MAINT: Stop using non-tuple indices internally - #10618

I think it is good if our code serves in part as showing best practice; in that respect, I'd prefer to go for tuples directly where possible rather than from a list. On the other hand, there is only so much time in a day, so I'm also happy with going with this as is.

@mhvk If you are happy with it, feel free to merge ;)

@mhvk: My thinking is that this patch is an externally visible change for subclasses, (should maybe be ENH?), but what you propose is just internal cleanup - I'd like to keep the two separate, if possible.

Additionally, I think we can make some big cleanups to gradient now that we're not getting any efficiency saving by reusing the same lists - something that would do better as a standalone MAINT pr so as not to produce a large diff to bisect in the distant future

Going back through issues, I saw this again. It really is fine to proceed in steps, so I'm happy to merge. Just in case: @eric-wieser - no change needed here, correct?

What bothers me is how bloated the code has become. Is there a plan that makes it prettier?

@mhvk: Yep, I'm happy with this as is

@charris: I plan to make gradient prettier, but there's not much we can do with the rest.

@charris - I think the bloatiness is not necessary if one were to start from defining things as tuples instead of lists and not explicitly rely on mutability. But that is a much larger effort and this PR does serve the purpose of ensuring we do not rely on something we hope to deprecate -- as ideally in the end lists always are interpreted as are arrays of indices.

@mhvk Go ahead in put this in if you think it ready. I can't think of any easy way to improve on things.

@mhvk: This is not just about working towards deprecating it - it also makes it easier for subclasses to overload getitem, and not have to implement list → tuple conversion heuristics to make the functions touched here work