Fix TypeError when a keypath sets a string subkey through a list value

benedict/dicts/keylist/keylist_util.py

@@ -28,7 +28,11 @@ def _get_item_key_and_value(item: Any, key: Any) -> tuple[Any, Any]:
 def _get_or_new_item_value(item: Any, key: Any, subkey: Any) -> Any:
     try:
         _, value = _get_item_key_and_value(item, key)
-        if not type_util.is_dict_or_list_or_tuple(value):
+        # a list can only hold an integer-index subkey; anything else needs a dict
+        keeps_subkey = type_util.is_dict(value) or (
+            type_util.is_list_or_tuple(value) and _get_index(subkey) is not None
+        )
+        if not keeps_subkey:
             raise TypeError
     except (IndexError, KeyError, TypeError):
         value = _new_item_value(subkey)

tests/dicts/keypath/test_keypath_dict.py

@@ -499,6 +499,23 @@ def test_setitem_override_existing_item(self) -> None:
         r = {"a": {"b": {"c": {"d": 3}}}}
         self.assertEqual(b, r)
 
+    def test_setitem_string_subkey_overrides_list_value(self) -> None:
+        # a string subkey cannot index a list, so it overrides the list like a scalar
+        b = KeypathDict({"a": [1, 2]})
+        b["a.x"] = 1
+        self.assertEqual(b, {"a": {"x": 1}})
+        b = KeypathDict({"a": [1, 2]})
+        b["a.x.y"] = 1
+        self.assertEqual(b, {"a": {"x": {"y": 1}}})
+        # a list-of-scalar element is likewise overridden by a string subkey
+        b = KeypathDict({"a": [5]})
+        b["a[0].x"] = 3
+        self.assertEqual(b, {"a": [{"x": 3}]})
+        # a valid integer index keeps the list
+        b = KeypathDict({"a": [1, 2]})
+        b["a[0]"] = 9
+        self.assertEqual(b, {"a": [9, 2]})
+
     def test_setitem_with_keys_list(self) -> None:
         d: dict[str, Any] = {
             "a": {