Why return None when doing recursion?

Question

I'm doing this problem Add Digits in recursion way. Why does it return None?

def addDigits(num):
    """
    :type num: int
    :rtype: int
    """
    shit = str(num)
    n = len(shit)
    if n == 1:
        return num
    else:
        num = 0
        for i in xrange(n):
            num += int(shit[i])
        addDigits(num)

Answer 1

You need to return the value of the recursive call in the last line:

return addDigits(num)

Without such a return you silently return None