append function in python

Question

I have a list X = ['xyz']

I use the below commands for appending to another variable.

L = X

L.append(X)

L

Out[109]: ['xyz', [...]]

I am unable to understand why the second element in the new L list is not having the value as 'xyz'

My question is not how to append or extend a list, but in fact about the functionality of Append function, which has been correctly explained by @sinsuren below.

Er, you seem to have appended a list to itself. What did you think would happen? — Daniel Roseman, Oct 07 '16 at 09:31
Why is this question marked as duplicate, I don't understand. :( :( My question is not how to append or extend a list, but in fact about the functionality of Append function, which has been correctly explained by @sinsuren below. — Sarang Manjrekar, Oct 07 '16 at 09:50

score 2 · Answer 1 · answered Oct 07 '16 at 09:33

2

append add X as an element to L. If you want the element inside X to be inserted to L, use extend instead:

>>> X = ['xyz']
>>> L = X
>>> L.extend(X)
>>> L
['xyz', 'xyz']

answered Oct 07 '16 at 09:33

Uriel

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score 1 · Accepted Answer · edited May 23 '17 at 10:34

1

Try this, it will extend the list.

L.extend(X)

But if you want to use append. Use a element like this L.append('abc'). It will give same result or L.append(X[0])

Edit:
You have Appended list to itself. It will recursively append to itself and due to which even L[1] will give you same response. Like L[1] = ['xyz', [...]] . and for more understanding Please refer What's exactly happening in infinite nested lists?

edited May 23 '17 at 10:34

Community

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answered Oct 07 '16 at 09:33

sinsuren

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Any rule about, when not to use 'append' function in specific ? – Sarang Manjrekar Oct 07 '16 at 09:34
2

If you want to put an element there, use append. If you want to make your list longer by chaining on another list at the end, use extend. – Moberg Oct 07 '16 at 09:35
@Moberg, why does it work OK then in the example below ? `>>> x = [1,2,3]` `>>> y = [4,5,6]` `>>> x.append(y)` `>>> x` `[1, 2, 3, [4, 5, 6]]` – Sarang Manjrekar Oct 07 '16 at 09:39
Ya, that was expected. But my initial question was, why the second element in the resultant list is blank ? – Sarang Manjrekar Oct 07 '16 at 09:42
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When you assigned L with X both share same reference. So you appended a list to itself which holds same reference. and it seems like infinite `Like Inception Movie` – sinsuren Oct 07 '16 at 09:46
https://ideone.com/OOYGP0 Here you can see this also. – sinsuren Oct 07 '16 at 09:47
@sinsuren, Thanks I got it now ! – Sarang Manjrekar Oct 07 '16 at 09:49
Sure, thanks again ! – Sarang Manjrekar Oct 07 '16 at 09:51

append function in python

2 Answers2