This is essentially a summary of a thread on the mailinglist: http://codespeak.net/pipermail/lxml-dev/2010-March/005329.html
For some XML processing tools I need to be able to change the namespace mapping, specifically to insert a new entry into the namespace map. My test code looked like this:
NS="http://xml.zope.org/namespaces/i18n"
tree=lxml.etree.parse(input)
root=tree.getroot()
count=1
if "i18n" not in root.nsmap:
root.nsmap["i18n"]=NS
for el in root.iter():
if "{%s}translate" % NS in el.attrib:
continue
if hasText(el):
el.attrib["{%s}translate" % NS]="string%d" % count
count+=1
print lxml.etree.tostring(tree)
This did not work since root.nsmap ignores any changes you make in it. An alternative implementation suggested by Simon Wiles created a new root element with the right nsmap and moved all children over, but approach looses data (specifcally the docinfo).
The missing lxml feature to make this possible is a way to change the namespace mapping.
This is critical to convert XML-like versions as GPX 1.0 to 1.1.