Make help message include choices information for StrOpt

Config option `StrOpt` accept choices parameter to indicate
valid values and include comments "# Allowed values:" when generating
sample config file, so it's unnecessary to add allowed values in hep
string of the config option. But it doesn't include choices information
when printing help for the config option. This commit makes consistency
for both of sample config file and help output of command line.

Closes-Bug:1727683

Change-Id: I962e49c81bdf44043a6e1233408a6ee5b883acde

oslo_config/cfg.py

@@ -1261,6 +1261,27 @@ class StrOpt(Opt):
                                          max_length=max_length),
                                      **kwargs)
 
+    def _get_choice_text(self, choice):
+        if choice is None:
+            return '<None>'
+        elif choice == '':
+            return "''"
+        return six.text_type(choice)
+
+    def _get_argparse_kwargs(self, group, **kwargs):
+        """Extends the base argparse keyword dict for the config dir option."""
+        kwargs = super(StrOpt, self)._get_argparse_kwargs(group)
+
+        if getattr(self.type, 'choices', None):
+            choices_text = ', '.join([self._get_choice_text(choice)
+                                      for choice in self.type.choices])
+            if kwargs['help'] is not None:
+                kwargs['help'] += (' Allowed values: %s\n' % choices_text)
+            else:
+                kwargs['help'] = (' Allowed values: %s\n' % choices_text)
+
+        return kwargs
+
 
 class BoolOpt(Opt):
 

oslo_config/tests/test_cfg.py

@@ -167,6 +167,32 @@ class HelpTestCase(BaseTestCase):
         self.assertIn('optional', f.getvalue())
         self.assertIn('-h, --help', f.getvalue())
 
+    def test_print_strOpt_with_choices_help(self):
+        f = moves.StringIO()
+        cli_opts = [
+            cfg.StrOpt('aa', short='a', default='xx',
+                       choices=['xx', 'yy', 'zz'],
+                       help='StrOpt with choices.'),
+            cfg.StrOpt('bb', short='b', default='yy',
+                       choices=[None, 'yy', 'zz'],
+                       help='StrOpt with choices.'),
+            cfg.StrOpt('cc', short='c', default='zz',
+                       choices=['', 'yy', 'zz'],
+                       help='StrOpt with choices.'),
+        ]
+        self.conf.register_cli_opts(cli_opts)
+        self.conf([])
+        self.conf.print_help(file=f)
+        self.assertIn('usage: test FOO BAR', f.getvalue())
+        self.assertIn('optional', f.getvalue())
+        self.assertIn('-h, --help', f.getvalue())
+        self.assertIn('StrOpt with choices. Allowed values: xx, yy, zz',
+                      f.getvalue())
+        self.assertIn('StrOpt with choices. Allowed values: <None>, yy, zz',
+                      f.getvalue())
+        self.assertIn("StrOpt with choices. Allowed values: '', yy, zz",
+                      f.getvalue())
+
     def test_print_sorted_help(self):
         f = moves.StringIO()
         self.conf.register_cli_opt(cfg.StrOpt('abc'))