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

Lib/test/test_str.py

@@ -465,6 +465,15 @@ class StrTest(
             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)
 

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 2.7.14?
 Core and Builtins
 -----------------
 
+- bpo-28598: Support __rmod__ for subclasses of str being called before
+  str.__mod__.  Patch by Martijn Pieters.
+
 - bpo-29602: Fix incorrect handling of signed zeros in complex constructor for
   complex subclasses and for inputs having a __complex__ method. Patch
   by Serhiy Storchaka.

Python/ceval.c

@@ -1446,10 +1446,14 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
         {
             w = POP();
             v = TOP();
-            if (PyString_CheckExact(v))
+            if (PyString_CheckExact(v)
+                && (!PyString_Check(w) || PyString_CheckExact(w))) {
+                /* fast path; string formatting, but not if the RHS is a str subclass
+                   (see issue28598) */
                 x = PyString_Format(v, w);
-            else
+            } else {
                 x = PyNumber_Remainder(v, w);
+            }
             Py_DECREF(v);
             Py_DECREF(w);
             SET_TOP(x);