Appending element into the list of list overrides the entire list

Question

Code is as shown below :

s = [[]] *4
s[0].append(1)

print(s)

it gives me output :

[[1],[1],[1],[1]]

but i want output like this :

[[1],[],[],[],[]]

How can i achieve that?

Possible duplicate of [How to clone or copy a list?](https://stackoverflow.com/questions/2612802/how-to-clone-or-copy-a-list) — Chris, Jun 21 '19 at 06:35
the reference remains the same so the operation on 1st row corresponds to all the row, create the 2D list using list comprehension as `[[] for _ in range(n)]` — arajshree, Jun 21 '19 at 06:42

score 6 · Accepted Answer · answered Jun 21 '19 at 06:39

6

You cannot use [[]] * 4 to create four lists. In this case, you are just creating one list and four references pointing to it.

So you should use [[] for _ in range(4)].

answered Jun 21 '19 at 06:39

Sraw

score 1 · Answer 2 · answered Jun 21 '19 at 06:53

Just an additional info to Sraw's reply:

>>> t = [[]] * 4 
>>> t
[[], [], [], []]
>>> id(t[0]) == id(t[1]) == id(t[2]) == id(t[3])
True
>>> l = [[], []]
>>> id(l[0]) == id(l[1])
False

You get 4 references to same list. That's why adding an element to any of the references show up in the others.

2 Answers2