Add auto-create support for OptGroup instances

Currently, it is possible to auto-create groups by passing a group name
to register_opt or register_opts. However, it is not possible to
auto-register groups when a OptGroup instance is passed instead.

This patch adds support for that to the _get_group method. There are
some advantages introduced by this patch:

    * It keeps consistency of oslo.config's public API.
    * It allow users to auto-register a group a group and
    re-use the instance:
        group = cfg.OptGroup("test")
        cfg.register_opts(..., group=group)
        # do something with the group
    * It allows users to auto-register groups with descriptions.

DocImpact

Change-Id: I9b9a74688c906d6bdea9c42044b47dc52e48c19d

2 files changed, 17 insertions, 8 deletions

diff --git a/oslo/config/cfg.py b/oslo/config/cfg.py
index 8092df1..7e74651 100644
--- a/oslo/config/cfg.py
+++ b/oslo/config/cfg.py
@@ -2038,6 +2038,10 @@ class ConfigOpts(collections.Mapping):
         objects, which will be a copy of any OptGroup object that users of
         the API have access to.
 
+        If autocreate is True, the group will be created if it's not found. If
+        group is an instance of OptGroup, that same instance will be
+        registered, otherwise a new instance of OptGroup will be created.
+
         :param group_or_name: the group's name or the OptGroup object itself
         :param autocreate: whether to auto-create the group if it's not found
         :raises: NoSuchGroupError
@@ -2046,10 +2050,10 @@ class ConfigOpts(collections.Mapping):
         group_name = group.name if group else group_or_name
 
         if group_name not in self._groups:
-            if group is not None or not autocreate:
+            if not autocreate:
                 raise NoSuchGroupError(group_name)
 
-            self.register_group(OptGroup(name=group_name))
+            self.register_group(group or OptGroup(name=group_name))
 
         return self._groups[group_name]
 
diff --git a/tests/test_cfg.py b/tests/test_cfg.py
index 086e55b..785a57e 100644
--- a/tests/test_cfg.py
+++ b/tests/test_cfg.py
@@ -1347,7 +1347,7 @@ class OptGroupsTestCase(BaseTestCase):
         self.assertTrue(hasattr(self.conf.blaa, 'foo'))
         self.assertEqual(self.conf.blaa.foo, 'bar')
 
-    def test_autocreate_group(self):
+    def test_autocreate_group_by_name(self):
         self.conf.register_cli_opt(cfg.StrOpt('foo'), group='blaa')
 
         self.conf(['--blaa-foo', 'bar'])
@@ -1356,6 +1356,16 @@ class OptGroupsTestCase(BaseTestCase):
         self.assertTrue(hasattr(self.conf.blaa, 'foo'))
         self.assertEqual(self.conf.blaa.foo, 'bar')
 
+    def test_autocreate_group_by_group(self):
+        group = cfg.OptGroup(name='blaa', title='Blaa options')
+        self.conf.register_cli_opt(cfg.StrOpt('foo'), group=group)
+
+        self.conf(['--blaa-foo', 'bar'])
+
+        self.assertTrue(hasattr(self.conf, 'blaa'))
+        self.assertTrue(hasattr(self.conf.blaa, 'foo'))
+        self.assertEqual(self.conf.blaa.foo, 'bar')
+
     def test_autocreate_title(self):
         blaa_group = cfg.OptGroup('blaa')
         self.assertEqual(blaa_group.title, 'blaa options')
@@ -2200,11 +2210,6 @@ class SadPathTestCase(BaseTestCase):
         self.conf.register_cli_opt(cfg.StrOpt('bar', short='f'))
         self.assertRaises(cfg.DuplicateOptError, self.conf, [])
 
-    def test_no_such_group(self):
-        group = cfg.OptGroup('blaa')
-        self.assertRaises(cfg.NoSuchGroupError, self.conf.register_cli_opt,
-                          cfg.StrOpt('foo'), group=group)
-
     def test_already_parsed(self):
         self.conf([])