enumerate() for sliced list does not work as expected

Question

enumerate() for a list works as expected.

def swap(nums, i, j):
    temp = nums[i]
    nums[i] = nums[j]
    nums[j] = temp

mylist = [0,1,22,33,4]

for pos, num in enumerate(mylist):
    print(pos,num)
    if pos == 0:
        swap(mylist,2,3)

In this example, num shows swapped value as expected.

0 0
1 1
2 33 <--
3 22 <-- 
4 4

But if I change mylist to mylist[:-1] like:

def swap(nums, i, j):
    temp = nums[i]
    nums[i] = nums[j]
    nums[j] = temp

mylist = [0,1,22,33,4]

for pos, num in enumerate(mylist[:-1]):
    print(pos,num)
    if pos == 0:
        swap(mylist,2,3)

num does not show swapped value.

0 0
1 1
2 22 <--
3 33 <--

Why does this happen?

Answer 1

The difference is that you are enumerating on a copy of the list. In your first example, you are enumerating on a direct reference to the list that you are altering, but in the second example, you used slicing on the list to enumerate over. Slicing returns a new list object.

This is important because you are altering what not-yet-iterated-over positions are referencing to. List iteration uses an ever-increasing index to yield values, altering the list as you iterate can mean that a next index references a different value.

That the iteration is done via the enumerate() function doesn't really matter. You'd have the same problem without enumerate():

for num in mylist:
    print(num)
    if num == 0:
        swap(mylist, 2, 3)

would print

0
1
33
22
4

and slicing would give you a copy, so:

for num in mylist[:-1]:
    print(num)
    if num == 0:
        swap(mylist, 2, 3)

outputs

0
1
22
33

If the goal is to skip the last element, then instead of creating a copy through slicing, you could use the enumerated position to break out of the loop early:

for pos, num in enumerate(mylist):
    if pos == len(mylist) - 1:
        break
    print(pos, num)
    if pos == 0:
        swap(mylist, 2, 3)

or you can use the itertools.islice() function

from itertools import islice

for pos, num in enumerate(islice(mylist, len(mylist) - 1)):
    print(pos, num)
    if pos == 0:
        swap(mylist, 2, 3)

or you could simply create your copy first, then alter the indices of that copy:

sliced = mylist[:-1]
for pos, num in enumerate(sliced):
    print(pos, num)
    if pos == 0:
        swap(sliced, 2, 3)