When you use a for loop on a list in Python, it iterates over a snapshot of the original list when the loop starts. That is, the variable i gets its value from list a based on the elements that existed in the list when the loop started, and not a modified version of the list during the iteration.

Let's look at the code.

First iteration:

○ i is 1 (the first element).

○ if a != []: evaluates to True since a is [1, 3, 4, 5].

○ a.pop() removes 5 (the last element in the list) and prints 5.

○ a is now [1, 3, 4].

Second iteration:

○ The next iteration, i now gets its next value, which was 3 in the original list.

○ if a != []: evaluates to True since a is [1, 3, 4].

○ a.pop() removes 4 (the last element in the current list) and prints 4. Now a is [1, 3].

-Third iteration:

Next iteration. i is 4 (next original element), but 4 is no longer in the modified list, so the for loop does not fetch that new value, so the iteration is skipped.

-Fourth iteration:

The last iteration. i is 5 (next original element), but 5 is no longer in the modified list, so the for loop does not fetch that new value, so the iteration ends here.

So a is [1, 3]