Question

I’m trying to learn how to do XPath queries from Python using this sample XML file: http://pastie.org/1333021 I just added a namespace to it because my actual application uses it.

Basically, I want to execute a top level query that returns a subset of nodes and then query the subset (on a much larger scale than this example)

So this is my code to first find all <food> nodes and then iterate over the description of each one.

#!/usr/bin/python2

import libxml2

doc = libxml2.parseFile("simple.xml")
context = doc.xpathNewContext()

context.xpathRegisterNs("db", "http://examplenamespace.com")
res = context.xpathEval("//db:food")

for node in res:
    # Query xmlNode here
    print "Got Food Node:"
    desc = node.xpathEval('db:description') # this is wrong?
    print desc

So it’s essentially a namespace problem, if I remove the xlns attribute from the XML file and use just basic XPATH queries without db: it works fine. The top query //db:food works fine, but the second one fails evaluating.

Please can someone correct my namespace/query syntax.

Thanks a lot

Answer 1

I don’t normally use libxml2, I much prefer lxml.etree.

Had a play around. The xpathEval method on your node creates a new context each time, apparently without the namespace you registered.

You can reset your context to different locations like this:

>>> import libxml2
>>> from urllib2 import urlopen
>>> data = urlopen('http://pastie.org/pastes/1333021/download').read()
>>>
>>> doc = libxml2.parseMemory(data,len(data))
>>>
>>> context = doc.xpathNewContext()
>>> context.xpathRegisterNs("db", "http://examplenamespace.com")
0
>>>
>>> for res in context.xpathEval("//db:food"):
...     context.setContextNode(res)
...     print "Got Food Node:"
...     desc = context.xpathEval('./db:description')[0]
...     print desc
...
Got Food Node:
<description>two of our famous Belgian Waffles with plenty of real maple syrup</description>
Got Food Node:
<description>light Belgian waffles covered with strawberries and whipped cream</description>
Got Food Node:
<description>light Belgian waffles covered with an assortment of fresh berries and whipped cream</description>
Got Food Node:
<description>thick slices made from our homemade sourdough bread</description>
Got Food Node:
<description>two eggs, bacon or sausage, toast, and our ever-popular hash browns</description>