python · berkerpeksag · Feb 27, 2017 · Feb 14, 2017 · Feb 27, 2017 · Feb 27, 2017
diff --git 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
@@ -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-29602: Fix incorrect handling of signed zeros in complex constructor for
  complex subclasses and for inputs having a __complex__ method. Patch
  by Serhiy Storchaka.

diff --git 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);