Copying element from one list to another in python

Question

Let's say that I created two different lists:

listA = [0, 0, 0]
listB = [1, 1, 1]

I want to make the an element in listB refer to the same object as an element in listA. So that the element in both lists change together.

listB[2] = listA[0]
>>listB[2]
>>0

listB[2] = 2
>>listA[0]
>>2

However doing the above just passes the value of the elements, so the lists are still referring to individual objects. Is there a method or a different data structure that could give me the desired effect?

Coincidently, This is pretty much what already happens when you say `listA = listB` or visa versa. See [here](http://stackoverflow.com/questions/2612802/how-to-clone-or-copy-a-list) for details. — Christian Dean, Dec 26 '16 at 21:57
Just make `listA = listB`, two references to *one list*. You can't share references between ordinary list objects otherwise, no. — Martijn Pieters, Dec 26 '16 at 22:00
if you are working with objects you can assume that, but not for integers. — motia, Dec 26 '16 at 22:00
@MartijnPieters I think he only wants _one_ element in each list to correspond, not all of them. — Christian Dean, Dec 26 '16 at 22:00
@leaf: I know, but you can't do that, so there are no other options. — Martijn Pieters, Dec 26 '16 at 22:00
if the element is an immutable then changing it won't update it in other list. — tihom, Dec 26 '16 at 22:02

score 1 · Accepted Answer · answered Dec 26 '16 at 22:04

Here's a simple idea to get you started, and also help you understand better what's going on:

>>> listA = [[0], [0], [0]]
>>> listB = [[1], [1], [1]]
>>> listA[0] = listB[0]
>>> listA
[[1], [0], [0]]
>>> listB[0][0] = 2
>>> listB
[[2], [1], [1]]
>>> listA
[[2], [0], [0]]

But be careful:

>>> listB[0] = [3]
>>> listB
[[3], [1], [1]]
>>> listA
[[2], [0], [0]]

You could write a class that makes this invisible so it looks more like a list and is easier to use without making mistakes.

score 1 · Answer 2 · answered Dec 26 '16 at 22:05

You can't do that. You can either make listA and listB themselves be the same object, in which case all their indices will reflect any changes made to either, or you can make them separate objects, in which case none of their indices will reflect changes. You can't make two lists that are "joined at the hip", sharing only some of their index references.

Suppose that the value of listB[2] is some object called obj. When you do something like listB[2] = 2, you aren't changing obj; you are changing listB (by making its index 2 point to some other object). So you can't make listA[2] "reflect" listB[2] unless listA and listB are the same object. You can certainly make listA[2] and listB[2] refer to the same object, but that won't affect what happens when you do listB[2] = 2, because that operation won't affect that shared object at all.

Copying element from one list to another in python

2 Answers2