human
Complete the below
def get_existing_cluster(conn, opts, cluster_name, die_on_error=True):

    ''''Get the EC2 instances in an existing cluster if available.
    Returns a tuple of lists of EC2 instance objects for the masters and slaves.'
    '''
 What is a the Python code you requested?
assistant
#Here is the Python code you requested.
    print('Searching    for    existing    cluster    {c}    in    region    {r}...'.format(c=cluster_name, r=opts.region))
    def get_instances(group_names):
        '\n                                Get    all    non-terminated    instances    that    belong    to    any    of    the    provided    security    groups.\n\n                                EC2    reservation    filters    and    instance    states    are    documented    here:\n                                                http://docs.aws.amazon.com/cli/latest/reference/ec2/describe-instances.html#options\n                                '
        reservations = conn.get_all_reservations(filters={'instance.group-name': group_names})
        instances = itertools.chain.from_iterable((r.instances for r in reservations))
        return [i for i in instances if (i.state not in ['shutting-down', 'terminated'])]
    master_instances = get_instances([(cluster_name + '-master')])
    slave_instances = get_instances([(cluster_name + '-slaves')])
    if any((master_instances, slave_instances)):
        print('Found    {m}    master{plural_m},    {s}    slave{plural_s}.'.format(m=len(master_instances), plural_m=('' if (len(master_instances) == 1) else 's'), s=len(slave_instances), plural_s=('' if (len(slave_instances) == 1) else 's')))
    if ((not master_instances) and die_on_error):
        print('ERROR:    Could    not    find    a    master    for    cluster    {c}    in    region    {r}.'.format(c=cluster_name, r=opts.region), file=sys.stderr)
        sys.exit(1)
    return (master_instances, slave_instances)