code: def _prefix_from_uri(self, uriname):
          
          uripart, tag = uriname.split('}', maxsplit=1)
          uri = uripart.replace('{', '')
          return self.REVERSE_NS[uri] + ':' + tag
docs: Given a fully qualified XML name, find a prefix
  
          e.g. {http://ns.adobe.com/pdf/1.3/}Producer -> pdf:Producer