How to program python variable to point different reference location but same value

Question

After thinking a while this concept came to my mind which I was gone through few days ago.

In Python if I did x=y then automatically x and y will point same object reference location but is there any way I can manage to change y reference location but with same value if x.

for example:

x=100
y=x

now x and y share same object reference location of value 100 but I want to have a different location for y.

Edit: What I am trying to do

l1=[1,2,3,4]
l2=l1
i=0
j=len(l1)-1

while j >= 0 :
    l1[i]=l2[j]
    i=i+1
    j=j-1

    print("L1=",l1,"L2",l2,"i=",i,"j=",j)

What I am getting as outout

L1= [4, 2, 3, 4] L2 [4, 2, 3, 4] i= 1 j= 2
L1= [4, 3, 3, 4] L2 [4, 3, 3, 4] i= 2 j= 1
L1= [4, 3, 3, 4] L2 [4, 3, 3, 4] i= 3 j= 0
L1= [4, 3, 3, 4] L2 [4, 3, 3, 4] i= 4 j= -1

Thank you.

You can't reliably achieve that. The value might e.g. be interned, with the system trying to avoid creating more than one instance. What are you trying to achieve with that? — Ulrich Eckhardt, Mar 27 '16 at 06:25
@UlrichEckhardt I am trying to achieve list swapping with iteration and I am failing then. — Raja G, Mar 27 '16 at 06:27
Still unclear. Give an example. In other words: You surely don't need this, there must be something else you're doing wrong. — Ulrich Eckhardt, Mar 27 '16 at 06:28
@Raja The answer linked shows that you cannot have a different memory location for integers between -5 and 256 as they are cached. Your problem is not about the integer cache. When you write `l2=l1` you are copying the reference only. Use `l2=l1[:]` or `copy.copy(l1)`. — Selcuk, Mar 27 '16 at 06:34

score 2 · Accepted Answer · edited May 23 '17 at 12:15

This is unlikely to occur with small numbers because numbers are immutable in Python and the runtime is almost assuredly going to make x and y "point to" the same object in the object store:

>>> x = 100
>>> id(x)
4534457488
>>> y = 100
>>> id(y)
4534457488

Note that even by making y reference a new copy of 100, I still got the same object.

We can try:

>>> y = 25 * (5 - 1)
>>> id(y)
4534457488

Same.

If we used something mutable like lists, we could do:

>>> x = [1, 2, 3]
>>> id(x)
4539581000
>>> y = [1, 2] + [3]
>>> id(y)
4539578440
>>> x == y
True
>>> id(x) == id(y)
False

And now you have two variables, each referencing a different object in the object store, but the object values are equal.

As wRAR points, out, copy.copy can do things much more cleanly:

>>> from copy import copy
>>> x = [1, 2, 3]
>>> id(x)
4539578440
>>> y = copy(x)
>>> id(y)
4539580232

Tada. This is more inline with what you want, but notice it might not work for immutables like numbers!

>>> from copy import copy 
>>> x = 100
>>> id(x)
4534457488
>>> y = copy(x)
>>> y
100
>>> id(y)
4534457488

My Python interpreter is just going to make one int object with value 100 in the object store.

For larger numbers, this may not be the case. I tried:

>>> x = 393
>>> id(x)
4537495408
>>> y = 393
>>> id(y)
4539235760

I found information in this S.O. question where an answerer experimented with integers on the object pool. The number of pre-cached integer objects is probably implementation dependent in Python, whereas the JVM does define caching behavior.

Excellent , but so everytime I need to slice and append y[-1] so I can point to different object location. — Raja G, Mar 27 '16 at 06:24
Well appending is a mutating operation which will not change the `id` of the list. You will want to copy or use `+` to make a new list. — Ray Toal, Mar 27 '16 at 06:27

Rudziankoŭ · Answer 2 · 2016-03-27T06:35:13.613

2

You can not do it with int. More over:

x = 100 
y = 100
x is y
>>> True

This is like a pool of small integers. They have only single representation in the memory.

With mutable objects you may do copy() operation:

a = [1]
b = copy(a)
a is b 
>>> False

edited Mar 27 '16 at 06:35

answered Mar 27 '16 at 06:24

Rudziankoŭ

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score 1 · Answer 3 · answered Mar 27 '16 at 06:19

1

You can use copy.copy() for this.

answered Mar 27 '16 at 06:19

wRAR

25,009
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I tried this but it did not do what the OP wants: >>> from copy import copy `>>> x = 100 >>> id(x) 4534457488 >>> y = copy(x) >>> y 100 >>> id(y) 4534457488` – Ray Toal Mar 27 '16 at 06:21
would you mind giving an example ? : EDIT : Ray mentioned in his answer. Its good. Thanks for pointing out wRAR. – Raja G Mar 27 '16 at 06:24
@Ray Toal, see my answer, pleae. We cannot copy "small" `int` it has single representation – Rudziankoŭ Mar 27 '16 at 06:26
Right. [More info here](http://davejingtian.org/2014/12/11/python-internals-integer-object-pool-pyintobject/). – Ray Toal Mar 27 '16 at 06:36

How to program python variable to point different reference location but same value

3 Answers3