Parsing XML with namespace in Python via 'ElementTree'

You need to give the .find(), findall() and iterfind() methods an explicit namespace dictionary:

namespaces = {'owl': 'http://www.w3.org/2002/07/owl#'} # add more as needed

root.findall('owl:Class', namespaces)

Prefixes are only looked up in the namespaces parameter you pass in. This means you can use any namespace prefix you like; the API splits off the owl: part, looks up the corresponding namespace URL in the namespaces dictionary, then changes the search to look for the XPath expression {http://www.w3.org/2002/07/owl}Class instead. You can use the same syntax yourself too of course:

root.findall('{http://www.w3.org/2002/07/owl#}Class')

Also see the Parsing XML with Namespaces section of the ElementTree documentation.

If you can switch to the lxml library things are better; that library supports the same ElementTree API, but collects namespaces for you in .nsmap attribute on elements and generally has superior namespaces support.

Here's how to do this with lxml without having to hard-code the namespaces or scan the text for them (as Martijn Pieters mentions):

from lxml import etree
tree = etree.parse("filename")
root = tree.getroot()
root.findall('owl:Class', root.nsmap)

UPDATE:

5 years later I'm still running into variations of this issue. lxml helps as I showed above, but not in every case. The commenters may have a valid point regarding this technique when it comes merging documents, but I think most people are having difficulty simply searching documents.

Here's another case and how I handled it:

<?xml version="1.0" ?><Tag1 xmlns="http://www.mynamespace.com/prefix">
<Tag2>content</Tag2></Tag1>

xmlns without a prefix means that unprefixed tags get this default namespace. This means when you search for Tag2, you need to include the namespace to find it. However, lxml creates an nsmap entry with None as the key, and I couldn't find a way to search for it. So, I created a new namespace dictionary like this

namespaces = {}
# response uses a default namespace, and tags don't mention it
# create a new ns map using an identifier of our choice
for k,v in root.nsmap.iteritems():
    if not k:
        namespaces['myprefix'] = v
e = root.find('myprefix:Tag2', namespaces)