diff --git a/py/builtinevex.c b/py/builtinevex.c
index 89fcc2fe1a..d9a3833ccc 100644
--- a/py/builtinevex.c
+++ b/py/builtinevex.c
@@ -78,7 +78,7 @@ STATIC mp_obj_t mp_builtin_compile(size_t n_args, const mp_obj_t *args) {
     (void)n_args;
 
     // get the source
-    mp_uint_t str_len;
+    size_t str_len;
     const char *str = mp_obj_str_get_data(args[0], &str_len);
 
     // get the filename
@@ -128,7 +128,7 @@ STATIC mp_obj_t eval_exec_helper(size_t n_args, const mp_obj_t *args, mp_parse_i
     }
     #endif
 
-    mp_uint_t str_len;
+    size_t str_len;
     const char *str = mp_obj_str_get_data(args[0], &str_len);
 
     // create the lexer
diff --git a/py/builtinimport.c b/py/builtinimport.c
index 8101d633fe..cf17d53053 100644
--- a/py/builtinimport.c
+++ b/py/builtinimport.c
@@ -111,7 +111,7 @@ STATIC mp_import_stat_t find_file(const char *file_str, uint file_len, vstr_t *d
         // go through each path looking for a directory or file
         for (mp_uint_t i = 0; i < path_num; i++) {
             vstr_reset(dest);
-            mp_uint_t p_len;
+            size_t p_len;
             const char *p = mp_obj_str_get_data(path_items[i], &p_len);
             if (p_len > 0) {
                 vstr_add_strn(dest, p, p_len);
@@ -265,7 +265,7 @@ mp_obj_t mp_builtin___import__(size_t n_args, const mp_obj_t *args) {
         }
     }
 
-    mp_uint_t mod_len;
+    size_t mod_len;
     const char *mod_str = mp_obj_str_get_data(module_name, &mod_len);
 
     if (level != 0) {
@@ -296,7 +296,7 @@ mp_obj_t mp_builtin___import__(size_t n_args, const mp_obj_t *args) {
         DEBUG_printf("\n");
 #endif
 
-        mp_uint_t this_name_l;
+        size_t this_name_l;
         const char *this_name = mp_obj_str_get_data(this_name_q, &this_name_l);
 
         const char *p = this_name + this_name_l;
diff --git a/py/modbuiltins.c b/py/modbuiltins.c
index f1d703a6d4..e8119456db 100644
--- a/py/modbuiltins.c
+++ b/py/modbuiltins.c
@@ -336,7 +336,7 @@ STATIC mp_obj_t mp_builtin_oct(mp_obj_t o_in) {
 MP_DEFINE_CONST_FUN_OBJ_1(mp_builtin_oct_obj, mp_builtin_oct);
 
 STATIC mp_obj_t mp_builtin_ord(mp_obj_t o_in) {
-    mp_uint_t len;
+    size_t len;
     const char *str = mp_obj_str_get_data(o_in, &len);
     #if MICROPY_PY_BUILTINS_STR_UNICODE
     if (MP_OBJ_IS_STR(o_in)) {
@@ -397,9 +397,9 @@ STATIC mp_obj_t mp_builtin_print(size_t n_args, const mp_obj_t *args, mp_map_t *
     mp_map_elem_t *sep_elem = mp_map_lookup(kwargs, MP_OBJ_NEW_QSTR(MP_QSTR_sep), MP_MAP_LOOKUP);
     mp_map_elem_t *end_elem = mp_map_lookup(kwargs, MP_OBJ_NEW_QSTR(MP_QSTR_end), MP_MAP_LOOKUP);
     const char *sep_data = " ";
-    mp_uint_t sep_len = 1;
+    size_t sep_len = 1;
     const char *end_data = "\n";
-    mp_uint_t end_len = 1;
+    size_t end_len = 1;
     if (sep_elem != NULL && sep_elem->value != mp_const_none) {
         sep_data = mp_obj_str_get_data(sep_elem->value, &sep_len);
     }
diff --git a/py/obj.h b/py/obj.h
index 65543b7856..580b814a8d 100644
--- a/py/obj.h
+++ b/py/obj.h
@@ -712,7 +712,7 @@ void mp_init_emergency_exception_buf(void);
 bool mp_obj_str_equal(mp_obj_t s1, mp_obj_t s2);
 qstr mp_obj_str_get_qstr(mp_obj_t self_in); // use this if you will anyway convert the string to a qstr
 const char *mp_obj_str_get_str(mp_obj_t self_in); // use this only if you need the string to be null terminated
-const char *mp_obj_str_get_data(mp_obj_t self_in, mp_uint_t *len);
+const char *mp_obj_str_get_data(mp_obj_t self_in, size_t *len);
 mp_obj_t mp_obj_str_intern(mp_obj_t str);
 void mp_str_print_quoted(const mp_print_t *print, const byte *str_data, size_t str_len, bool is_bytes);
 
