From c9ca1b2856e49e5a1161b46710476b2fbd9b97b9 Mon Sep 17 00:00:00 2001
From: santhreal <64453045+santhreal@users.noreply.github.com>
Date: Fri, 17 Jul 2026 23:44:06 -0700
Subject: [PATCH] Fix IndexError on trailing dotted keys with default_box

box_dots paths that end with a trailing dot leave an empty children
segment; children[0] then raised IndexError while creating the
intermediate container. Use children[:1] so empty children create a Box
and set the empty-string key, matching existing parent paths.
---
 box/box.py       |  3 ++-
 box/box_list.py  |  3 ++-
 test/test_box.py | 11 +++++++++++
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/box/box.py b/box/box.py
index 8252643..105be19 100644
--- a/box/box.py
+++ b/box/box.py
@@ -669,7 +669,8 @@ def __setitem__(self, key, value):
                 if hasattr(self[first_item], "__setitem__"):
                     return self[first_item].__setitem__(children, value)
             elif self._box_config["default_box"]:
-                if children[0] == "[":
+                # children may be "" for trailing dots (e.g. "a."); treat as Box + empty key.
+                if children[:1] == "[":
                     super().__setitem__(first_item, box.BoxList(**self.__box_config(extra_namespace=first_item)))
                 else:
                     super().__setitem__(
diff --git a/box/box_list.py b/box/box_list.py
index 72609ff..adc4ed4 100644
--- a/box/box_list.py
+++ b/box/box_list.py
@@ -106,7 +106,8 @@ def __setitem__(self, key, value):
                 return super().__setitem__(pos, value)
             children = key[len(list_pos.group()) :].lstrip(".")
             if self.box_options.get("default_box"):
-                if children[0] == "[":
+                # children may be "" after a trailing dot on a list path (e.g. "[0].").
+                if children[:1] == "[":
                     super().__setitem__(pos, box.BoxList(**self.box_options))
                 else:
                     super().__setitem__(pos, self.box_options.get("box_class")(**self.box_options))
diff --git a/test/test_box.py b/test/test_box.py
index e5c56e0..685aa74 100644
--- a/test/test_box.py
+++ b/test/test_box.py
@@ -344,6 +344,17 @@ def test_set_default_box_dots(self):
         assert t == Box(a=1, b=[1, 2])
         assert t.setdefault("c", [{"d": 2}]) == BoxList([{"d": 2}])
 
+    def test_box_dots_trailing_dot_default_box(self):
+        # Trailing dots leave an empty children segment; must set empty-string key, not IndexError.
+        b = Box(default_box=True, box_dots=True)
+        b["a."] = 1
+        assert b["a"][""] == 1
+        b2 = Box(default_box=True, box_dots=True)
+        b2["a[0]."] = 2
+        assert b2.a[0][""] == 2
+        b3 = Box(default_box=True, box_dots=True)
+        assert isinstance(b3[".."], Box)
+
     def test_from_json_file(self):
         bx = Box.from_json(filename=data_json_file)
         assert isinstance(bx, Box)