py/runtime: Properly handle passing user mappings to ** keyword args.

py/runtime.c

@@ -714,13 +714,18 @@ void mp_call_prepare_args_n_kw_var(bool have_self, mp_uint_t n_args_n_kw, const
             }
         }
     } else {
-        // generic mapping
+        // generic mapping:
-        // TODO is calling 'items' on the mapping the correct thing to do here?
+        // - call keys() to get an iterable of all keys in the mapping
-        mp_obj_t dest[2];
+        // - call __getitem__ for each key to get the corresponding value
-        mp_load_method(kw_dict, MP_QSTR_items, dest);
+
+        // get the keys iterable
+        mp_obj_t dest[3];
+        mp_load_method(kw_dict, MP_QSTR_keys, dest);
         mp_obj_t iterable = mp_getiter(mp_call_method_n_kw(0, 0, dest));
-        mp_obj_t item;
+
-        while ((item = mp_iternext(iterable)) != MP_OBJ_STOP_ITERATION) {
+        mp_obj_t key;
+        while ((key = mp_iternext(iterable)) != MP_OBJ_STOP_ITERATION) {
+            // expand size of args array if needed
             if (args2_len + 1 >= args2_alloc) {
                 uint new_alloc = args2_alloc * 2;
                 if (new_alloc < 4) {
@@ -729,15 +734,20 @@ void mp_call_prepare_args_n_kw_var(bool have_self, mp_uint_t n_args_n_kw, const
                 args2 = m_renew(mp_obj_t, args2, args2_alloc, new_alloc);
                 args2_alloc = new_alloc;
             }
-            mp_obj_t *items;
+
-            mp_obj_get_array_fixed_n(item, 2, &items);
             // the key must be a qstr, so intern it if it's a string
-            mp_obj_t key = items[0];
             if (MP_OBJ_IS_TYPE(key, &mp_type_str)) {
                 key = mp_obj_str_intern(key);
             }
+
+            // get the value corresponding to the key
+            mp_load_method(kw_dict, MP_QSTR___getitem__, dest);
+            dest[2] = key;
+            mp_obj_t value = mp_call_method_n_kw(1, 0, dest);
+
+            // store the key/value pair in the argument array
             args2[args2_len++] = key;
-            args2[args2_len++] = items[1];
+            args2[args2_len++] = value;
         }
     }
 

tests/basics/fun_calldblstar3.py

@@ -0,0 +1,16 @@
+# test passing a user-defined mapping as the argument to **
+
+def foo(**kw):
+    print(sorted(kw.items()))
+
+class Mapping:
+    def keys(self):
+        return ['a', 'b', 'c']
+
+    def __getitem__(self, key):
+        if key == 'a':
+            return 1
+        else:
+            return 2
+
+foo(**Mapping())