Double-Duty Recursive Functions in Python

Functional programming encourages the use of recursive functions in order to tackle bigger problems, allowing you to avoid mutable state and express the problem as small problems. One of the most popular example of recursive functions is the factorial function, which can be written in python as follows:

def factorial(num):
    """
    num must be an integer greater than 0
    """
    if num == 1:
        return num
    else:
        return num * factorial(num - 1)

That’s the old-school approach to recursive functions and an effective enough function for typical usage. But the biggest problem with typical old-school recursive functions is the stack buildup. If the input is too large, a recursive function will cause a stack overflow.