Python operator, built-in function problem

The phenomenon and background of the problem

function main() Receive a positive integer num, Requires a positive integer to be returned num The sum of your numbers .
No modules can be imported , Circular structures and any form of derivation cannot be used , You can't use built-in functions eval().

Problem related code , Do not paste screenshots

def main(num):

What I want to achieve

main(123)
6