Dayly programing (Project Euler:#20)

euler20.py

#!/usr/bin/python
from math import factorial

def digit_sum(num,sum=0):
    if num > 0:
        num,sum = int(num / 10), sum + (num % 10)
        return digit_sum(num,sum)
    else:
        return sum

print(digit_sum(factorial(100)))

#=>648