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 635f413dc74be8..fd25a5f325077e 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-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 7b405188d38c66..d3239089284c48 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);
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