input: Write a function to calculate the absolute difference between two given numbers. If the numbers given are identical, the function should return 0. If only one of the numbers is provided, the function should return the absolute value of that number.
  assert absolute_difference(9, 5)==4
  assert absolute_difference(5, 5)==0
  assert absolute_difference(-3, 3)==6
  assert absolute_difference(10, None)==10
  assert absolute_difference(None, -4)==4
output: def absolute_difference(a=None, b=None):
      if a is None and b is None:
          return 0
      elif a is None:
          return abs(b)
      elif b is None:
          return abs(a)
      else:
          return abs(a - b)