'Modifies an existing DB subnet group. DB subnet groups must
contain at least one subnet in at least two AZs in the region.
:type db_subnet_group_name: string
:param db_subnet_group_name: The name for the DB subnet group. This
value is stored as a lowercase string.
Constraints: Must contain no more than 255 alphanumeric characters or
hyphens. Must not be "Default".
Example: `mySubnetgroup`
:type db_subnet_group_description: string
:param db_subnet_group_description: The description for the DB subnet
group.
:type subnet_ids: list
:param subnet_ids: The EC2 subnet IDs for the DB subnet group.'

def modify_db_subnet_group(self, db_subnet_group_name, subnet_ids, db_subnet_group_description=None):

    params = {'DBSubnetGroupName': db_subnet_group_name}
    self.build_list_params(params, subnet_ids, 'SubnetIds.member')
    if (db_subnet_group_description is not None):
        params['DBSubnetGroupDescription'] = db_subnet_group_description
    return self._make_request(action='ModifyDBSubnetGroup', verb='POST', path='/', params=params)
