为什么这个数字根函数不起作用？

Question

我正在做a challenge on codewars，我应该在Python中编写数字根函数。我无法弄清楚它为什么不起作用。这是我第一次尝试递归。我真的很想绕过这个。

def digital_root(num):
        #Check to see if num has more than one digit
        if num > 9:
            x = 0
            z = 1
            #Create an array of the digits in num
            digits = [int(d) for d in str(num)]

            #Convert string elements in digits[] to ints
            for n in digits:
                digits[x] = int(x)
                x = x + 1

            #Add each element in digits[] to digits[0]
            for n in digits:
                digits[0] = digits[0] + digits[z]

                #If digits[0] has more than one digit, then run digital_root with digits[0] in the parameters
                if digits[0] > 9:
                    digital_root(digits[0])
                else:
                    return digits[0]
        else:
            return num

digital_root(15)


>> Program finished with exit code 0