Here is a way to test for overflow which is correct for any C implementation:

static PyObject *
int_add(PyIntObject *v, PyIntObject *w)
{
	register long a, b;
	CONVERT_TO_LONG(v, a);
	CONVERT_TO_LONG(w, b);
	if (((a>0)&&(b>0)&&((LONG_MAX-a)<b))
		||((a<0)&&(b<0)&&((LONG_MIN-a)>b))) {
		/* would overflow the long type */
	    return PyLong_Type.tp_as_number->nb_add((PyObject *)v, (PyObject *)w);
	}
	
	return PyInt_FromLong(a+b);
}