fix: Table header missing

Fixes #374

Signed-off-by: Frost Ming <me@frostming.com>

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [unreleased]
 
+### Fixed
+
+- Fix the `Table.is_super_table()` check for tables with dotted key as the only child. ([#374](https://github.com/python-poetry/tomlkit/issues/374))
+
 ## [0.13.0] - 2024-07-10
 
 ### Changed

tests/test_items.py

@@ -985,3 +985,11 @@ def test_no_extra_minus_sign():
     assert doc.as_string() == "a = +1.5"
     doc["a"] *= -1
     assert doc.as_string() == "a = -1.5"
+
+
+def test_serialize_table_with_dotted_key():
+    child = api.table()
+    child.add(api.key(("b", "c")), 1)
+    parent = api.table()
+    parent.add("a", child)
+    assert parent.as_string() == "[a]\nb.c = 1\n"

tomlkit/items.py

@@ -1624,8 +1624,18 @@ def is_super_table(self) -> bool:
         # If the table has only one child and that child is a table, then it is a super table.
         if len(self) != 1:
             return False
-        only_child = next(iter(self.values()))
-        return isinstance(only_child, (Table, AoT))
+        k, only_child = next(iter(self.items()))
+        if not isinstance(k, Key):
+            k = SingleKey(k)
+        index = self.value._map[k]
+        if isinstance(index, tuple):
+            return False
+        real_key = self.value.body[index][0]
+        return (
+            isinstance(only_child, (Table, AoT))
+            and real_key is not None
+            and not real_key.is_dotted()
+        )
 
     def as_string(self) -> str:
         return self._value.as_string()