NS_MAP = {'http://www.red-dove.com/ns/abc': 'rdc',
    'http://www.adobe.com/2006/mxml': 'mx',
    'http://www.red-dove.com/ns/def': 'oth'}
DATA = """<?xml version="1.0" encoding="utf-8"?>
<rdc:container xmlns:mx="http://www.adobe.com/2006/mxml"
                 xmlns:rdc="http://www.red-dove.com/ns/abc"
                 xmlns:oth="http://www.red-dove.com/ns/def">
  <mx:Style>
    <oth:style1/>
  </mx:Style>
  <mx:Style>
    <oth:style2/>
  </mx:Style>
  <mx:Style>
    <oth:style3/>
  </mx:Style>
</rdc:container>"""
tree = ET.parse(StringIO(DATA))
some_node = tree.getroot().getchildren()[1]
print(ET.fixtag(some_node.tag, NS_MAP))
some_node = some_node.getchildren()[0]
print(ET.fixtag(some_node.tag, NS_MAP))
Изменить префикс пространства имен с помощью ElementTree в Python