How to get part of a list in Python without creating a new list?

Question

I have a list a = [1,2,3,4,5]. And I have a function, say, Func(x). I know that if I do Func(a) then the reference of a will be passed into Func(x). And if I do Func(a[:]), a new list will be created and passed into Func(x).

So my question is: Is it possible to only pass the first three elements into Func(x) by reference, like Func(a) (I don't want to pass the whole list a into function due to certain reason)? If I do Func(a[:4]) a new list will be created and that's what I want to avoid.

The only way I can think about is to pass a and the indexes into Func(x), like Func(a, start, end).

Why do you want to avoid creating a new list? Is there any specific reason? — thefourtheye, Feb 22 '15 at 03:38
This is a recursive function. If I create a new list every time, the stack will overflow. — stanleyli, Feb 22 '15 at 03:40
But I wouldn't worry about that, unless you get memory exception. — thefourtheye, Feb 22 '15 at 03:43
Basic slicing on numpy arrays generate views that meet your requirements, but there is nothing in the language or standard library — AChampion, Feb 22 '15 at 03:48
I would add that it's not _too_ hard to create your own [immutable list view](http://stackoverflow.com/a/3485490/577088) class. — senderle, Feb 22 '15 at 03:57

score 1 · Accepted Answer · answered Feb 22 '15 at 03:44

There is no way to create a 'window' on a list, no.

Your only options are to create a slice, or to pass in start and end indices to the function and have the function honour those.

The latter is what the bisect module functions do for example; each function takes a lo and hi parameter that default to 0 and len(list) respectively:

def func(lst, lo=0, hi=None):
    if hi is None:
        hi = len(lst)

score 0 · Answer 2 · answered Feb 22 '15 at 03:44

Why not create a second argument so that Func (a) becomes Func (a, n) where a is the reference to your array and n is the position in the array to which you want to evaluate to?

Something like:

Func (a, 2)

With that example the first three elements are evaluated.

How to get part of a list in Python without creating a new list?

2 Answers2