I agree with Raymond about the Py_INCREF.  I could see more uses for this macro without the Py_INCREF, especially for things like in-place arithmetic operations.  The following pattern appears a lot in code like Objects/longobject.c:

Old code:

/* add one to x */
temp = PyNumber_Add(x, one);
Py_DECREF(x);
x = temp;
if (x == NULL)
    return NULL;


With a non-INCREF version of Py_SETREF:

/* add one to x */
Py_SETREF(x, PyNumber_Add(x, one));
if (x == NULL)
    return NULL;