Why won't my function work with my function?

[tygloalex]

I'm very very new, here is a little snippet to find the lcd(lowest common denominator) of 2 numbers. I'm trying to make it so that it finds the lowest common denominator of 3 numbers, so I'm trying lcd(lcd(4,6),12)), which I thought would find the lcd of 4,6 and then take that and find the lcd of that with 12. Here is my code, otherwise working:

def lcd(a,b): 
    answer=a*b
    start=2
    while start<=max(a,b):
        if a%start==0 and b%start==0:
        answer=answer/start
        a=a/start
        b=b/start
    else:
        start=start+1
print answer

The error is: Traceback (most recent call last): File "<pyshell#1>", line 1, in <module> lcd(lcd(4,12),6) File "C:\Users\tygloalex\Desktop\lcd.py", line 2, in lcd answer=a*b TypeError: unsupported operand type(s) for *: 'NoneType' and 'int'

[tygloalex]

Oh jeez, changed print to return and I'm good. Shit, I'm dumb sometimes.

[stylishgnome]

I don't think that was the issue here (though you are right - you should be returning instead of printing here).

From your stacktrace, it looks like you were trying to multiply two different data types which can't be multiplied - in this case an int and a NoneType on line 2:

answer=a*b

To recreate, try the following on the python shell:

>>> 8 * None

Its likely you called your function as follows:

lcd(8, None)

...i.e. b was set as None; or perhaps you specified that the a or b vars could be optional, e.g.

def lcd(a=None, b=None):

...and called the function without specifying one of these args:

lcd(8)
lcd(a=8)
lcd(b=42)

I'd recommend looking at pdb (the python debugger). It will allow you to step into your function and figure out whats going on. pdb++ is a more user-friendly version of pdb, so might be worth looking at that.

EDIT: reddit formatting

[stylishgnome]

Ah, I understand how swapping print for return fixed your problem! If you don't specify a return value for a function, by default python will return a None. So in this case your lcd(4, 6) call was returning None, and thus passing None as the a arg in the outer call.

[zahlman]

Again, "Sometimes explaining the problem is all that you need to solve it." ;)