Prevent instantiation of ShardRangeOuterBound

Only the subclasses should be instantiated.

Co-Authored-By: Tim Burke <tim.burke@gmail.com>
Change-Id: I9b90863bb502ca8b95e626e83775475c352a7a4d
Related-Change: I21292e7991e93834b35cda6f5daea4c552a8e999

swift/common/utils.py

@@ -4972,11 +4972,15 @@ except TypeError:
 
 class ShardRangeOuterBound(object):
     """
-    A custom singleton type used for the outer bounds of ShardRanges.
+    A custom singleton type to be subclassed for the outer bounds of
+    ShardRanges.
     """
     _singleton = None
 
     def __new__(cls):
+        if cls is ShardRangeOuterBound:
+            raise TypeError('ShardRangeOuterBound is an abstract class; '
+                            'only subclasses should be instantiated')
         if cls._singleton is None:
             cls._singleton = super(ShardRangeOuterBound, cls).__new__(cls)
         return cls._singleton

test/unit/common/test_utils.py

@@ -7732,6 +7732,9 @@ class TestShardRange(unittest.TestCase):
         self.ts_iter = make_timestamp_iter()
 
     def test_min_max_bounds(self):
+        with self.assertRaises(TypeError):
+            utils.ShardRangeOuterBound()
+
         # max
         self.assertEqual(utils.ShardRange.MAX, utils.ShardRange.MAX)
         self.assertFalse(utils.ShardRange.MAX > utils.ShardRange.MAX)