bpo-25652: Fix __rmod__ of UserString (GH-13326)

The ``__rmod__`` method of ``collections.UserString`` class had a bug that made it unusable. 


https://bugs.python.org/issue25652

2 files changed, 14 insertions, 3 deletions

diff --git a/Lib/collections/__init__.py b/Lib/collections/__init__.py
index 960d82a..cb7f1bb 100644
--- a/Lib/collections/__init__.py
+++ b/Lib/collections/__init__.py
@@ -1214,9 +1214,8 @@ class UserString(_collections_abc.Sequence):
     __rmul__ = __mul__
     def __mod__(self, args):
         return self.__class__(self.data % args)
-    def __rmod__(self, format):
-        return self.__class__(format % args)
-
+    def __rmod__(self, template):
+        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):
diff --git a/Lib/test/test_userstring.py b/Lib/test/test_userstring.py
index 7152822..19b0acf 100644
--- a/Lib/test/test_userstring.py
+++ b/Lib/test/test_userstring.py
@@ -39,6 +39,18 @@ class UserStringTest(
         # we don't fix the arguments, because UserString can't cope with it
         getattr(object, methodname)(*args)
 
+    def test_rmod(self):
+        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')
+
 
 if __name__ == "__main__":
     unittest.main()