Treat empty CLI args as error and show usage

src/spoc_cli.py

@@ -79,8 +79,7 @@ def prune():
 
 def parse_args(args=None):
     parser = argparse.ArgumentParser(description='SPOC application manager')
-    parser.set_defaults(action=None)
-    subparsers = parser.add_subparsers()
+    subparsers = parser.add_subparsers(dest='action', required=True)
 
     parser_list = subparsers.add_parser('list')
     parser_list.set_defaults(action=listing)

tests/test_cli.py

@@ -317,6 +317,7 @@ def test_main_invalid(prune, login, stop_all, start_autostarted, status, stop, s
     ['autostart', 'someapp'],
     ['login'],
     ['invalid'],
+    [],
 ])
 def test_main_systemexit(argv):
     argv.insert(0, 'foo')