bpo-28598: Support __rmod__ for RHS subclasses of str in % string formatting operations (GH-366)

diff --git a/Lib/test/test_str.py b/Lib/test/test_str.py
index 5bb9f48..8b306f4 100644
--- a/Lib/test/test_str.py
+++ b/Lib/test/test_str.py
@@ -465,6 +465,15 @@
             self.assertIn('str', exc)
             self.assertIn('tuple', exc)
 
+    def test_issue28598_strsubclass_rhs(self):
+        # A subclass of str with an __rmod__ method should be able to hook
+        # into the % operator
+        class SubclassedStr(str):
+            def __rmod__(self, other):
+                return 'Success, self.__rmod__({!r}) was called'.format(other)
+        self.assertEqual('lhs %% %r' % SubclassedStr('rhs'),
+                         "Success, self.__rmod__('lhs %% %r') was called")
+
 def test_main():
     test_support.run_unittest(StrTest)