python · zhangyangyu · Mar 1, 2017 · Mar 1, 2017
diff --git a/Lib/test/test_str.py b/Lib/test/test_str.py
@@ -465,6 +465,15 @@ def test_startswith_endswith_errors(self):
            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)


diff --git a/Misc/NEWS b/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.

diff --git a/Python/ceval.c b/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);