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).

py/modbuiltins.c

@@ -634,6 +634,9 @@ STATIC const mp_map_elem_t mp_module_builtins_globals_table[] = {
 
     // built-in objects
     { MP_OBJ_NEW_QSTR(MP_QSTR_Ellipsis), (mp_obj_t)&mp_const_ellipsis_obj },
+    #if MICROPY_PY_BUILTINS_NOTIMPLEMENTED
+    { MP_OBJ_NEW_QSTR(MP_QSTR_NotImplemented), MP_OBJ_SENTINEL },
+    #endif
 
     // built-in user functions
     { MP_OBJ_NEW_QSTR(MP_QSTR_abs), (mp_obj_t)&mp_builtin_abs_obj },

py/mpconfig.h

@@ -479,6 +479,11 @@ typedef double mp_float_t;
 #define MICROPY_PY_BUILTINS_REVERSED (1)
 #endif
 
+// Whether to define "NotImplemented" special constant
+#ifndef MICROPY_PY_BUILTINS_NOTIMPLEMENTED
+#define MICROPY_PY_BUILTINS_NOTIMPLEMENTED (0)
+#endif
+
 // Whether to set __file__ for imported modules
 #ifndef MICROPY_PY___FILE__
 #define MICROPY_PY___FILE__ (1)

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);

py/qstrdefs.h

@@ -115,6 +115,9 @@ Q(builtins)
 
 Q(Ellipsis)
 Q(StopIteration)
+#if MICROPY_PY_BUILTINS_NOTIMPLEMENTED
+Q(NotImplemented)
+#endif
 
 Q(BaseException)
 Q(ArithmeticError)

unix/mpconfigport.h

@@ -64,6 +64,7 @@
 #define MICROPY_PY_BUILTINS_MEMORYVIEW (1)
 #define MICROPY_PY_BUILTINS_FROZENSET (1)
 #define MICROPY_PY_BUILTINS_COMPILE (1)
+#define MICROPY_PY_BUILTINS_NOTIMPLEMENTED (1)
 #define MICROPY_PY_MICROPYTHON_MEM_INFO (1)
 #define MICROPY_PY_ALL_SPECIAL_METHODS (1)
 #define MICROPY_PY_ARRAY_SLICE_ASSIGN (1)