[Python-Dev] Re: [Python-ideas] Re: reversed enumerate

Before jumping in: In many cases, when you want to reverse an enumerate, it’s small and fixed-sized, so there’s a trivial way to do this: Just store the enumerate iterator in a tuple, and tuples are reversible. for idx, value in reversed(tuple(enumerate(stuff))): But of course there are some cases where this isn’t appropriate, like enumerating a fixed-size but huge input.

On Apr 1, 2020, at 19:23, Steven D'Aprano wrote:

[Ilya]

...

I needed reversed(enumerate(x: list)) in my code, and have discovered that it wound't work. This is disappointing because operation is well defined.

It isn't really well-defined, since enumerate can operate on infinite iterators, and you cannot reverse an infinite stream.

...

However, having said that, I think that your idea is not unreasonable. `enumerate(it)` in the most general case isn't reversable, but if `it` is reversable and sized, there's no reason why `enumerate(it)` shouldn't be too.

Agreed—but this is just a small piece of a much wider issue. Today, enumerate is always an Iterator. It’s never reversible. But it’s also not sized, or subscriptable, or in-testable, even if you give it inputs that are. And it’s not just enumerate—the same is true for map, filter, zip, itertools.islice, itertools.dropwhile, etc. There’s no reason these things couldn’t all be views, just like the existing dict views (and other things like memoryview and third-party things like numpy array slices). In fact, they already are in Swift, and will be in C++20.

My personal opinion is that this is a fairly obvious and straightforward enhancement, one which (hopefully!) shouldn't require much, if any, debate. I don't think we need a new class for this, I think enhancing enumerate to be reversable if its underlying iterator is reversable makes good sense.

Actually, that doesn’t work—it has to be Sized as well. More generally, it’s rarely _quite_ as simple as just “views support the same operations as the things they view”. An enumerate can be a Sequence if its input is, but a filter can’t. A map with multiple inputs isn’t Reversible unless they’re all not just Reversible but Sized, although a map with only one input doesn’t need it to be Sized. And so on. But none of these things are hard, it’s just a bunch of work to go through all the input types for all the view types and write up the rules. (Or steal them from another language or library that already did that work…)

But if you can show some concrete use-cases, especially one or two from the standard library, that would help your case. Or some other languages which offer this functionality as standard.

Agreed. I don’t think we need to wait until someone designs and writes a complete viewtools library and submits it for stdlib inclusion before we can consider adding just one extension to one iterator. But I do think we want to add the one(s) that are most useful if any, not just whichever ones people think of first. I’ve personally wanted to reverse a map or a filter more often than an enumerate, but examples would easily convince me that that’s just me, and reversing enumerate is more needed.

One potentially serious question: what should `enumerate.__reversed__` do when given a starting value?

reversed(enumerate('abc', 1))

I don’t think this is a problem. When you reversed(tuple(enumerate('abc', 1))) today, what do you get? You presumably don’t even need to look that up or try it out. It would be pretty confusing if it were different without the tuple.