diff --git a/py/objcomplex.c b/py/objcomplex.c
index 426e76e5f7..8118fb8138 100644
--- a/py/objcomplex.c
+++ b/py/objcomplex.c
@@ -80,7 +80,7 @@ STATIC mp_obj_t complex_make_new(const mp_obj_type_t *type_in, size_t n_args, si
         case 1:
             if (MP_OBJ_IS_STR(args[0])) {
                 // a string, parse it
-                mp_uint_t l;
+                size_t l;
                 const char *s = mp_obj_str_get_data(args[0], &l);
                 return mp_parse_num_decimal(s, l, true, true, NULL);
             } else if (MP_OBJ_IS_TYPE(args[0], &mp_type_complex)) {
diff --git a/py/objfloat.c b/py/objfloat.c
index 9292490a9e..6d80d6c0e2 100644
--- a/py/objfloat.c
+++ b/py/objfloat.c
@@ -89,7 +89,7 @@ STATIC mp_obj_t float_make_new(const mp_obj_type_t *type_in, size_t n_args, size
         default:
             if (MP_OBJ_IS_STR(args[0])) {
                 // a string, parse it
-                mp_uint_t l;
+                size_t l;
                 const char *s = mp_obj_str_get_data(args[0], &l);
                 return mp_parse_num_decimal(s, l, false, false, NULL);
             } else if (mp_obj_is_float(args[0])) {
diff --git a/py/objfun.c b/py/objfun.c
index 2ce33cb21c..08d031c8d8 100644
--- a/py/objfun.c
+++ b/py/objfun.c
@@ -501,7 +501,7 @@ STATIC mp_uint_t convert_obj_for_inline_asm(mp_obj_t obj) {
         return mp_obj_int_get_truncated(obj);
     } else if (MP_OBJ_IS_STR(obj)) {
         // pointer to the string (it's probably constant though!)
-        mp_uint_t l;
+        size_t l;
         return (mp_uint_t)mp_obj_str_get_data(obj, &l);
     } else {
         mp_obj_type_t *type = mp_obj_get_type(obj);
diff --git a/py/objint.c b/py/objint.c
index ce091549d4..2e8fe4f22d 100644
--- a/py/objint.c
+++ b/py/objint.c
@@ -56,7 +56,7 @@ STATIC mp_obj_t mp_obj_int_make_new(const mp_obj_type_t *type_in, size_t n_args,
                 return args[0];
             } else if (MP_OBJ_IS_STR_OR_BYTES(args[0])) {
                 // a string, parse it
-                mp_uint_t l;
+                size_t l;
                 const char *s = mp_obj_str_get_data(args[0], &l);
                 return mp_parse_num_integer(s, l, 0, NULL);
 #if MICROPY_PY_BUILTINS_FLOAT
@@ -72,7 +72,7 @@ STATIC mp_obj_t mp_obj_int_make_new(const mp_obj_type_t *type_in, size_t n_args,
         default: {
             // should be a string, parse it
             // TODO proper error checking of argument types
-            mp_uint_t l;
+            size_t l;
             const char *s = mp_obj_str_get_data(args[0], &l);
             return mp_parse_num_integer(s, l, mp_obj_get_int(args[1]), NULL);
         }
diff --git a/py/objstr.c b/py/objstr.c
index 682bd26930..60a26d45b1 100644
--- a/py/objstr.c
+++ b/py/objstr.c
@@ -513,7 +513,7 @@ mp_obj_t mp_obj_str_split(size_t n_args, const mp_obj_t *args) {
             bad_implicit_conversion(sep);
         }
 
