Python recursion

A recursive function calls itself on a smaller version of the problem, until it reaches a case small enough to answer directly (the base case).

Trust the smaller call to be right, and only work out how to build this answer from it.

def factorial(n):
    if n == 0:
        return 1
    return n * factorial(n - 1)

print(factorial(5))

Watch for: Without a base case, or if the problem doesn't get smaller, it never stops: Python gives up with a RecursionError.