/types:td[1]
[<Element {http://www.w3schools.com/furniture}td at 0x44bdac8>]
/df:root/df:table/df:tr/types:td[2]
[<Element {http://www.w3schools.com/furniture}td at 0x44bdb88>]