output list reducing each time through loop

[buran]

You are getting list of empty lists, not empty list

Output:
[[], [], [], [], []]
>>>

Check this link https://nedbatchelder.com/text/names.html
lists are mutable. when you append a to output on line 5, every list you append refers to same elements. when you pop element on line 6 you pop it out from all the lists. To fix this, you need to append copy of a to output

a = [1,2,3,4,5]
output = []
 
while a:
  output.append(a[:])
  a.pop(0)
 
print(output)

Output:
[[1, 2, 3, 4, 5], [2, 3, 4, 5], [3, 4, 5], [4, 5], [5]]
>>>