def _get_db_subnet_group_arn(region, current_aws_account_id, db_subnet_group_name):
    """
    Return an ARN for the DB subnet group name by concatenating the account name and region.
    This is done to avoid another AWS API call since the describe_db_instances boto call does not return the DB subnet
    group ARN.
    Form is arn:aws:rds:{region}:{account-id}:subgrp:{subnet-group-name}
    as per https://docs.aws.amazon.com/general/latest/gr/aws-arns-and-namespaces.html
    """
    return f"arn:aws:rds:{region}:{current_aws_account_id}:subgrp:{db_subnet_group_name}"