inside nested loops appending list to a new list gives unexpected result

I have tried to write my error issue in reproducible manner for platform to be seen and guided. I cannot see my logic gap why this error happens.
I have a inner loop which brings new elements while scraping and appends it to list named list_inner. Then in outer loop list named list_outer appends that new list. But final result gives amount of members right, but elements of list list_outer are same, the last list element of list list_inner. How can this happen? If it will be one elemented list I will understand.

import random
list_inner=[]
list_outer=[]
for i in range(5):
    for r in range(random.randint(1,10)):
        list_inner.append(r)
        print(r)
    list_outer.append(list_inner)
    print(list_outer)
print(list_outer)

I am sharing for two results, as giving idea what is in real and what I was expecting. I got this result:

0
1
2
3
[[0, 1, 2, 3]]
0
1
2
3
4
[[0, 1, 2, 3, 0, 1, 2, 3, 4], [0, 1, 2, 3, 0, 1, 2, 3, 4]]

But I was expecting this result:

[[0,1,2,3],[0,1,2,3,4]]

>Solution :

There are really two problems here:

• You are not clearing list_inner at the end of each outer loop, so it retains all the elements from all the iterations so far
• You are appending list_inner directly to list_outer instead of appending a copy of list_inner. This means that when list_inner gets changed, the reference to it that has already been put inside list_outer gets changed as well. So you end up with the same thing repeated in list_outer.

All you need to do is change your code to copy list_inner before appending it, and also clear list_inner at the end of each outer loop:

import random

list_inner = []
list_outer = []
for i in range(5):
    for r in range(random.randint(1,10)):
        list_inner.append(r)
    list_outer.append(list_inner.copy())
    list_inner.clear()

print(list_outer)

Example output:

[[0, 1, 2, 3, 4, 5, 6, 7, 8], [0, 1, 2], [0, 1, 2, 3, 4, 5, 6], [0, 1], [0, 1, 2, 3]]