Don't understand part of the binary search algorithm?

Question

Python

There is a code

def binary_search(mylist, item):
    start = 0
    stop = len(my_list) - 1

    while start <= stop:
        mid = (start + stop) // 2
        guess = mylist[mid]
        if guess  == item:
            return mid
        if guess > item:
            stop = mid - 1
        else:
            start = mid + 1
    return None

my_list = [1,2,3,4,5,6,7,8,9,11,12,13,14,15,16,17,19]
print(binary_search(my_list, 3))

I can't understand why in the line
stop = len(my_list) - 1
do "-1"? why not just write stop = len(my_list)?

Answer 1

Probably because in an array, indices start from 0. And therefore, the index of the last element of the array will be equal to the length of the array minus 1
because in this case it is possible to crash with an error when accessing a non-existent element of the array

Answer 2

That's why

def binary_search(mylist, item):
    start = 0
    stop = len(my_list) # верни -1

    while start <= stop:
        mid = (start + stop) // 2
        guess = mylist[mid]
        if guess  == item:
            return mid
        if guess > item:
            stop = mid - 1
        else:
            start = mid + 1
    return None

my_list = [1,2,3,4,5,6,7,8,9,11,12,13,14,15,16,17,19]
print(binary_search(my_list, 20))