modbuiltins: Add NotImplemented builtin constant.

From https://docs.python.org/3/library/constants.html#NotImplemented :
"Special value which should be returned by the binary special methods
(e.g. __eq__(), __lt__(), __add__(), __rsub__(), etc.) to indicate
that the operation is not implemented with respect to the other type;
may be returned by the in-place binary special methods (e.g. __imul__(),
__iand__(), etc.) for the same purpose. Its truth value is true."

Some people however appear to abuse it to mean "no value" when None is
a legitimate value (don't do that).

1 files changed, 8 insertions, 0 deletions

diff --git a/py/obj.c b/py/obj.c
index 81adbe3933..1928fad98f 100644
--- a/py/obj.c
+++ b/py/obj.c
@@ -62,6 +62,14 @@ void mp_obj_print_helper(const mp_print_t *print, mp_obj_t o_in, mp_print_kind_t
         return;
     }
 #endif
+
+    #if MICROPY_PY_BUILTINS_NOTIMPLEMENTED
+    if (o_in == MP_OBJ_SENTINEL) {
+        mp_printf(print, "%q", MP_QSTR_NotImplemented);
+        return;
+    }
+    #endif
+
     mp_obj_type_t *type = mp_obj_get_type(o_in);
     if (type->print != NULL) {
         type->print((mp_print_t*)print, o_in, kind);