commands: Internalise handling of help action

This means help will always be handled by the root command. Additionally this
adds an exit helper.

mopidy/utils/command.py

@@ -18,6 +18,23 @@ class ArgumentParser(argparse.ArgumentParser):
         raise CommandError(message)
 
 
+class _HelpError(Exception):
+    """Internal exception used to trigger help code path."""
+
+
+class _HelpAction(argparse.Action):
+    def __init__(self, option_strings, dest=None, help=None):
+        super(_HelpAction, self).__init__(
+            option_strings=option_strings,
+            dest=dest or argparse.SUPPRESS,
+            default=argparse.SUPPRESS,
+            nargs=0,
+            help=help)
+
+    def __call__(self, parser, namespace, values, option_string=None):
+        raise _HelpError()
+
+
 class Command(object):
     def __init__(self):
         self._children = collections.OrderedDict()
@@ -27,6 +44,7 @@ class Command(object):
     def _build(self):
         actions = []
         parser = ArgumentParser(add_help=False)
+        parser.register('action', 'help', _HelpAction)
 
         for args, kwargs in self._arguments:
             actions.append(parser.add_argument(*args, **kwargs))
@@ -44,6 +62,9 @@ class Command(object):
     def set_defaults(self, **kwargs):
         self._defaults.update(kwargs)
 
+    def exit(self, return_code):
+        sys.exit(return_code)
+
     def format_usage(self, prog=None):
         actions = self._build()[1]
         prog = prog or os.path.basename(sys.argv[0])
@@ -99,8 +120,12 @@ class Command(object):
 
     def parse(self, args, prog=None):
         prog = prog or os.path.basename(sys.argv[0])
-        return self._parse(
-            args, argparse.Namespace(), self._defaults.copy(), prog)
+        try:
+            return self._parse(
+                args, argparse.Namespace(), self._defaults.copy(), prog)
+        except _HelpError:
+            print self.format_help(prog)
+            self.exit(0)
 
     def _parse(self, args, namespace, defaults, prog):
         defaults.update(self._defaults)

tests/utils/command_test.py

@@ -197,6 +197,16 @@ class CommandParsingTest(unittest.TestCase):
         result = cmd.parse(['command'])
         self.assertEqual(result.foo, 'bar')
 
+    def test_help_action_works(self):
+        cmd = command.Command()
+        cmd.add_argument('-h', action='help')
+        cmd.format_help = mock.Mock()
+        cmd.exit = mock.Mock()
+
+        cmd.parse(['-h'])
+        cmd.format_help.assert_called_once_with(mock.ANY)
+        cmd.exit.assert_called_once_with(0)
+
 
 class UsageTest(unittest.TestCase):
     @mock.patch('sys.argv')