Adding up digits of an input number with recursion in Python

There are at least two mistakes in your code:

inputNum = inputNum % 10
digit_sum(inputNum / 10)

First, you're changing the value of inputNum when you still need that value in the next statement. Second, you're making a common recursion beginner's error in that if your function returns a value, you need to deal with that returned value when you call your function recursively. A working version of your code might read:

def digit_sum(inputNum):
    if inputNum < 10:
        return inputNum
    
    return inputNum % 10 + digit_sum(inputNum // 10)

We could also implement a different approach using the often forgotten divmod function:

def digit_sum(number):
    if number < 10:
        return number

    quotient, remainder = divmod(number, 10)
    return remainder + digit_sum(quotient)

There are two similar recursive digit summing problems that turn up on SO. One simply sums the digits. The other keeps reapplying the summing logic until the result boils down to a single digit. Providing example input and output with your quesion can help clarify which algorithm you're implementing.

This can be done simply with while loop like this,

def digit_sum(inputNum):
    result = 0
    while len(str(inputNum)) > 1:
        result += inputNum % 10
        inputNum //= 10
    return inputNum + result 

If you are only comfortable with recursion, try this.

def digit_sum(inputNum, result=0):
    if inputNum >= 10:
        result = inputNum % 10
        inputNum //= 10
        return result + digit_sum(inputNum, result)
    else:
        return inputNum

I found several issues:

• elif could be replaced with else
• return requires in else section
• else logic is incorrect

Here is corrected code:

def run():
    input_num = int(input("Enter an int: "))
    print(f"sum of digits of {input_num} is {digit_sum(input_num)}.")


def digit_sum(input_num):
    if input_num < 10:
        return input_num
    else:
        return int(input_num % 10) + digit_sum(int(input_num/10))


if __name__ == "__main__":
    run()

Sample output:

Enter an int: 678
sum of digits of 678 is 21.

Check the below code it's easy hopefully it will help

    def run():
       num = int(input("Enter an int: "))
       result = sum_of_digit(num) 
       print("Sum of digits in",num,"is", result) 

    def sum_of_digit( n ): 
       if n == 0: 
          return 0
       return (n % 10 + sum_of_digit(int(n / 10))) 




    if __name__ == "__main__":
        run()

As Green Cloak Guy says you are not returning any value in your elif statement. Therefore you will be getting a "None" result.

Also looking at your code it will only return a single value based on the initial value divided by ten. You may want to try:

retNum = inputNum % 10
baseNum = inputNum - retNum
return retNum + digit_sum(baseNum/10)

This will add the modulo value to whatever is returned by the recursion method. Hope I have been able to help