code: def namespace_for_prefix(self, prefix):
          
          try:
              ni = self.__lookup_prefix(prefix)
          except PrefixNotFoundError:
              return None
          else:
              return ni.uri
docs: Get the namespace the given prefix maps to.
  
          Args:
              prefix (str): The prefix
  
          Returns:
              str: The namespace, or None if the prefix isn't mapped to
                  anything in this set.