py3: display help if no subcommand provided to manage-shard-ranges

That way we don't traceback when you just provide a DB file.

At some point, py3 switched to having optional subparsers. In py37,
they added a kwarg to say whether a subparser is optional or required,
but for the sake of earlier versions, we have to check whether it was
set manually and cannot rely on argparse doing it for us. See also:

- https://bugs.python.org/issue9253
- https://github.com/python/cpython/commit/aaf6fc0
- https://bugs.python.org/issue33109
- https://github.com/python/cpython/commit/8ebf5ce

Change-Id: Iac8048ad0e23ffb28b914fe880c4b6538bc41c86

swift/cli/manage_shard_ranges.py

@@ -438,7 +438,7 @@ def _make_parser():
     parser.add_argument('--verbose', '-v', action='count', default=0,
                         help='Increase output verbosity')
     subparsers = parser.add_subparsers(
-        help='Sub-command help', title='Sub-commands')
+        dest='subcommand', help='Sub-command help', title='Sub-commands')
 
     # find
     find_parser = subparsers.add_parser(
@@ -503,6 +503,15 @@ def _make_parser():
 def main(args=None):
     parser = _make_parser()
     args = parser.parse_args(args)
+    if not args.subcommand:
+        # On py2, subparsers are required; on py3 they are not; see
+        # https://bugs.python.org/issue9253. py37 added a `required` kwarg
+        # to let you control it, but prior to that, there was no choice in
+        # the matter. So, check whether the destination was set and bomb
+        # out if not.
+        parser.print_help()
+        print('\nA sub-command is required.')
+        return 1
     logger = get_logger({}, name='ContainerBroker', log_to_console=True)
     broker = ContainerBroker(args.container_db, logger=logger,
                              skip_commits=True)