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 changeset b4f0e980b6084e9a994e3f069269fac2471e0d78 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 changeset bc144f0abff2b36595171377ee847c0266596ab2 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 changeset 53039ad3814a8918c5311f37bd654428b9843fcc 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 changeset d7e64337ef45085792b382a09f5b3a45d3687c8c 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