Why does somelist[len(somelist)] generate an IndexError but not somelist[len(somelist):]?

Question

I understand that somelist[len(somelist)] cannot access an index that is outside of the defined list - this makes sense.

But why then does Python allow you to do somelist[len(somelist):]?

I've even read that somelist[len(somelist):] = [1] is equivalent to somelist.append(1)

But why does slice notation change the fact that the index "len(somelist)" is still outside the range of the list?

Answer 1

Here's something from the documentation. There are specific rules around slicing of any iterable; of particular note is #4, emphasis mine:

The slice of s from i to j is defined as the sequence of items with index k such that i <= k < j. If i or j is greater than len(s), use len(s). If i is omitted or None, use 0. If j is omitted or None, use len(s). If i is greater than or equal to j, the slice is empty.

Answer 2

There's nothing at the index len(somelist) (list indices start at 0 in python). Therefore, trying to access a non-existing element raises an error.

However, list slicing (with the myList[i:] syntax) returns a new list containing the elements including and after i. Since there are no elements in the list at index i (or after), an empty list is returned