# HG changeset patch
# User Martijn Pieters <mjpieters@fb.com>
# Date 1478194992 0
#      Thu Nov 03 17:43:12 2016 +0000
# Node ID 9551d8f5c522567fad4793a8aaa664d07c70bdcc
# Parent  fca5c4a63251410c87f010beb99b25b348466f37
Allow a str subclass hook into % with __rmod__ (issue 28598)

diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -1441,6 +1441,15 @@
         with self.assertRaises(ValueError):
             result = format_string % 2.34
 
+    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")
+
     @support.cpython_only
     def test_formatting_huge_precision_c_limits(self):
         from _testcapi import INT_MAX
diff --git a/Python/ceval.c b/Python/ceval.c
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -1409,9 +1409,15 @@
         TARGET(BINARY_MODULO) {
             PyObject *divisor = POP();
             PyObject *dividend = TOP();
-            PyObject *res = PyUnicode_CheckExact(dividend) ?
-                PyUnicode_Format(dividend, divisor) :
-                PyNumber_Remainder(dividend, divisor);
+            PyObject *res;
+            if (PyUnicode_CheckExact(dividend) && !(
+                  PyUnicode_Check(divisor) && !PyUnicode_CheckExact(divisor))) {
+              // fast path; string formatting, but not if the RHS is a str subclass
+              // (see issue28598)
+              res = PyUnicode_Format(dividend, divisor);
+            } else {
+              res = PyNumber_Remainder(dividend, divisor);
+            }
             Py_DECREF(divisor);
             Py_DECREF(dividend);
             SET_TOP(res);