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

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):

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()

Misc/NEWS.d/next/Library/2019-05-14-21-39-52.bpo-25652.xLw42k.rst

@@ -0,0 +1 @@
+Fix bug in ``__rmod__`` of ``UserString`` - by Batuhan Taskaya.