a:b为名不行吧，要展开为{URI}b这种形式，看看下面小例子取出的tag名称：

# -*- coding: utf-8 -*-

from xml.etree import ElementTree as ET
import cStringIO

xml = """\
<?xml version="1.0"?>
<root xmlns = " http://default-namespace.org/ "
xmlns:py = " http://www.python.org/ns/ ">
<py:elem1 />
<elem2 xmlns="" />
</root>
"""
f = cStringIO.StringIO(xml)

#find all elements and print tag's name.
tree = ET.parse(f)
print repr(tree.getroot().tag)
elems = tree.findall('.//*')
for elem in elems:
print repr(elem.tag)

#same as above, but using iterparse.
f.seek(0)
for event, elem in ET.iterparse(f, ("start",)):
print repr(elem.tag)

输出：
'{ http://default-namespace.org/ }root'
'{ http://www.python.org/ns/ }elem1'
'elem2'
'{ http://default-namespace.org/ }root'
'{ http://www.python.org/ns/ }elem1'
'elem2'