Make paasta without args compat with py3

paasta_tools/cli/cli.py

@@ -84,6 +84,10 @@ def get_argparser():
     )
 
     subparsers = parser.add_subparsers(help="[-h, --help] for subcommand help")
+    subparsers.required = True
+    # https://stackoverflow.com/questions/18282403/argparse-with-required-subcommands/18283730#18283730
+    subparsers.dest = 'command'
+
     # Adding a separate help subparser allows us to respont to "help" without --help
     help_parser = subparsers.add_parser('help', add_help=False)
     help_parser.set_defaults(command=None)
@@ -115,11 +119,11 @@ def main(argv=None):
     logging.basicConfig()
     try:
         args, parser = parse_args(argv)
-        if hasattr(args, 'command'):
-            return_code = args.command(args)
-        else:
+        if args.command is None:
             parser.print_help()
             return_code = 0
+        else:
+            return_code = args.command(args)
     except KeyboardInterrupt:
         return_code = 1
     sys.exit(return_code)