diff --git a/py/persistentcode.c b/py/persistentcode.c
index f64e383a6..c9cfc3b5c 100644
--- a/py/persistentcode.c
+++ b/py/persistentcode.c
@@ -198,29 +198,29 @@ STATIC qstr load_qstr(mp_reader_t *reader) {
 STATIC mp_obj_t load_obj(mp_reader_t *reader) {
     byte obj_type = read_byte(reader);
     #if MICROPY_EMIT_MACHINE_CODE
-    if (obj_type == 't') {
+    if (obj_type == MP_PERSISTENT_OBJ_FUN_TABLE) {
         return MP_OBJ_FROM_PTR(&mp_fun_table);
     } else
     #endif
-    if (obj_type == 'e') {
+    if (obj_type == MP_PERSISTENT_OBJ_ELLIPSIS) {
         return MP_OBJ_FROM_PTR(&mp_const_ellipsis_obj);
     } else {
         size_t len = read_uint(reader);
-        if (len == 0 && obj_type == 'b') {
+        if (len == 0 && obj_type == MP_PERSISTENT_OBJ_BYTES) {
             read_byte(reader); // skip null terminator
             return mp_const_empty_bytes;
         }
         vstr_t vstr;
         vstr_init_len(&vstr, len);
         read_bytes(reader, (byte *)vstr.buf, len);
-        if (obj_type == 's' || obj_type == 'b') {
+        if (obj_type == MP_PERSISTENT_OBJ_STR || obj_type == MP_PERSISTENT_OBJ_BYTES) {
             read_byte(reader); // skip null terminator
-            return mp_obj_new_str_from_vstr(obj_type == 's' ? &mp_type_str : &mp_type_bytes, &vstr);
-        } else if (obj_type == 'i') {
+            return mp_obj_new_str_from_vstr(obj_type == MP_PERSISTENT_OBJ_STR ? &mp_type_str : &mp_type_bytes, &vstr);
+        } else if (obj_type == MP_PERSISTENT_OBJ_INT) {
             return mp_parse_num_integer(vstr.buf, vstr.len, 10, NULL);
         } else {
-            assert(obj_type == 'f' || obj_type == 'c');
-            return mp_parse_num_decimal(vstr.buf, vstr.len, obj_type == 'c', false, NULL);
+            assert(obj_type == MP_PERSISTENT_OBJ_FLOAT || obj_type == MP_PERSISTENT_OBJ_COMPLEX);
+            return mp_parse_num_decimal(vstr.buf, vstr.len, obj_type == MP_PERSISTENT_OBJ_COMPLEX, false, NULL);
         }
     }
 }
@@ -498,16 +498,16 @@ STATIC void save_qstr(mp_print_t *print, qstr qst) {
 STATIC void save_obj(mp_print_t *print, mp_obj_t o) {
     #if MICROPY_EMIT_MACHINE_CODE
     if (o == MP_OBJ_FROM_PTR(&mp_fun_table)) {
-        byte obj_type = 't';
+        byte obj_type = MP_PERSISTENT_OBJ_FUN_TABLE;
         mp_print_bytes(print, &obj_type, 1);
     } else
     #endif
     if (mp_obj_is_str_or_bytes(o)) {
         byte obj_type;
         if (mp_obj_is_str(o)) {
-            obj_type = 's';
+            obj_type = MP_PERSISTENT_OBJ_STR;
         } else {
-            obj_type = 'b';
+            obj_type = MP_PERSISTENT_OBJ_BYTES;
         }
         size_t len;
         const char *str = mp_obj_str_get_data(o, &len);
@@ -515,21 +515,21 @@ STATIC void save_obj(mp_print_t *print, mp_obj_t o) {
         mp_print_uint(print, len);
         mp_print_bytes(print, (const byte *)str, len + 1); // +1 to store null terminator
     } else if (MP_OBJ_TO_PTR(o) == &mp_const_ellipsis_obj) {
-        byte obj_type = 'e';
+        byte obj_type = MP_PERSISTENT_OBJ_ELLIPSIS;
         mp_print_bytes(print, &obj_type, 1);
     } else {
         // we save numbers using a simplistic text representation
         // TODO could be improved
         byte obj_type;
         if (mp_obj_is_int(o)) {
-            obj_type = 'i';
+            obj_type = MP_PERSISTENT_OBJ_INT;
         #if MICROPY_PY_BUILTINS_COMPLEX
         } else if (mp_obj_is_type(o, &mp_type_complex)) {
-            obj_type = 'c';
+            obj_type = MP_PERSISTENT_OBJ_COMPLEX;
         #endif
         } else {
             assert(mp_obj_is_float(o));
-            obj_type = 'f';
+            obj_type = MP_PERSISTENT_OBJ_FLOAT;
         }
         vstr_t vstr;
         mp_print_t pr;
diff --git a/py/persistentcode.h b/py/persistentcode.h
index 1991ba26f..7426029ad 100644
--- a/py/persistentcode.h
+++ b/py/persistentcode.h
@@ -102,6 +102,16 @@ enum {
     MP_NATIVE_ARCH_XTENSAWIN,
 };
 
