[Clients] Add test for osclients.cached

osclients.cached missed unit test, so this patch adds one.
Also, there is a small refactoring in this function.

Change-Id: I76301fbfb4abb8323b5d76888f93984d23981d5b

rally/osclients.py

@@ -46,10 +46,8 @@ def cached(func):
         key = "{0}{1}{2}".format(func.__name__,
                                  str(args) if args else "",
                                  str(kwargs) if kwargs else "")
-
-        if key in self.cache:
-            return self.cache[key]
-        self.cache[key] = func(self, *args, **kwargs)
+        if key not in self.cache:
+            self.cache[key] = func(self, *args, **kwargs)
         return self.cache[key]
 
     return wrapper

tests/unit/test_osclients.py

@@ -26,6 +26,28 @@ from tests.unit import fakes
 from tests.unit import test
 
 
+class CachedTestCase(test.TestCase):
+
+    def test_cached(self):
+        foo_client = mock.Mock(
+            __name__="foo_client",
+            side_effect=lambda ins, *args, **kw: (args, kw))
+        ins = mock.Mock(cache={})
+        cached = osclients.cached(foo_client)
+        self.assertEqual(((), {}), cached(ins))
+        self.assertEqual({"foo_client": ((), {})}, ins.cache)
+        self.assertEqual((("foo",), {"bar": "spam"}),
+                         cached(ins, "foo", bar="spam"))
+        self.assertEqual(
+            {"foo_client": ((), {}),
+             "foo_client('foo',){'bar': 'spam'}": (("foo",),
+                                                   {"bar": "spam"})},
+            ins.cache)
+        ins.cache["foo_client('foo',){'bar': 'spam'}"] = "foo_cached"
+        self.assertEqual(
+            "foo_cached", cached(ins, "foo", bar="spam"))
+
+
 class TestCreateKeystoneClient(test.TestCase):
 
     def setUp(self):