improve tests

Lib/collections/__init__.py

@@ -1215,9 +1215,7 @@ class UserString(_collections_abc.Sequence):
     def __mod__(self, args):
         return self.__class__(self.data % args)
     def __rmod__(self, template):
-        if isinstance(template, str):
-            return self.__class__(template % self)
-        return NotImplemented
+        return self.__class__(str(template) % self)
     # the following methods are defined in alphabetical order:
     def capitalize(self): return self.__class__(self.data.capitalize())
     def casefold(self):

Lib/test/test_collections.py

@@ -71,10 +71,17 @@ class TestUserObjects(unittest.TestCase):
         self._copy_test(obj)
 
     def test_str_rmod(self):
-        arg = UserString("python")
-        template = "I love %s"
-        self.assertEqual(arg.__rmod__(template), "I love python")
-        self.assertIs(arg.__rmod__(type('Dummy', (), {})), NotImplemented)
+        class ustr2(UserString):
+            pass
+
+
+        class ustr3(ustr2):
+            def __rmod__(self, other):
+                return super().__rmod__(other)
+
+        fmt2 = ustr2('value is %s')
+        str3 = ustr3('TEST')
+        self.assertEqual(fmt2 % str3, 'value is TEST')
 
 ################################################################################
 ### ChainMap (helper class for configparser and the string module)