I can't help but feel that I'm making a rookie mistake here but can't figure it out.
I'm trying to use curator_cli (version 5.5.3) to list snapshots on an ES cluster (version 6.2.4). It will happily show_indices and I can see the snapshots using curl. I've reduced it to the bare minimum (no config files any more) and still can't get it to work.
I have a repository called s3_backup. This is what happens when I use curl directly to list the snapshots:
$ curl -s http://localhost:9200/_snapshot/s3_backup/_all | jq .snapshots[].snapshot
"curator-20180510031502"
"curator-20180517031501"
"curator-20180518031543"
"curator-20180519031542"
"curator-20180520031542"
"curator-20180521031542"
"curator-20180522031542"
"curator-20180523031542"
When I try to do what I believe is the same thing using curator_cli I get nothing:
$ curator_cli --host 127.0.0.1 --port 9200 show_snapshots --repository s3_backup --filter_list '{"filtertype":"none"}'
If I do a show_indices this works as expected...
$ curator_cli --host 127.0.0.1 --port 9200 show_indices --filter_list '{"filtertype":"none"}'
.kibana
logstash-2018.05.16
logstash-2018.05.17
logstash-2018.05.18
logstash-2018.05.19
logstash-2018.05.20
logstash-2018.05.21
logstash-2018.05.22
logstash-2018.05.23
Adding --loglevel DEBUG makes this super noisy for show_indices:
curator_cli --loglevel DEBUG --host 127.0.0.1 --port 9200 show_indices --filter_list '{"filtertype":"none"}'
2018-05-23 12:43:23,475 DEBUG curator.cli_singletons.cli_action.show_indices check_filters:117 Validating provided filters: [{'filtertype': 'none'}]
2018-05-23 12:43:23,475 DEBUG curator.validators.SchemaCheck __init__:26 Schema: <function Filters.<locals>.f at 0x7f0477acfd90>
2018-05-23 12:43:23,475 DEBUG curator.validators.SchemaCheck __init__:27 "filters" config: [{'filtertype': 'none'}]
2018-05-23 12:43:23,475 DEBUG curator.validators.SchemaCheck __init__:26 Schema: {'filtertype': Any(In(['age', 'alias', 'allocated', 'closed', 'count', 'forcemerged', 'ilm', 'kibana', 'none', 'opened', 'pattern', 'period', 'space', 'state']), msg="filtertype must be one of ['age', 'alias', 'allocated', 'closed', 'count', 'forcemerged', 'ilm', 'kibana', 'none', 'opened', 'pattern', 'period', 'space', 'state']")}
2018-05-23 12:43:23,475 DEBUG curator.validators.SchemaCheck __init__:27 "filter" config: {'filtertype': 'none'}
2018-05-23 12:43:23,476 DEBUG curator.validators.filters f:48 Filter #0: {'filtertype': 'none'}
2018-05-23 12:43:23,476 DEBUG curator.utils get_client:803 kwargs = {'hosts': ['127.0.0.1'], 'port': 9200, 'aws_sign_request': False, 'client_cert': None, 'timeout': 30, 'client_key': None, 'aws_secret_key': None, 'aws_key': None, 'master_only': False, 'http_auth': None, 'ssl_no_validate': False, 'url_prefix': '', 'aws_token': None, 'certificate': None, 'use_ssl': False}
2018-05-23 12:43:23,480 DEBUG curator.utils get_client:880 Not using "requests_aws4auth" python module to connect.
2018-05-23 12:43:23,484 DEBUG curator.utils check_version:689 Detected Elasticsearch version 6.2.4
2018-05-23 12:43:23,484 DEBUG curator.indexlist __get_indices:65 Getting all indices
2018-05-23 12:43:23,491 DEBUG curator.utils get_indices:644 Detected Elasticsearch version 6.2.4
...
<snip>
However if I add --loglevel DEBUG for show_snapshots it doesn't log any debug lines at all. I still get absolutely no output:
curator_cli --loglevel DEBUG --host 127.0.0.1 --port 9200 show_snapshots --repository s3_backup --filter_list '{"filtertype":"none"}'
strace shows that it is doing something, but doesn't suggest anything obvious.
strace curator_cli --loglevel DEBUG --host 127.0.0.1 --port 9200 show_snapshots --repository s3_backup --filter_list '{"filtertype":"none"}'
execve("/usr/bin/curator_cli", ["curator_cli", "--loglevel", "DEBUG", "--host", "127.0.0.1", "--port", "9200", "show_snapshots", "--repository", "s3_backup", "--filter_list", "{\"filtertype\":\"none\"}"], [/* 20 vars */]) = 0
brk(NULL) = 0x11f7000
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
readlink("/proc/self/exe", "/opt/elasticsearch-curator/curat"..., 4096) = 38
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
open("/opt/elasticsearch-curator/tls/x86_64/libpthread.so.0", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
stat("/opt/elasticsearch-curator/tls/x86_64", 0x7fff9893b120) = -1 ENOENT (No such file or directory)
open("/opt/elasticsearch-curator/tls/libpthread.so.0", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
stat("/opt/elasticsearch-curator/tls", 0x7fff9893b120) = -1 ENOENT (No such file or directory)
open("/opt/elasticsearch-curator/x86_64/libpthread.so.0", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
...
<SNIP>
...
open("/opt/elasticsearch-curator/lib/library.zip", O_RDONLY) = 4
ioctl(4, FIOCLEX) = 0
fstat(4, {st_mode=S_IFREG|0664, st_size=912451, ...}) = 0
lseek(4, 0, SEEK_SET) = 0
read(4, "PK\3\4\24\0\0\0\10\0f\254\265L=.\nD\327\0\0\0\35\1\0\0\23\0\0\0BU"..., 2568) = 2568
read(4, "PK\3\4\24\0\0\0\10\0\256\243\265L\7\240\302\242s\2\0\0\332\3\0\0\17\0\0\0_b"..., 4096) = 4096
lseek(4, 6664, SEEK_SET) = 6664
close(4) = 0
stat("/root/.curator/curator.yml", 0x7fff989387d0) = -1 ENOENT (No such file or directory)
stat("/root/.curator/curator.yml", 0x7fff989395d0) = -1 ENOENT (No such file or directory)
rt_sigaction(SIGINT, {SIG_DFL, [], SA_RESTORER, 0x7f4ef7768390}, {0x5761f0, [], SA_RESTORER, 0x7f4ef7768390}, 8) = 0
munmap(0x7f4ef2987000, 262144) = 0
sigaltstack(NULL, {ss_sp=0x1215400, ss_flags=0, ss_size=8192}) = 0
sigaltstack({ss_sp=NULL, ss_flags=SS_DISABLE, ss_size=0}, NULL) = 0
close(3) = 0
exit_group(0) = ?
+++ exited with 0 +++
If anyone can shed any light on what I'm doing wrong that would be really helpful. In the meanwhile I'm going to prototype using curl as I feel like I've reached the end of a confusing road. 
Thanks for reading!
Simon