fix: preserve existing keys on dotted list-element writes

box/box_list.py

@@ -106,9 +106,15 @@ def __setitem__(self, key, value):
                 return super().__setitem__(pos, value)
             children = key[len(list_pos.group()) :].lstrip(".")
             if self.box_options.get("default_box"):
+                # Only replace the element at ``pos`` with a fresh container when it is
+                # not already the right one; otherwise a second dotted write into an
+                # existing element (e.g. ``a[0].y`` after ``a[0].x``) would discard the
+                # data already stored there. Mirrors the guard in Box.__setitem__.
+                current = super().__getitem__(pos)
                 if children[0] == "[":
-                    super().__setitem__(pos, box.BoxList(**self.box_options))
-                else:
+                    if not isinstance(current, box.BoxList):
+                        super().__setitem__(pos, box.BoxList(**self.box_options))
+                elif not isinstance(current, box.Box):
                     super().__setitem__(pos, self.box_options.get("box_class")(**self.box_options))
             return super().__getitem__(pos).__setitem__(children, value)
         super().__setitem__(key, value)

test/test_box_list.py

@@ -241,6 +241,24 @@ def test_box_list_default_dots(self):
         box_3["a.b[0]"] = 42
         assert box_3.a.b[0] == 42
 
+        # A second dotted write into an existing list element must not discard
+        # the keys already written there (regression).
+        box_4 = Box(default_box=True, box_dots=True)
+        box_4["a[0].x"] = 1
+        box_4["a[0].y"] = 2
+        assert box_4.a[0].to_dict() == {"x": 1, "y": 2}
+
+        box_5 = Box(default_box=True, box_dots=True)
+        box_5["a[0][0].x"] = 1
+        box_5["a[0][1].y"] = 2
+        assert box_5.a[0].to_list() == [{"x": 1}, {"y": 2}]
+
+        # A scalar already at the position is still overwritten.
+        box_6 = Box(default_box=True, box_dots=True)
+        box_6["a[0]"] = 5
+        box_6["a[0].x"] = 1
+        assert box_6.a[0].to_dict() == {"x": 1}
+
     def test_box_config_propagate(self):
         structure = Box(a=[Box(default_box=False)], default_box=True, box_inherent_settings=True)
         assert structure._box_config["default_box"] is True