If Python slice copy the reference, why can't I use it to modify the original list?

Question

I know that Slicing lists does not generate copies of the objects in the list; it just copies the references to them.

But if that\'s the case, then why doesn\'t this wor

Answer 1

For the sake of explaining it better, assume you had written

l = [1, 2, 3]
k = l[0:2]
k[-1] = 10

I hope you can agree that this is equivalent.

Now let's break down the individual statements:

l = [1, 2, 3]

This creates the following objects and references:

id  object
--  --------
0   
1   
2   
3   

name  →  id
----     --
l     →  3
l[0]  →  0
l[1]  →  1
l[2]  →  2

k = l[0:2]

This creates a new list containing copies of the references contained in l:

id  object
--  --------
0   
1   
2   
3   
4   

name  →  id
----     --
l     →  3
l[0]  →  0
l[1]  →  1
l[2]  →  2
k     →  4
k[0]  →  0  (copy of l[0])
k[1]  →  1  (copy of l[1])

k[-1] = 10

First, index −1 resolves to index 1 (because k has length 2), so this is equivalent to k[1] = 10. This assignment means that the objects and references are updated as such:

id  object
--  --------
0   
1   
2   
3   
4   
5   

name  →  id
----     --
l     →  3
l[0]  →  0
l[1]  →  1
l[2]  →  2
k     →  4
k[0]  →  0
k[1]  →  5

Note how l and l[0] to l[2] are not affected by this. QED.