[Python-ideas] lazy list

[Steven D'Aprano]

On Thu, May 14, 2015 at 03:52:55PM +0100, Alexander Atkins wrote:
> Hi, I'm new to this mailing list.
> 
> I needed a lazy list implementation for something, so I created one.  I was
> a little bit surprised to find that there wasn't one in the *itertools*
> module 

Why? It's not really an iterator tool. The things in itertools are tools 
for processing streams, not containers.


> and it seemed like quite a basic thing to me, as someone who has
> used Haskell before, so I thought probably I should share it.  I'm
> wondering whether something like this should be part of the standard
> library?
> 
> A fuller explanation is in the README, which is here:
> https://github.com/jadatkins/python-lazylist
> The gist of it is that it allows you to index into a generator.  Previously
> evaluated elements are remembered, so foo[5] returns the same thing each
> time, and you can later call foo[4] and get the previous element.  There
> are many uses for such a thing, but if you're not expecting it in the
> language, you might not necessarily think of them.

What sort of uses? Can you give some examples?

I'm having trouble thinking of a situation where I might use something 
like that. If I want random access, I'd use a list, or a computed 
sequence like (x)range. I don't think I would want something which acts 
like a generator but quietly holds onto all the items it has seen 
before, whether I need them or not.


> Warning: it may contain bugs, especially the stuff to do with slicing,
> which is not really what it's for.

A slice is just a subsequence of indexed values. If you can index it, 
you should be able to slice it.

assert spam[start:end:step] == [spam[i] for i in range(start, end, step)]



-- 
Steve