On Fri, Sep 25, 2020, 5:49 PM Steven D'Aprano 
Since both None and () are likely to be legitimate indexes, and NotImplemented is less likely to be such, I think this supports using NotImplemented.

I think your arguments for NotImplemented vs None or () are solid. But I'm having trouble crossing my eyes in just the way that will make those words fit the purpose.

What about instead subclassing NotImplemented as maybe NoIndex? That would make sense in reading, and have the same favorable characteristics. Moreover, being a brand new class, we can be certain no one is already using it.