input: Write a function to create a Python program that takes in a positive integer and uses a while loop to output the numbers counting down to 1. If the input is not a positive integer, the program should print an error message.
  def count_down(num):
      if num > 0:
          while num > 0:
              print(num)
              num -= 1
          print("Done!")
      else:
          print("Invalid input: Please enter a positive integer.")
  
  assert count_down(6)==None
  assert count_down(-1)=="Invalid input: Please enter a positive integer."
  assert count_down(0)=="Invalid input: Please enter a positive integer."
  assert count_down(123)==None
output: def count_down(num):
      if num > 0:
          while num > 0:
              print(num)
              num -= 1
          print("Done!")
      else:
          print("Invalid input: Please enter a positive integer.")