Function returns none instead of the sum of list

Question

Why does this code return none in Python 2? However if I replace return statement with print statement ie print sum in the function itself, it gives the right answer. Why is that so? I know it is a silly question but i cant figure it out on my own.

import math
n=int(raw_input().strip())
temp = [5]
arr=[]
def cal(arr):
    arr.append(int(math.floor(temp[-1]/2)))
    temp.append(arr[-1]*3)
    if len(arr)==n:
        return sum(arr)
    cal(arr)
print cal(arr)

Answer 1

You need to

return cal(arr)

otherwise the function will just call itself recursively and return None by default (instead of the calculated result).

Answer 2

The end of the function should read

if len(arr)==n:
    return sum(arr)
else:    
    return cal(arr)

shouldn't it? You get None as a return value if your function ends without returning a value.