This was a problem a student brought to me. It was very interesting! It suggests 2 approaches of different “trickiness”.

The natural approach suggested by the problem’s description is to use a for-loop. This is complicated by that, in an unusual twist, the for loop (over the provided index parameter) isn’t really related to the length of the array. So while you have the i or whatever in your for loop, you want to resist the temptation to do a[i].

Ultimately you’ll want a secondary variable. While i tracks the provided index, j handles the “wrap-around” logic:

def norm_index_test(seq, ind):
    if seq == []:
        return None
    isneg = ind < 0
    ind = abs(ind)
    j = 0
    for i in range(ind):
        j += 1
        if j == len(seq):
            j = 0
    assert(j < len(seq))
    if isneg and j != 0:
        j = len(seq) - j
    return seq[j]

This is actually probably the hardest form of the question, complicated by the negative indexes. If I were asking this problem, I would not ask about negative indexes. Ugh!

Once you can trust modular arithmetic (and maybe Python makes it too easy? Not sure if all languages define module of a negative number the same), it’s a one-liner. Cute, but OK.

def norm_index_test(seq, ind): 
    if seq == []:
        return None
    return seq[ind%len(seq)]