diff --git a/Lib/test/test_str.py b/Lib/test/test_str.py
index 5bb9f4867bcb0e..71061973bbd14d 100644
--- a/Lib/test/test_str.py
+++ b/Lib/test/test_str.py
@@ -38,6 +38,15 @@ def test_formatting(self):
string_tests.MixinStrUnicodeUserStringTest.test_formatting(self)
self.assertRaises(OverflowError, '%c'.__mod__, 0x1234)
+ 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")
+
@test_support.cpython_only
def test_formatting_huge_precision(self):
from _testcapi import INT_MAX
diff --git a/Misc/NEWS b/Misc/NEWS
index 4523df5a936155..868b2a2b995af1 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,9 @@ What's New in Python 2.7.14?
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
index 2af78ff8747411..68f2a398acece4 100644
--- 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);
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