[Python-ideas] Question about `list.insert`

[Ram Rachum]

Thanks for your answers everyone! It was interesting for me.
On Feb 7, 2014 12:37 PM, "spir" <denis.spir at gmail.com> wrote:

> On 02/07/2014 11:14 AM, Ram Rachum wrote:
>
>> Okay, now that I've posted on python-list, and among the 80% of joke posts
>> in my thread, there was a 20% of posts that assured me that Python's list
>> has no such behavior and that I should use deque, I now ask on
>> Python-ideas: Is there a good reason why `list.insert(whatever, 0)`
>> doesn't
>> opportunistically try to allocate more space at the left side of the list,
>> so as to save the expensive operation of moving all the items? I'm not
>> saying it should reserve space there, just check if that space is
>> available, and if so use it. Is there any reason why not?
>>
>
> It is not possible, because it would change the pointer (to the actual
> "stock" of items), that must point to the first item, and be also known by
> the memory allocator as such. In principle, one can have a "fake" pointer
> with some part of the data actually positioned *before* the address it
> points to. This is in fact the way "Pascal arrays" (and strings) are
> commonly implemented, with their sizes prefixed to the first item. Bit this
> introduces some complication (eg one must recompute the original pointer
> for dealloc) and must be designed right from the start at the lowest level.
>
> The only practicle way would be to systematically reserve memory space
> before the start item [*], for such cases. It is not worth it for a very
> specific operation like like.insert_anywhere (even less
> list.insert_at_start), which is not (in my view) the proper point of lists
> (arrays, in fact). We should use proper collections whenever we need
> inserting (and removing) at arbitrary locations. 99% lists do not need
> that. As I see it, lists are for, in order of importance:
>
> * put new item (at the end) (also when used as a stack, =push)
> * iterate (in order)
>
> * read item (anywhere)
> * change item (anywhere)
>
> * take or remove last item (only when used only as a stack, =pop)
>
> The deal may possibly be different if arrays were python's only collection
> (like link lists in lisp or tables in lua).
>
> d
>
> [*] I sometimes wish strings would reserve place for exactly one more code
> at the _end_, for cases when one must ensure a string (often read from
> file) terminates with a newline ;-)
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