+ 4

# Python list

why does this happen when using extend() and then insert()? a = [1, 2, 3, 4, 5] print(a) # Output ==> [1, 2, 3, 4, 5] a.extend([44]) print(a) # Output ==> [1, 2, 3, 4, 5, 44] a.insert(-1, len(a)) print(a) # Output ==> [1, 2, 3, 4, 5, 6, 44] If you notice, the extended number (44) goes to the end even if I have extended it before inserting the length of my list; len(a) == 6. Why is that happening?

7th Jul 2019, 8:20 AM
Dolan
16 odpowiedzi
+ 10
using insert with lists does always have the same behaviour: a = [0, 1, 2, 3, 4] # now insert “99” at index 2: a. insert(2, 99) # result is: [0, 1, 99, 2, 3, 4] # now insert “33” at index -1: a.insert(-1, 33) # result is: [0, 1, 99, 2, 3, 33, 4] So insert at index 2 is correct. insert creates a new empty element at index 2 and then fill it with the defined value. All the other elements will be shifted to the tight. The same will happen if your index is -1 which means the last position in your list. at this place it creates a new emty element, and therefor the original element will be shifted to the right.
7th Jul 2019, 9:11 AM
Lothar
+ 5
a short and final remark: if index given for insert is >= len(list) the element will be inserted at the real end of the list. As there is no index error if “out of index” value of 99 as index for a list with len = 5 causes no problem.
7th Jul 2019, 11:37 AM
Lothar
+ 4
if you will later have 789 as a real existing index it will work as explained in my sample - so something inserted there will appear as the second last position in the list.
7th Jul 2019, 11:49 AM
Lothar
+ 4
a.insert(-1, len(a)) print(a) # python takes list a[1,2,3,4,5,44], your argument len(a) (that equal 6 on that moment) and pastes it on choosing you position (before "-1" element). Output [1, 2, 3, 4, 5, 6, 44]
7th Jul 2019, 8:17 PM
Антон Попов
+ 2
This seems to work too: a.insert(len(a), len(a))
7th Jul 2019, 8:37 AM
Paul
+ 2
I tried it. That’s true. and why is it like that!!!? a = [1,2,3,4] a.insert(789,55) #Output ==> [1,2,3,4,55] what if later we had a real index with this index number(789)?
7th Jul 2019, 11:44 AM
Dolan
+ 1
Thanks Drax . It is clear that I can use append(). but just wanted to make sure if is it because I didn’t save the extend() result into a variable that this is happening or what!? thanks anyway for your answer
7th Jul 2019, 8:29 AM
Dolan
+ 1
You don't need to save the result of extend because it modify the original list ;)
7th Jul 2019, 8:33 AM
Drax
+ 1
thanks Lothar it is quite obvious and clear. thanks for your description.
7th Jul 2019, 9:39 AM
Dolan
+ 1
quite interesting :-)
7th Jul 2019, 11:51 AM
Dolan
+ 1
I don't understand how it will be shifted when insert function applied?
8th Feb 2020, 11:59 AM
Urooj Nazir
0
so why the insert() result will come before the extend() one! the extend() numbers should come before the insert() ones!
7th Jul 2019, 8:36 AM
Dolan
0
thanks Paul Jacobs your offer works perfectly even there are much more ways to solve the problem. Just wanted to know the answer of the question actually! why is it like that!?
7th Jul 2019, 8:42 AM
Dolan
0
8th Feb 2020, 12:04 PM
Dolan
0
a = [0,1,2,3,4,5] if I insert a number at third index, it will shift the others to right side. a.insert(3, 99) OUTPUT will be [0,1,2,99,3,4,5]
8th Feb 2020, 12:07 PM
Dolan
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