I'll repeat my advice to take this to the python-ideas mailing list. You'll get real reasons why this shouldn't (or should) be included in __builtins__. As fa

I'll repeat my advice to take this to the python-ideas mailing list. You'll get real reasons why this shouldn't (or should) be included in __builtins__. As far as two libraries agreeing on using the same instance, one can define it the way you've done with `NotSet = object()`, and they can both use it just like they share any other named object (classes, functions, constants, sentinels). Better yet, it can carry a more meaningful name when "NotSet" doesn't really describe how they're using the sentinel.