<s>[INST] How to use Xpath in Python? What is the library? Is there a full implementation? How is the library used? Where is its website?
 [/INST] libxml2 has a number of advantages:

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

Downsides include:

Compliance to the spec. It's strict. Things like default namespace handling are easier in other libraries.
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.
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

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