Negative index in python for loop [duplicate]

Answer 1

This is because in Python, this:

a = [0, 1, 2, 3]
for i in a:
    ... statements ...

can be understood as this:

a = [0, 1, 2, 3]
it = iter(a)
i = next(it)
... statements ...
i = next(it)
... statements ...
i = next(it)
... statements ...
i = next(it)
... statements ...

Now if you replace i by a[index], then you end up replacing a[index] by the last element you iterated over, which is the last element in your list.