py/objdict: Fix fromkeys to return the right type.

Fixes #8173

It looks like a small fix, and mostly independent of upstream
plans around micropython#6173

I also filed an issue upstream
micropython#12011

Author: kbsriram

py/objdict.c

@@ -110,15 +110,22 @@ STATIC void dict_print(const mp_print_t *print, mp_obj_t self_in, mp_print_kind_
     }
 }
 
-mp_obj_t mp_obj_dict_make_new(const mp_obj_type_t *type, size_t n_args, size_t n_kw, const mp_obj_t *args) {
-    mp_obj_t dict_out = mp_obj_new_dict(0);
+// This is a helper function to initialize an empty, but typed dictionary with
+// a given number of slots.
+STATIC mp_obj_t dict_new_typed(const mp_obj_type_t *type, const size_t n) {
+    mp_obj_t dict_out = mp_obj_new_dict(n);
     mp_obj_dict_t *dict = MP_OBJ_TO_PTR(dict_out);
     dict->base.type = type;
     #if MICROPY_PY_COLLECTIONS_ORDEREDDICT
     if (type == &mp_type_ordereddict) {
         dict->map.is_ordered = 1;
     }
     #endif
+    return dict_out;
+}
+
+mp_obj_t mp_obj_dict_make_new(const mp_obj_type_t *type, size_t n_args, size_t n_kw, const mp_obj_t *args) {
+    mp_obj_t dict_out = dict_new_typed(type, 0);
     if (n_args > 0 || n_kw > 0) {
         mp_obj_t args2[2] = {dict_out, args[0]}; // args[0] is always valid, even if it's not a positional arg
         mp_map_t kwargs;
@@ -264,6 +271,7 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_1(dict_copy_obj, mp_obj_dict_copy);
 #if MICROPY_PY_BUILTINS_DICT_FROMKEYS
 // this is a classmethod
 STATIC mp_obj_t dict_fromkeys(size_t n_args, const mp_obj_t *args) {
+    mp_obj_type_t *type = MP_OBJ_TO_PTR(args[0]);
     mp_obj_t iter = mp_getiter(args[1], NULL);
     mp_obj_t value = mp_const_none;
     mp_obj_t next = MP_OBJ_NULL;
@@ -277,9 +285,9 @@ STATIC mp_obj_t dict_fromkeys(size_t n_args, const mp_obj_t *args) {
     mp_obj_t len = mp_obj_len_maybe(args[1]);
     if (len == MP_OBJ_NULL) {
         /* object's type doesn't have a __len__ slot */
-        self_out = mp_obj_new_dict(0);
+        self_out = dict_new_typed(type, 0);
     } else {
-        self_out = mp_obj_new_dict(MP_OBJ_SMALL_INT_VALUE(len));
+        self_out = dict_new_typed(type, MP_OBJ_SMALL_INT_VALUE(len));
     }
 
     mp_obj_dict_t *self = MP_OBJ_TO_PTR(self_out);

tests/basics/ordereddict1.py

@@ -41,3 +41,14 @@
     d.popitem()
 except:
     print('empty')
+
+# fromkeys returns the correct type and order
+d = dict.fromkeys('abcdefghij')
+print(type(d) == dict)
+d = OrderedDict.fromkeys('abcdefghij')
+print(type(d) == OrderedDict)
+print(''.join(d))
+
+# fromkey handles ordering with duplicates
+d = OrderedDict.fromkeys('abcdefghijjihgfedcba')
+print(''.join(d))