sub.c

#define ENSURE_CONSTANT(NODE) \
    if ((NODE) != NULL && (NODE)->kind != Constant_kind) \
        return 1;

#define GET_CONSTANT(NODE) (((NODE) != NULL) ? (NODE)->v.Constant.value : NULL)

static int
fold_subscr(expr_ty node, PyArena *arena, int optimize)
{
    PyObject *newval, *key;
    expr_ty arg;
    slice_ty slice;

    arg = node->v.Subscript.value;
    slice = node->v.Subscript.slice;
    if (node->v.Subscript.ctx != Load)
    {
        return 1;
    }
    ENSURE_CONSTANT(arg)
    switch (slice->kind) {
    case Index_kind:
        ENSURE_CONSTANT(slice->v.Index.value)
        key = GET_CONSTANT(slice->v.Index.value);
        break;
    case Slice_kind:
        ENSURE_CONSTANT(slice->v.Slice.lower)
        ENSURE_CONSTANT(slice->v.Slice.upper)
        ENSURE_CONSTANT(slice->v.Slice.step)
        key = PySlice_New(GET_CONSTANT(slice->v.Slice.lower),
                          GET_CONSTANT(slice->v.Slice.upper),
                          GET_CONSTANT(slice->v.Slice.step));
        break;
    default:
        return 1;
    }

    newval = PyObject_GetItem(GET_CONSTANT(arg), key);
    return make_const(node, newval, arena);
}