docs: def _get_mapper(self, mapper_name):
          """ Return the mapper based on the given name.
  
          :returns: the mapper based on the given ``mapper_name``
          :raises: NotAcceptable if we don't support the requested format.
          """
  
          if mapper_name in self._datamappers:
              # mapper found
              return self._datamappers[mapper_name]
          else:
              # unsupported format
              return self._unknown_format(mapper_name)