Get a part of a list and change the original when the new list changes

Question

I want to know if it's possible do that:

a = [0, 1, 2, 3, 4]
b = a[2:]
b[0] = -1
print(a)
print(b)

and get this:

[0, 1, -1, 3, 4]
[-1, 3, 4]

Normally, you will get this:

[0, 1, 2, 3, 4]
[-1, 3, 4]

Your question is unclear... You provided a code and its output, now, what would you *like* it to do that it currently doesn't ? — Nir Alfasi, Jul 17 '17 at 23:47
So you want `a` and `b` to point to the same list object and update together? — Christian Dean, Jul 17 '17 at 23:49
Yes, I do. The problem's you get a copy when you slice the original list. I need when I change 'b' changes 'a'. — rvcristiand, Jul 17 '17 at 23:51
@downshift He apparently doesn't want to hard code it. He wants the update to happen automatically. — Christian Dean, Jul 17 '17 at 23:52
But if I do that I must made the changes two times... The idea is change only 'b' and changes 'a' automatically. — rvcristiand, Jul 17 '17 at 23:53
might want to edit the "little" in title to that which *actually* changes — chickity china chinese chicken, Jul 17 '17 at 23:58

Mr_U4913 · Answer 1 · 2017-07-18T00:11:53.630

4

Briefly explain what I understand this, let me know if I make any mistakes.

The answer is NO.

In python, the variable is a tag linked to the object. If we do

a = [0, 1, 2, 3, 4]
b = a
id(a) #199598920
id(b) #199598920

The b is just a name tag name linked to the object.It shares the same object with a

To your question,

a = [0, 1, 2, 3, 4]
id(a) #199598920
a[2:] # [2, 3, 4]
id(a[2:]) #199581576

a[2:] gives us a slice of a list by creating a new list and copying a part of the first list into the new list.

In this case, you cannot automatically update you b[0] and list a

edited Jul 18 '17 at 00:11

answered Jul 18 '17 at 00:06

Mr_U4913

I understood. It's not possible, I need coded it. Thank you ! – rvcristiand Jul 18 '17 at 00:11
Comparing the `id` of the original and the slice isn't conclusive - because even if they would share the underlying "data" the id has to be different (simply because one instance can't represent itself and a slice of itself). Otherwise great answer - I really like the images! :) – MSeifert Jul 18 '17 at 00:27

MSeifert · Accepted Answer · 2017-07-18T00:16:33.117

4

No, with plain lists this is not possible because slicing returns a shallow copy of the slice.

However if you have NumPy it's possible because there slices of an array return views not copies:

import numpy as np
a = np.array([0, 1, 2, 3, 4])
b = a[2:]
b[0] = -1
print(a)   # [ 0  1 -1  3  4]
print(b)   # [-1  3  4]

edited Jul 18 '17 at 00:16

answered Jul 18 '17 at 00:13

MSeifert

score 0 · Answer 3 · answered Jul 18 '17 at 00:05

0

Do you mean something like that? ;)

a = [0, 1, 2, 3, 4]
b = a[2:]
b[0] = a[2] = -1 
print(a)
print(b)

answered Jul 18 '17 at 00:05

ZRTSIM

3 Answers3