Fix appends after standalone array comments

.github/workflows/tests.yml

@@ -18,6 +18,13 @@ jobs:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
         python-version: [3.9, "3.10", 3.11, 3.12, 3.13, 3.14]
+        exclude:
+          - os: ubuntu-latest
+            python-version: 3.9
+        include:
+          # setup-python has no Python 3.9 build for Ubuntu 26.04.
+          - os: ubuntu-24.04
+            python-version: 3.9
 
     steps:
       - uses: actions/checkout@v6

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Add a missing separator when appending to an array whose last value is followed by standalone comments.
+
 ## [0.15.2] - 2026-10-16
 
 ### Changed

tests/test_items.py

@@ -521,6 +521,28 @@ def test_modify_array_with_comment() -> None:
     assert doc.as_string() == "x = [\n    2\n]"
 
 
+@pytest.mark.parametrize("trailing_comma", [False, True])
+@pytest.mark.parametrize("operation", ["append", "extend", "insert"])
+@pytest.mark.parametrize("comments", ["# trailing\n", "# first\n# trailing\n"])
+def test_append_after_standalone_array_comments(
+    trailing_comma: bool, operation: str, comments: str
+) -> None:
+    doc = parse("x = [\n1" + ("," if trailing_comma else "") + "\n" + comments + "]\n")
+    array = doc["x"]
+    if operation == "append":
+        array.append(2)
+    elif operation == "extend":
+        array.extend([2, 3])
+    else:
+        array.insert(len(array) + 5, 2)
+
+    expected = [1, 2, 3] if operation == "extend" else [1, 2]
+    rendered = doc.as_string()
+    assert array == expected
+    assert parse(rendered)["x"] == expected
+    assert comments in rendered
+
+
 def test_append_to_multiline_array_with_comment() -> None:
     doc = parse(
         """\

tomlkit/items.py

@@ -1675,6 +1675,10 @@ def insert(self, pos: int, value: Any) -> None:  # type: ignore[override]
             indent = self._value[idx].indent
         if idx > 0:
             last_item = self._value[idx - 1]
+            last_value_item = last_item
+            if pos > 0 and length:
+                # Standalone comments may follow the preceding array value.
+                last_value_item = self._value[self._index_map[min(pos, length) - 1]]
             if indent is None:
                 indent = last_item.indent
             if not isinstance(last_item.value, Null) and "\n" in default_indent:
@@ -1683,14 +1687,14 @@ def insert(self, pos: int, value: Any) -> None:  # type: ignore[override]
                 comma = last_item.comma
             comma_in_indent = indent is not None and "," in indent.s
             if (
-                last_item.comma is None
-                and not isinstance(last_item.value, Null)
+                last_value_item.comma is None
+                and not isinstance(last_value_item.value, Null)
                 and not comma_in_indent
             ):
                 # Add comma to the last item to separate it from the following
                 # items, unless the new item's indent already starts with a
                 # comma (comma-first style).
-                last_item.comma = Whitespace(",")
+                last_value_item.comma = Whitespace(",")
         if indent is not None and "," in indent.s:
             # Comma-first style: the item that will follow the new one brings
             # its own leading comma, so the new item must not add a trailing