Python append to list of list

Question

I have a list which contains list of words.

Example:

[['A','B','C'],['D'],['E']]

To implement this i have create an empty list of list of size 4.

list1=[list()] *4

and then tried append element one by one.

list1[0].append('A')
list1[0].append('B')
list1[0].append('C')
list1[1].append('D')
list1[2].append('E')

Output:

[['A', 'B', 'C', 'D', 'E'], ['A', 'B', 'C', 'D', 'E'], ['A', 'B', 'C', 'D', 'E'], ['A', 'B', 'C', 'D', 'E']]

Expected was:

[['A','B','C'],['D'],['E']]

Can you not use a for loop as suggested here: https://stackoverflow.com/questions/41452819/list-append-in-for-loop — py_noob, Apr 25 '20 at 08:07
`print([id(x) for x in list1])` will show they are all the same object. — RoadRunner, Apr 25 '20 at 08:16

score 2 · Answer 1 · answered Apr 25 '20 at 08:11

Your issue is that here :

list1=[list()] *4

you are making copies of the same list object. You are essentially adding the elements to the same list every time.

To fix:

list1=[[] for i in range(3)]


list1[0].append('A')
list1[0].append('B')
list1[0].append('C')
list1[1].append('D')
list1[2].append('E')

Which gives your expected result.

score 0 · Answer 2 · answered Apr 25 '20 at 08:14

0

The initialization method you used creates a list of four pointers that all point to the same list. You shouldn't use the asterisk operator in this case.

answered Apr 25 '20 at 08:14

Samuel O.D.

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Python append to list of list

2 Answers2