> something[::-1] = something

I was thinking this for a while, and this is the actual reason that I didn't propose this until I saw someone's comment under Raymond's tweet about 'slices are not hashable' (https://twitter.com/4skvictor/status/1330433911646785539). At least for optimization, IMHO it worth taking the shot. 

> I could definitely see code using duck-typing via slices to distinguish sequences from other iterables and mappings

That sounds a bit unorthodox. It is a very common practice to use ABCs for doing these checks but I've never seen a piece of code where they test if something is a mapping or not by throwing unhashable things in it.