Recursion

A recursive function calls itself on a smaller input. Every recursive function needs a base case that stops the calls.

factorial(4) is 4 * factorial(3), and so on until factorial(1), which simply returns 1.
Deep recursion uses memory and Python limits its depth. Use loops when the repetition is simple.

def countdown(n):
    if n == 0:
        return
    print(n)
    countdown(n - 1)