Python: List of lists output

Question

I am trying to make a list of 5 lists.

I did that by web = [[]]*5

Now I want to update the first list in the 'list of lists' with values 1 and 2.

web[0].append(1)
web[0].append(2)

But my output is

[[1, 2],
[1, 2],
[1, 2],
[1, 2],
[1, 2],
[1, 2],
[1, 2],
[1, 2],
[1, 2],
[1, 2]]

But shouldn't the first list be updated only? My desired output is as below

[[1, 2],
[],
[],
[],
[],
[],
[],
[],
[],
[]]

Alex Fung · Answer 1 · 2017-02-28T06:24:41.783

4

When you create web = [[]]*5, you are actually creating an array of references to the same object (a list of []s). So when you change the object via one reference, this changes all entries in the list.

This is a very common Python 'gotcha' for beginners.

try this instead:

web = [[] for x in xrange(5)]

edited Feb 28 '17 at 06:24

answered Feb 28 '17 at 06:23

Alex Fung

1,996
13
21

can you point me to a references for more explanation of this – Reynard Asis Feb 28 '17 at 06:24
@ReynardAsis check out [here](http://stackoverflow.com/questions/240178/list-of-lists-changes-reflected-across-sublists-unexpectedly) – Alex Fung Feb 28 '17 at 06:25
Instead of answering a "common Python 'gotcha'" and linking to the canonical page for it, you can flag the question for closure as a duplicate. This reduces fragmentation and makes the site easier to use. – TigerhawkT3 Feb 28 '17 at 06:27

Taufiq Rahman · Answer 2 · 2017-02-28T06:30:33.277

-3

Creating list that way, creates duplicates only.You can rather creating this way:

n = 5 #range
web = [[] for _ in range(n)]

web[0].append(1)
web[0].append(2)
print(web)

Ouput:

[
 [1, 2],
 [],
 [],
 [],
 []
]

edited Feb 28 '17 at 06:30

answered Feb 28 '17 at 06:20

Taufiq Rahman

5,600
2
36
44

He's duplicating the same array which is because of how he is trying to initialize the array, so any append to one of the lists will append to them all, because they are the same object. – TemporalWolf Feb 28 '17 at 06:22
You are right. I edited my answer. – Taufiq Rahman Feb 28 '17 at 06:31

Python: List of lists output

2 Answers2