+enum {
+    MP_PERSISTENT_OBJ_FUN_TABLE = 0,
+    MP_PERSISTENT_OBJ_ELLIPSIS,
+    MP_PERSISTENT_OBJ_STR,
+    MP_PERSISTENT_OBJ_BYTES,
+    MP_PERSISTENT_OBJ_INT,
+    MP_PERSISTENT_OBJ_FLOAT,
+    MP_PERSISTENT_OBJ_COMPLEX,
+};
+
 mp_compiled_module_t mp_raw_code_load(mp_reader_t *reader, mp_module_context_t *ctx);
 mp_compiled_module_t mp_raw_code_load_mem(const byte *buf, size_t len, mp_module_context_t *ctx);
 mp_compiled_module_t mp_raw_code_load_file(const char *filename, mp_module_context_t *ctx);
diff --git a/tools/mpy-tool.py b/tools/mpy-tool.py
index 84c09a0c6..fc0b7c09c 100755
--- a/tools/mpy-tool.py
+++ b/tools/mpy-tool.py
@@ -121,6 +121,14 @@ MP_NATIVE_ARCH_ARMV7EMDP = 8
 MP_NATIVE_ARCH_XTENSA = 9
 MP_NATIVE_ARCH_XTENSAWIN = 10
 
+MP_PERSISTENT_OBJ_FUN_TABLE = 0
+MP_PERSISTENT_OBJ_ELLIPSIS = 1
+MP_PERSISTENT_OBJ_STR = 2
+MP_PERSISTENT_OBJ_BYTES = 3
+MP_PERSISTENT_OBJ_INT = 4
+MP_PERSISTENT_OBJ_FLOAT = 5
+MP_PERSISTENT_OBJ_COMPLEX = 6
+
 MP_SCOPE_FLAG_VIPERRELOC = 0x10
 MP_SCOPE_FLAG_VIPERRODATA = 0x20
 MP_SCOPE_FLAG_VIPERBSS = 0x40
@@ -1104,26 +1112,26 @@ def read_qstr(reader, segments):
 
 
 def read_obj(reader, segments):
-    obj_type = reader.read_bytes(1)
-    if obj_type == b"t":
+    obj_type = reader.read_byte()
+    if obj_type == MP_PERSISTENT_OBJ_FUN_TABLE:
         return MPFunTable()
-    elif obj_type == b"e":
+    elif obj_type == MP_PERSISTENT_OBJ_ELLIPSIS:
         return Ellipsis
     else:
         ln = reader.read_uint()
         start_pos = reader.tell()
         buf = reader.read_bytes(ln)
-        if obj_type in (b"s", b"b"):
+        if obj_type in (MP_PERSISTENT_OBJ_STR, MP_PERSISTENT_OBJ_BYTES):
             reader.read_byte()  # read and discard null terminator
-        if obj_type == b"s":
+        if obj_type == MP_PERSISTENT_OBJ_STR:
             obj = str_cons(buf, "utf8")
-        elif obj_type == b"b":
+        elif obj_type == MP_PERSISTENT_OBJ_BYTES:
             obj = buf
-        elif obj_type == b"i":
+        elif obj_type == MP_PERSISTENT_OBJ_INT:
             obj = int(str_cons(buf, "ascii"), 10)
-        elif obj_type == b"f":
+        elif obj_type == MP_PERSISTENT_OBJ_FLOAT:
             obj = float(str_cons(buf, "ascii"))
-        elif obj_type == b"c":
+        elif obj_type == MP_PERSISTENT_OBJ_COMPLEX:
             obj = complex(str_cons(buf, "ascii"))
         else:
             raise MPYReadError(reader.filename, "corrupt .mpy file")