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.

从链接的 typing 规范中的“Support for singleton types in unions”小节开始。明确说明 object() 实例本身并不是 singleton，解释为什么 singleton 类型标注需要 Enum，并保留它与仅在其模块内检查身份的私有 object 之间的区别。