sum function returns inconsistent values when used with slicing

Question

lst=[2,5]
sum(lst)

This returns 7 as expected

lst[0]

This returns 2 as expected

lst[1]

This returns 5 as expected

sum(lst[0:-1])

why does this return 2 instead of 7??

Answer 1

This is as expected as lst[0:-1] selects everything from the first element, up to one before the last element:

lst[0:-1]
>>> [2]