>>> def fun():
...   x is y
...   x is not y
... 
>>> import dis
>>> dis.dis(fun)
  2           0 LOAD_GLOBAL              0 (x)
              2 LOAD_GLOBAL              1 (y)
              4 COMPARE_OP               8 (is)
              6 POP_TOP
  3           8 LOAD_GLOBAL              0 (x)
             10 LOAD_GLOBAL              1 (y)
             12 COMPARE_OP               9 (is not)
             14 POP_TOP
             16 LOAD_CONST               0 (None)
             18 RETURN_VALUE
>>>

case TARGET(COMPARE_OP): {
    PyObject *right = POP();
    PyObject *left = TOP();
    PyObject *res = cmp_outcome(tstate, oparg, left, right);
    Py_DECREF(left);
    Py_DECREF(right);
    SET_TOP(res);
    if (res == NULL)
        goto error;
    PREDICT(POP_JUMP_IF_FALSE);
    PREDICT(POP_JUMP_IF_TRUE);
    DISPATCH();
}

static PyObject *
cmp_outcome(PyThreadState *tstate, int op, PyObject *v, PyObject *w)
{
    int res = 0;
    switch (op) {
    case PyCmp_IS:
        res = (v == w);
        break;
    case PyCmp_IS_NOT:
        res = (v != w);
        break;
        ...
    }
    v = res ? Py_True : Py_False;
    Py_INCREF(v);
    return v;
}

def _is(a, b):
    return id(a) == id(b)

def _is_not(a, b):
    return id(a) != id(b)