-        mp_uint_t sep_len;
+        size_t sep_len;
         const char *sep_str = mp_obj_str_get_data(sep, &sep_len);
 
         if (sep_len == 0) {
@@ -611,7 +611,7 @@ STATIC mp_obj_t str_rsplit(size_t n_args, const mp_obj_t *args) {
     if (sep == mp_const_none) {
         mp_not_implemented("rsplit(None,n)");
     } else {
-        mp_uint_t sep_len;
+        size_t sep_len;
         const char *sep_str = mp_obj_str_get_data(sep, &sep_len);
 
         if (sep_len == 0) {
@@ -1306,12 +1306,12 @@ STATIC vstr_t mp_obj_str_format_helper(const char *str, const char *top, int *ar
             switch (type) {
                 case '\0': // no explicit format type implies 's'
                 case 's': {
-                    mp_uint_t slen;
+                    size_t slen;
                     const char *s = mp_obj_str_get_data(arg, &slen);
                     if (precision < 0) {
                         precision = slen;
                     }
-                    if (slen > (mp_uint_t)precision) {
+                    if (slen > (size_t)precision) {
                         slen = precision;
                     }
                     mp_print_strn(&print, s, slen, flags, fill, width);
@@ -1452,7 +1452,7 @@ not_enough_args:
         switch (*str) {
             case 'c':
                 if (MP_OBJ_IS_STR(arg)) {
-                    mp_uint_t slen;
+                    size_t slen;
                     const char *s = mp_obj_str_get_data(arg, &slen);
                     if (slen != 1) {
                         mp_raise_TypeError("%%c requires int or char");
@@ -2097,7 +2097,7 @@ const char *mp_obj_str_get_str(mp_obj_t self_in) {
     }
 }
 
-const char *mp_obj_str_get_data(mp_obj_t self_in, mp_uint_t *len) {
+const char *mp_obj_str_get_data(mp_obj_t self_in, size_t *len) {
     if (MP_OBJ_IS_STR_OR_BYTES(self_in)) {
         GET_STR_DATA_LEN(self_in, s, l);
         *len = l;
diff --git a/py/repl.c b/py/repl.c
index 997d800054..6d8f7cca46 100644
--- a/py/repl.c
+++ b/py/repl.c
@@ -153,7 +153,7 @@ mp_uint_t mp_repl_autocomplete(const char *str, mp_uint_t len, const mp_print_t
             mp_obj_t obj = MP_OBJ_NULL;
             for (mp_uint_t i = 0; i < dict->map.alloc; i++) {
                 if (MP_MAP_SLOT_IS_FILLED(&dict->map, i)) {
-                    mp_uint_t d_len;
+                    size_t d_len;
                     const char *d_str = mp_obj_str_get_data(dict->map.table[i].key, &d_len);
                     if (s_len == d_len && strncmp(s_start, d_str, d_len) == 0) {
                         obj = dict->map.table[i].value;
@@ -197,7 +197,7 @@ mp_uint_t mp_repl_autocomplete(const char *str, mp_uint_t len, const mp_print_t
             mp_uint_t match_len = 0;
             for (mp_uint_t i = 0; i < dict->map.alloc; i++) {
                 if (MP_MAP_SLOT_IS_FILLED(&dict->map, i)) {
-                    mp_uint_t d_len;
+                    size_t d_len;
                     const char *d_str = mp_obj_str_get_data(dict->map.table[i].key, &d_len);
                     if (s_len <= d_len && strncmp(s_start, d_str, s_len) == 0) {
                         if (match_str == NULL) {
@@ -247,7 +247,7 @@ mp_uint_t mp_repl_autocomplete(const char *str, mp_uint_t len, const mp_print_t
             int line_len = MAX_LINE_LEN; // force a newline for first word
             for (mp_uint_t i = 0; i < dict->map.alloc; i++) {
                 if (MP_MAP_SLOT_IS_FILLED(&dict->map, i)) {
-                    mp_uint_t d_len;
+                    size_t d_len;
                     const char *d_str = mp_obj_str_get_data(dict->map.table[i].key, &d_len);
                     if (s_len <= d_len && strncmp(s_start, d_str, s_len) == 0) {
                         int gap = (line_len + WORD_SLOT_LEN - 1) / WORD_SLOT_LEN * WORD_SLOT_LEN - line_len;
diff --git a/py/runtime.c b/py/runtime.c
index 73886e10f9..1df6740b2b 100644
--- a/py/runtime.c
+++ b/py/runtime.c
@@ -1330,7 +1330,7 @@ import_error:
     }
 
     mp_load_method_maybe(module, MP_QSTR___name__, dest);
-    mp_uint_t pkg_name_len;
+    size_t pkg_name_len;
     const char *pkg_name = mp_obj_str_get_data(dest[0], &pkg_name_len);
 
     const uint dot_name_len = pkg_name_len + 1 + qstr_len(name);