output list reducing each time through loop

[buran]

on line 9 you don't mutate start, you create new list, i.e. start now points to different list:

start = [1,2,3,4,5]
print(f'id of start: {id(start)}')
new = []
print(f'id of new: {id(new)}')
t = True
for s in start:
    new.append(start)
    if t:
        start.pop(0)
        t = False
start = ['test']
print(f'id of start: {id(start)}')
print(new)
print(f'id of new: {id(new)}')

Output:
id of start: 4088456
id of new: 32132424
id of start: 32124040
[[2, 3, 4, 5], [2, 3, 4, 5], [2, 3, 4, 5], [2, 3, 4, 5]]
id of new: 32132424
>>>

as you can see the id of start changes, while the id of new is the same


Now, here is what you were looking/expecting:

start = [1,2,3,4,5]
print(f'id of start: {id(start)}')
new = []
print(f'id of new: {id(new)}')
t = True
for s in start:
    new.append(start)
    if t:
        start.pop(0)
        t = False
print(new)
start[0] = 'test'
print(f'id of start: {id(start)}')
print(new)
print(f'id of new: {id(new)}')

Output:
id of start: 4153992
id of new: 31997384
[[2, 3, 4, 5], [2, 3, 4, 5], [2, 3, 4, 5], [2, 3, 4, 5]]
id of start: 4153992
[['test', 3, 4, 5], ['test', 3, 4, 5], ['test', 3, 4, 5], ['test', 3, 4, 5]]
id of new: 31997384
>>>

here you mutate the original start (id is not changed) and you can see that change is also reflected in new