Why does this if-elif-else statement print 42?

Question

I cannot understand why this returns 42. I've asked a friend, I've traced it in PythonTutor...still makes no sense. Why doesn't it just return 9 bc 6 - 1 = 5, 6 - 2 = 4, 5 + 4 = 9? Why does it keep looping through? What is waiting for? How does it know where to stop?

def mystery(n):
    if n == 0:
        return 2 
    elif n == 1:
        return 4
    else: 
        return mystery(n-1) + mystery(n-2)

print(mystery(6))

Answer 1

This is called a recursive function. It calls itself (with new parameters) until it hits one of the "base cases" n==0 or n==1 .

mystery(0) == 2
mystery(1) == 4
mystery(2) = mystery(1) + mystery(0) = 6
mystery(3) = mystery(2) + mystery(1) = 10
mystery(4) = mystery(3) + mystery(2) = 16
mystery(5) = mystery(4) + mystery(3) = 26
mystery(6) = mystery(5) + mystery(4) = 42

Answer 2

as @Christian Slater wrote, you wrote a recursive function, you probably are looking for:

def mystery(n):
    if n == 0:
        return 2
    elif n == 1:
        return 4
    else:
        return (n-1) + (n-2)

print(mystery(6))

output: 9