From 4ea4e2059804d471b82c538ad8b6a8ed2a3e8387 Mon Sep 17 00:00:00 2001
From: Martijn Pieters <mjpieters@fb.com>
Date: Tue, 14 Feb 2017 08:33:07 +0000
Subject: [PATCH 1/2] [backport to 3.5] bpo-28598: Support __rmod__ for RHS
 subclasses of str in % string formatting operations

---
 Lib/test/test_unicode.py |  9 +++++++++
 Misc/NEWS                |  3 +++
 Python/ceval.c           | 12 +++++++++---
 3 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
index 3136ea1a1bad3e..9dbded2d99c530 100644
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -1424,6 +1424,15 @@ def test_formatting_huge_precision(self):
         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/Misc/NEWS b/Misc/NEWS
index fe50135c01615c..7dc66808178c05 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,9 @@ Release date: XXXX-XX-XX
 Core and Builtins
 -----------------
 
+- Issue #28598: Support __rmod__ for subclasses of str being called before
+  str.__mod__. Patch by Martijn Pieters.
+
 - bpo-29438: Fixed use-after-free problem in key sharing dict.
 
 - Issue #29319: Prevent RunMainFromImporter overwriting sys.path[0].
diff --git a/Python/ceval.c b/Python/ceval.c
index 7b405188d38c66..ff44f541c7a5e1 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -1545,9 +1545,15 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
         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);

From b2e718b23ddfaab093c9b0026814bd833be0b3ae Mon Sep 17 00:00:00 2001
From: Martijn Pieters <mjpieters@fb.com>
Date: Mon, 27 Feb 2017 14:39:16 +0000
Subject: [PATCH 2/2] Use C89-compatible comments

---
 Python/ceval.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Python/ceval.c b/Python/ceval.c
index ff44f541c7a5e1..d3239089284c48 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -1548,8 +1548,8 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
             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)
+              /* 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);