From 75de0cbce27330a8e2fbdcfd23a8bf5f25b842b5 Mon Sep 17 00:00:00 2001
From: fei <204683769+feiiiiii5@users.noreply.github.com>
Date: Sun, 25 Oct 2026 11:17:12 +0800
Subject: [PATCH] Carry _table_keys through Container.__copy__

The copy dropped the table-key bookkeeping that __setstate__ preserves
on pickle, so the merge heuristic in _handle_merge crashed with
IndexError when appending a table whose name already existed in the
copy.
---
 CHANGELOG.md                |  4 ++++
 tests/test_toml_document.py | 16 ++++++++++++++++
 tomlkit/container.py        |  1 +
 3 files changed, 21 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index f414f212..a6608988 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- `Container.__copy__` now carries over `_table_keys`, so appending a table to a copied container no longer raises `IndexError` when a table with the same name already exists.
+
 ## [0.15.2] - 2026-10-16
 
 ### Changed
diff --git a/tests/test_toml_document.py b/tests/test_toml_document.py
index 84b114b9..f5f3204a 100644
--- a/tests/test_toml_document.py
+++ b/tests/test_toml_document.py
@@ -486,6 +486,22 @@ def test_toml_document_can_be_copied() -> None:
     assert json.loads(json.dumps(doc)) == {"foo": {"bar": 1}}
 
 
+def test_toml_document_copy_keeps_table_keys() -> None:
+    doc = parse("[a.b]\nx = 1\n")
+
+    doc_copy = doc.copy()
+    doc_copy.append("a", tomlkit.table(True).append("d", 3))
+
+    # The copy accepts the append instead of raising IndexError, and the
+    # merged rendering stays parseable with both tables intact. (The copy is
+    # shallow, so table objects are shared with the original — only the
+    # container's own key bookkeeping is what __copy__ must duplicate.)
+    reloaded = parse(doc_copy.as_string())
+    assert reloaded["a"]["b"]["x"] == 1
+    assert reloaded["a"]["d"] == 3
+    assert doc._table_keys == ["a"]
+
+
 def test_getting_inline_table_is_still_an_inline_table() -> None:
     content = """\
 [tool.poetry]
diff --git a/tomlkit/container.py b/tomlkit/container.py
index 9265c740..63b9289e 100644
--- a/tomlkit/container.py
+++ b/tomlkit/container.py
@@ -1103,6 +1103,7 @@ def __copy__(self) -> Self:
         c._body += self.body
         c._map.update(self._map)
         c._out_of_order_keys |= self._out_of_order_keys
+        c._table_keys += self._table_keys
 
         return c