Here's the code for PyNumber_Remainder: PyObject * PyNumber_Remainder(PyObject *v, PyObject *w) { if (PyString_Check(v)) return PyString_Format(v, w); #ifdef Py_USING_UNICODE else if (PyUnicode_Check(v)) return PyUnicode_Format(v, w); #endif return binary_op(v, w, NB_SLOT(nb_remainder), "%"); } Is there any good reason why str.__mod__ != PyString_Format? I want to make a subclass of str that overrides the format operator. I guess one side effect would be that PyNumber_Check(astring) would start returning true. Should I file a bug saying "can't override __mod__ on str and unicode subclasses"? I guess the fix would be to check for nb_remainder first and then fallback to PyString_Format or PyUnicode_Format. Neil