The `BINARY_MODULO` operator hardcodes a test for `PyUnicode`:        TARGET(BINARY_MODULO) {            PyObject *divisor = POP();            PyObject *dividend = TOP();            PyObject *res = PyUnicode_CheckExact(dividend) ?                PyUnicode_Format(dividend, divisor) :                PyNumber_Remainder(dividend, divisor);This means that a RHS subclass of str can't override the operator:>>> class Foo(str):...     def __rmod__(self, other):...         return self % other...>>> "Bar: %s" % Foo("Foo: %s")'Bar: Foo %s'The expected output there is "Foo: Bar %s".This works correctly for `bytes`:>>> class FooBytes(bytes):...     def __rmod__(self, other):...         return self % other...>>> b"Bar: %s" % FooBytes(b"Foo: %s")b'Foo: Bar: %s'and for all other types where the RHS is a subclass.Perhaps there should be a test to see if `divisor` is a subclass, and in that case take the slow path?

Here's a proposed patch for tip; what versions would it be worth backporting this to?(Note, there's no NEWS update in this patch).

I'm not sure if issues are linked automatically yet. I put the patch up as a pull request on GitHub:https://github.com/python/cpython/pull/51

Is 2.7 free from this bug?

> Is 2.7 free from this bug?No, 2.7 is affected too:>>> class SubclassedStr(str):...     def __rmod__(self, other):...         return 'Success, self.__rmod__({!r}) was called'.format(other)...>>> 'lhs %% %r' % SubclassedStr('rhs')"lhs % 'rhs'"Expected output is "Success, self.__rmod__('lhs %% %r') was called"On the plus side, unicode is not affected:>>> class SubclassedUnicode(unicode):...     def __rmod__(self, other):...         return u'Success, self.__rmod__({!r}) was called'.format(other)...>>> u'lhs %% %r' % SubclassedUnicode(u'rhs')u"Success, self.__rmod__(u'lhs %% %r') was called"

New changesetb4f0e980b6084e9a994e3f069269fac2471e0d78 by Xiang Zhang in branch '2.7':bpo-28598: Support __rmod__ for RHS subclasses of str in % string formatting operations (GH-366)https://github.com/python/cpython/commit/b4f0e980b6084e9a994e3f069269fac2471e0d78

New changesetbc144f0abff2b36595171377ee847c0266596ab2 by Berker Peksag (Martijn Pieters) in branch '3.5':bpo-28598: Support __rmod__ for RHS subclasses of str in % string formatting operations (#94)https://github.com/python/cpython/commit/bc144f0abff2b36595171377ee847c0266596ab2

New changeset53039ad3814a8918c5311f37bd654428b9843fcc by Berker Peksag (Martijn Pieters) in branch '3.6':bpo-28598: Support __rmod__ for RHS subclasses of str in % string formatting operations (#95)https://github.com/python/cpython/commit/53039ad3814a8918c5311f37bd654428b9843fcc

New changesetd7e64337ef45085792b382a09f5b3a45d3687c8c by Serhiy Storchaka (Martijn Pieters) in branch 'master':bpo-28598: Support __rmod__ for RHS subclasses of str in % string formatting operations (#51)https://github.com/python/cpython/commit/d7e64337ef45085792b382a09f5b3a45d3687c8c