Ensure subparser is required. (ros-infrastructure#683)

Signed-off-by: Kenji Brameld <[email protected]>

bloom/commands/generate.py

@@ -58,15 +58,16 @@ def load_generator_description(generator_name):
 
 def create_subparsers(parser, generator_cmds):
     metavar = '[' + ' | '.join(generator_cmds) + ']'
-    subparser = parser.add_subparsers(
+    subparsers = parser.add_subparsers(
         title='generate commands',
         metavar=metavar,
         description='Call `bloom-generate {0} -h` for help on a each generate command.'.format(metavar),
-        dest='generator_cmd'
+        dest='generator_cmd',
+        required=True
     )
     for generator_cmd in generator_cmds:
         desc = load_generator_description(generator_cmd)
-        cmd_parser = subparser.add_parser(desc['title'], description=desc['description'])
+        cmd_parser = subparsers.add_parser(desc['title'], description=desc['description'])
         cmd_parser = desc['prepare_arguments'](cmd_parser)
         cmd_parser.set_defaults(func=desc['main'])
         add_global_arguments(cmd_parser)