Parsing XML with namespace in Python via 'ElementTree'

I have the following XML which I want to parse using Python's ElementTree:

<rdf:RDF xml:base="http://dbpedia.org/ontology/"    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"    xmlns:owl="http://www.w3.org/2002/07/owl#"    xmlns:xsd="http://www.w3.org/2001/XMLSchema#"    xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"    xmlns="http://dbpedia.org/ontology/"><owl:Class rdf:about="http://dbpedia.org/ontology/BasketballLeague"><rdfs:label xml:lang="en">basketball league</rdfs:label><rdfs:comment xml:lang="en">          a group of sports teams that compete against each other          in Basketball</rdfs:comment></owl:Class></rdf:RDF>

I want to find all owl:Class tags and then extract the value of all rdfs:label instances inside them. I am using the following code:

tree = ET.parse("filename")root = tree.getroot()root.findall('owl:Class')

Because of the namespace, I am getting the following error.

SyntaxError: prefix 'owl' not found in prefix map

I tried reading the document at http://effbot.org/zone/element-namespaces.htm but I am still not able to get this working since the above XML has multiple nested namespaces.

Kindly let me know how to change the code to find all the owl:Class tags.