input: Write a python function to prompt the user to input a number and handle invalid input by re-prompting until a valid integer is entered.
  assert isinstance(input_num(), int)
  assert input_num() != None
output: def input_num():
      while True:
          try:
              return int(input("Please enter a number: "))
          except ValueError:
              print("That's not a valid number. Please try again.")