What happens when you try to delete a list element while iterating over it

Question

I\'m iterating a list as follows:

some_list = [1, 2, 3, 4]
another_list = [1, 2, 3, 4]

for idx, item in enumerate(some_list):
    del some_list[idx]

for it         


        

Answer 1

I want to know what exactly happens behind the scenes

As we know, every item in a list lives at its own unique index; which are in order, starting from 0. If we remove an item, any item with an index greater than the one we've removed has now been shifted down.

And here's why that matters:

foo = ['a', 'b', 'c', 'd']
for index in range(len(foo)):
    del foo[index]

In this loop, we're removing all the elements, so we should end up with foo == [], right? This is not the case. In our first trip through the loop, we remove the item at index 0, and the item at index 1 becomes the item at index 0. Our next time through the loop, we remove the item at index 1 which was previously the item at index 2.

In just the first two iterations, we've removed 'a' and 'c' from the array, *but we've neglected to remove 'b'. Once we get to the third iteration (where we though we'd remove index 2), there is no longer an element at index 2; only indices 0 and 1. An exception is raised when we try to remove the non-existent item at index 2, and the loop is stopped. The result is a mangled array that looks like this: ['a', 'd'].