python 2d list/array problem

Question

I have this 2d list (list of list) initialized with same values.
Now if I try to change first column in first row(arr[0][0]) it changes all the rows.

https://code.sololearn.com/cip0ZaTS7HkH/?ref=app

Answer 1

Jan Markus the reference is not to None, but to the array formed from `[None] * 3`. This is because None is not a reference type while list is. That is why when you change 1 array, all arrays are changed. See this

https://code.sololearn.com/ciT1hkyu0J84/?ref=app

EDIT:
KUMAR SHANU another thing. Jan Markus gave you a solution for the problem. Just wanted to add that instead of
arr = [[None for i in range(3)] for j in range(3)]
just `arr = [[None] * 3 for i in range(3)]` will also work. I edited my code so you can see why

Answer 2

The lists after the multiplication are clones of each other. They refer to the same object. That's how * works here, it's called shallow copy.

Answer 3

Kevin ★ Jan Markus I got it 👍
Thank you for your help.

Answer 4

@Jan Markus I don't know if I understand you correctly but it seems to be due to the fact that None is inmutable while lists are not.

Answer 5

Kevin ★ if I use any other value instead of None. The problem is same

Answer 6

if u say [[arr]*3]*3], it will multiply everything by 3. u have to say for i in y do *3 if u want to specify.

Answer 7

XXX good explained 👍