input: Write a function to output a countdown from a given positive integer down to 1 using a while loop.
  def countdown(num):
      while num > 0:
          print(num)
          num -= 1
      print("Done!")
  
  assert countdown(6) # Output: 6 \n 5 \n 4 \n 3 \n 2 \n 1 \n Done!
output: def countdown(num):
      while num > 0:
          print(num)
          num -= 1
      print("Done!")