Not sure why this works...

[goldtiger6]

I'm working on a project for class and I was not sure why this didn't work:

Input:
0
9

def hcf(a, b):
  
  for i in range(a):
    if a > b:
	    a -= b
    elif b > a:
    	b -= a
  print(a)

hcf(int(input()), int(input()))

Output:
0 (Answer should be 9)

I then started adding random things to my code and discovered that the following code worked:
Input:
0
9

def hcf(a, b):
  
  for i in range(a):
    if a > b:
	    a -= b
    elif b > a:
    	b -= a
  print(a or b)

hcf(int(input()), int(input()))

Output:
9

I know this is an odd question but I would like a better understanding of how that changed the code, making it work.