input: Write a function to take in a positive integer and use a while loop to output the numbers counting down to 1. If the input is not a positive integer, return an appropriate message.
  assert countdown_to_one(6)==None
  assert countdown_to_one(0)=="Please enter a positive integer."
  assert countdown_to_one(-5)=="Please enter a positive integer."
  assert countdown_to_one(1)==None
  assert countdown_to_one(10)==None
output: def countdown_to_one(n):
      if n <= 0:
          return "Please enter a positive integer."
      while n > 0:
          print(n)
          n -= 1