Private identity objects are not singletons.

In the subsection describing singletons in unions, widely used object() example is described as a singleton, but it isn't one. It should be clearly stated that the problem for typing system is that this instance is not a singleton, that's why Enum is needed.

_empty = object() works with is check, because identity of _empty doesn't leak outside of module. There is nothing singleton about that. Typing spec can allow requiring _empty to be one, to allow use with is, but that should be clear it's the case.

リンクされた typing 仕様の「Support for singleton types in unions」サブセクションから始めてください。object() インスタンスは本質的にシングルトンではないことを明確にし、シングルトンの型付けに Enum が必要な理由を説明し、モジュール内でのみ同一性が検査される private な object との違いを維持してください。