Python Problem.

The digit sum of a number is the sum of the digits in the number when it is written out. For example, the digit sum of 52 is 7 (5 + 2) and the digit sum of 1000 is 1 (1 + 0 + 0 + 0).

Write a function that takes as argument a number and returns its digit sum.

(please add comments for understanding the code)