def describe_cluster_subnet_groups(self, subnet_identifier=None):
        subnet_groups = self.subnet_groups.values()
        if subnet_identifier:
            if subnet_identifier in self.subnet_groups:
                return [self.subnet_groups[subnet_identifier]]
            else:
                raise ClusterSubnetGroupNotFoundError(subnet_identifier)
        return subnet_groups
dataset/ETHPy150Open spulec/moto/moto/redshift/models.py/RedshiftBackend.describe_cluster_subnet_groups/original