Recursion

AlPHABET = "abcdefghijklmnopqrstuvwxyz"

def rotate(text, key):
    if text == "":
        return ""
    first_letter, rest = text[0], text[1:]
    if first_letter.isalpha():
        if first_letter.isupper():
            return AlPHABET[(AlPHABET.index(first_letter.lower()) + key) % 26].upper() + rotate(rest, key)
        else:
            return AlPHABET[(AlPHABET.index(first_letter) + key) % 26] + rotate(rest, key)
    else:
        return first_letter + rotate(rest, key)

This approach uses the same logic as that of the alphabet approach, but instead of a loop, recursion is used to parse the text and solve the problem.

Recursion is a programming technique where a function calls itself. It is powerful, but can be more tricky to implement than a while loop or for loop. Recursive techniques are more common in functional programming languages like Elixir, Haskell, and Clojure than they are in Python.

Solving this exercise with recursion removes the need for a result variable and the instantiation of a loop. If the number is not equal to one, we call <letter> + rotate(rest). Then the rotate function can execute the same code again with new values. We can build a long chain or "stack" of <letter> + rotate(rest) calls until the rest variable is exhausted and the code adds "". That translates to something like this: <letter> + <letter> + <letter> + <letter> + "".