input: Write a function to calculate the sum of the digits in a given number. The function should return the sum of the digits. If the input number is negative, it should be treated as a positive number for calculation.
  assert sum_digits(123)==6
  assert sum_digits(4567)==22
  assert sum_digits(9999)==36
  assert sum_digits(0)==0
  assert sum_digits(-123)==6
output: def sum_digits(num):
      num = abs(num)
      return sum(int(digit) for digit in str(num))