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It seems PyUnicode_FromFormatV wrongly assumes that the return value of
PyObject_Str and PyObject_Repr is a unicode object. It looks like the
%S and %R feature was backported from 3.x without updating the code for 2.x.
PyObject *
PyUnicode_FromFormatV(const char *format, va_list vargs)
{
...
 case 'S':
 {
 PyObject *obj = va_arg(count, PyObject *);
 PyObject *str;
 assert(obj);
 str = PyObject_Str(obj);
 if (!str)
 goto fail;
 n += PyUnicode_GET_SIZE(str);
 /* Remember the str and switch to the next slot */
 *callresult++ = str;
 break;
 }
 case 'R':
 {
 PyObject *obj = va_arg(count, PyObject *);
 PyObject *repr;
 assert(obj);
 repr = PyObject_Repr(obj);
 if (!repr)
 goto fail;
 n += PyUnicode_GET_SIZE(repr);
 /* Remember the repr and switch to the next slot */
 *callresult++ = repr;
 break;
 }
...
}