py: Fix bug in type_store_attr, trying to store to ROM.

py/map.h

@@ -3,6 +3,11 @@ typedef struct _mp_map_elem_t {
     mp_obj_t value;
 } mp_map_elem_t;
 
+// TODO maybe have a truncated mp_map_t for fixed tables, since alloc=used
+// put alloc last in the structure, so the truncated version does not need it
+// this would save 1 ROM word for all ROM objects that have a locals_dict
+// would also need a trucated dict structure
+
 typedef struct _mp_map_t {
     machine_uint_t all_keys_are_qstrs : 1;
     machine_uint_t table_is_fixed_array : 1;

py/objtype.c

@@ -325,14 +325,18 @@ STATIC bool type_store_attr(mp_obj_t self_in, qstr attr, mp_obj_t value) {
 
     if (self->locals_dict != NULL) {
         assert(MP_OBJ_IS_TYPE(self->locals_dict, &dict_type)); // Micro Python restriction, for now
-        mp_map_t *locals_map = ((void*)self->locals_dict + sizeof(mp_obj_base_t)); // XXX hack to get map object from dict object
-        mp_map_lookup(locals_map, MP_OBJ_NEW_QSTR(attr), MP_MAP_LOOKUP_ADD_IF_NOT_FOUND)->value = value;
+        mp_map_t *locals_map = mp_obj_dict_get_map(self->locals_dict);
+        mp_map_elem_t *elem = mp_map_lookup(locals_map, MP_OBJ_NEW_QSTR(attr), MP_MAP_LOOKUP_ADD_IF_NOT_FOUND);
+        // note that locals_map may be in ROM, so add will fail in that case
+        if (elem != NULL) {
+            elem->value = value;
             return true;
-    } else {
-        return false;
         }
     }
 
+    return false;
+}
+
 const mp_obj_type_t mp_type_type = {
     { &mp_type_type },
     .name = MP_QSTR_type,