From 22d94db8bee027319cc56592d93f7755f4b3a267 Mon Sep 17 00:00:00 2001
From: Vincent Gao <gaobing1230@gmail.com>
Date: Sat, 20 Jun 2026 15:02:47 +0200
Subject: [PATCH] fix: preserve existing keys on dotted list writes

BoxList.__setitem__ unconditionally replaced the element at the target
index with a fresh empty container before recursing, so a second dotted
write into an existing element (a[0].y after a[0].x) discarded the keys
already stored there. Only replace the element when it is not already the
right container type, matching the guard in Box.__setitem__.
---
 box/box_list.py       | 10 ++++++++--
 test/test_box_list.py | 18 ++++++++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/box/box_list.py b/box/box_list.py
index 72609ff..819c205 100644
--- a/box/box_list.py
+++ b/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)
diff --git a/test/test_box_list.py b/test/test_box_list.py
index 9cd90e0..0fd7de8 100644
--- a/test/test_box_list.py
+++ b/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