Exercise 2 solution

Cost: items[1:] builds a new list of length n-1, then n-2, and so on, so the
recursion copies about n*n/2 references in total, far more than the n
additions the sum needs. (It also hits Python's recursion limit for long
lists.)

Rewrite with an index, no slices:

    def total(items, i=0):
        if i == len(items):
            return 0
        return items[i] + total(items, i + 1)

Check:

    def total_slicing(items):
        return 0 if not items else items[0] + total_slicing(items[1:])

    data = list(range(500))
    print(total_slicing(data), total(data))

Output:
    124750 124750

Both agree; the second makes no copies. (A plain loop or sum() is better
still, and avoids deep recursion.)