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.

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)