Explain nested list in python

Question

I need an insight into understanding the below code. The first print gives me output: 'a' whereas on changing the value of y[0][0] to "p" it changes the value of y[0][0], y[1][0], y[2][0] and y[3][0] as well. I was expecting an output like [['p', 'b', 'c'], ['a', 'b', 'c'], ['a', 'b', 'c'], ['a', 'b', 'c']] but instead got [['p', 'b', 'c'], ['p', 'b', 'c'], ['p', 'b', 'c'], ['p', 'b', 'c']]

x=["a","b","c"]
y = [x] * 4
# first print
print(y[0][0])

y[0][0] = "p"
# second print
print(y)

Try doing `y = [list("abc"), list("abc"), list("abc"), list("abc")]`. I dont know if there is a better pythonic way to write this. — shahkalpesh, Dec 28 '18 at 05:39
`y=[x.copy() for i in range(4)]` use this. then you will able t0 change individual vlaues — sahasrara62, Dec 28 '18 at 05:42

U13-Forward · Accepted Answer · 2018-12-28T05:59:09.940

1

Because * operator is not exact, it's kind of funny, so use range.

It will work by replacing the below line:

y = [x] * 4

With:

y = [x.copy() for i in range(4)]

Use copy which creates a copy of something else, and actually copy is creating a same thing when printing it, but different id, different object really, so your code won't do the above reproduction again.

Also, you've got your question closed as a duplicate of:

List of lists changes reflected across sublists unexpectedly

Which have much better explanations.

edited Dec 28 '18 at 05:59

answered Dec 28 '18 at 05:31

U13-Forward

69,221
14
89
114

1

`y = [x for i in range(4)]` creates the reference to the same list. If you do a `id(y[0])` and `id(y[1])`, it returns same number. I didn't downvote, if that helps. – shahkalpesh Dec 28 '18 at 05:37
Both of your lines of code have the same effect. Also, "it's kind of funny" is not a helpful statement. – Richard Möhn Dec 28 '18 at 05:40
it's creating a shallow copy(all element reference to same memory loc) – sahasrara62 Dec 28 '18 at 05:43
1

@shahkalpesh Edited mine – U13-Forward Dec 28 '18 at 05:43
@RichardMöhn Edited mine. – U13-Forward Dec 28 '18 at 05:44
@prashantrana Edited mine. – U13-Forward Dec 28 '18 at 05:44
Thanks for all of you telling me why the donw-vote – U13-Forward Dec 28 '18 at 05:44
@U9-Forward Dude, also explain deep copy and shallow copy . It will be helpful to others. – sahasrara62 Dec 28 '18 at 05:51
@prashantrana Okay will edit. – U13-Forward Dec 28 '18 at 05:53
1

@prashantrana Finshed my edit. – U13-Forward Dec 28 '18 at 05:54
1

It's still hard to understand. And https://stackoverflow.com/questions/240178/list-of-lists-changes-reflected-across-sublists-unexpectedly has much better answers, so why bother? – Richard Möhn Dec 28 '18 at 05:57
@RichardMöhn How about referring to the link? – U13-Forward Dec 28 '18 at 05:59
1

Don't answer obvious duplicates. It fragments information and dilutes the quality of content on site. – wim Dec 28 '18 at 06:12
@wim I won't do next time.\ – U13-Forward Dec 28 '18 at 06:13
@U9-Forward. Thanks for the link. That helped with the explanation. – boom_itsme Dec 28 '18 at 06:57

Explain nested list in python

1 Answers1