(Hoping I figured out how to reply in the thread now... ) Yes, sorry for the cryptic message but it looks like you got it right. The example isn't impressive but just like Steven is saying the value comes in larger applications. In a web application for instance only the parsing layer will have to deal with the runtime validation of the type. The business logic and database layers can use static type checking to make sure only the narrowed value is allowed. So it's kind of like a NewType plus an "instantiation predicate" (I'm failing to find the correct term, nothing is actually instantiated). I thought it's relevant here since I imagine a NotType would also have to rely a lot on runtime narrowing. If a downstream API takes Sequence & Not[str] and we have a Sequence available, we'd have to narrow it with something like assert not isinstance(..., str).