Allow table setdefault calls without an explicit default

CHANGELOG.md

@@ -9,6 +9,8 @@
 
 ### Fixed
 
+- Allow `Table.setdefault()` and `InlineTable.setdefault()` to retrieve existing values without supplying a default, matching document and mapping behavior.
+
 - Fix `item()` and `dumps()` writing a `datetime` whose UTC offset is not a whole number of minutes (e.g. the historical LMT offsets `zoneinfo` gives, such as `-00:44:30`) as invalid TOML like `1900-01-01T12:00:00Z:30`, which no parser accepts. Such a value is now written as the same instant in UTC; this also covers `DateTime.replace()` and `astimezone()`.
 - Allow `DateTime.astimezone()` to use the local timezone when no timezone argument is supplied, matching `datetime.datetime`.
 - Arithmetic and `replace()` results for `Integer`, `Float`, `String`, `DateTime`, `Date` and `Time` no longer share the original item's `Trivia`, so changing the result's comment or indentation no longer rewrites the item still stored in the document.

tests/test_items.py

@@ -1901,3 +1901,35 @@ def test_out_of_order_table_proxy_membership() -> None:
         _ = 123 in table
     # membership must not resolve values or mutate the document
     assert doc.as_string() == content
+
+
+@pytest.mark.parametrize("inline", [False, True])
+@pytest.mark.parametrize("literal, expected", [("0", 0), ("false", False), ('""', "")])
+def test_table_setdefault_existing_key_without_default(
+    inline: bool, literal: str, expected: int | bool | str
+) -> None:
+    if inline:
+        source = f"settings = {{ value = {literal} }} # preserve formatting\n"
+    else:
+        source = f"[settings] # preserve formatting\nvalue = {literal}\n"
+    document = parse(source)
+    settings = document["settings"]
+    assert isinstance(settings, (Table, InlineTable))
+
+    assert settings.setdefault("value") == expected
+    assert api.dumps(document) == source
+
+
+@pytest.mark.parametrize("inline", [False, True])
+def test_table_setdefault_missing_key_without_default_preserves_document(
+    inline: bool,
+) -> None:
+    from tomlkit.exceptions import ConvertError
+
+    source = "settings = { value = 1 }\n" if inline else "[settings]\nvalue = 1\n"
+    document = parse(source)
+    settings = document["settings"]
+    assert isinstance(settings, (Table, InlineTable))
+    with pytest.raises(ConvertError):
+        settings.setdefault("missing")
+    assert api.dumps(document) == source

tomlkit/items.py

@@ -1909,7 +1909,7 @@ def remove(self: AT, key: Key | str) -> AT:
     def item(self, key: Key | str) -> Item | OutOfOrderTableProxy:
         return self._value.item(key)
 
-    def setdefault(self, key: Key | str, default: Any) -> Any:  # type: ignore[override]
+    def setdefault(self, key: Key | str, default: Any = None) -> Any:
         super().setdefault(key, default)
         return self[key]