input: Write a function to output numbers counting down from a positive integer to 1 using a while loop. If the integer is not positive, the function should not output any numbers but display a message indicating the input was invalid.
  
  def count_down(num):
      if num <= 0:
          print("Invalid input: please enter a positive integer.")
          return
      while num > 0:
          print(num)
          num -= 1
      print("Done!")
  
  assert count_down(6)
  # Expected output:
  # 6
  # 5
  # 4
  # 3
  # 2
  # 1
  # Done!
  
  assert count_down(-3)
  # Expected output:
  # Invalid input: please enter a positive integer.
output: def count_down(num):
      if num <= 0:
          print("Invalid input: please enter a positive integer.")
          return
      while num > 0:
          print(num)
          num -= 1
      print("Done!")