Fix: reject single keys that aren't strings (#416)

* Fix: accept ints as keys (interpret them as strings)

* fix: throw type error instead of casting to str

Author: savente93

tests/test_toml_document.py

@@ -15,6 +15,7 @@
 from tomlkit._utils import _utc
 from tomlkit.api import document
 from tomlkit.exceptions import NonExistentKey
+from tomlkit.toml_document import TOMLDocument
 
 
 def test_document_is_a_dict(example):
@@ -1187,3 +1188,8 @@ def test_overwrite_out_of_order_table_key():
 
 """
     )
+
+
+def test_set_default_int():
+    with pytest.raises(TypeError):
+        TOMLDocument().setdefault(4, 5)

tomlkit/items.py

@@ -380,6 +380,9 @@ def __init__(
         sep: str | None = None,
         original: str | None = None,
     ) -> None:
+        if not isinstance(k, str):
+            raise TypeError("Keys must be strings")
+
         if t is None:
             if not k or any(
                 c not in string.ascii_letters + string.digits + "-" + "_" for c in k