input: Write a function to check if a string contains only digits and return True if it does, otherwise return False. If the string is empty, return True.
  def is_digit_only(string):
      for char in string:
          if not char.isdigit():
              return False
      return True
  
  assert is_digit_only("0123")==True
  assert is_digit_only("1a3b")==False
  assert is_digit_only("456")==True
  assert is_digit_only("")==True
  assert is_digit_only("7890")==True
output: def is_digit_only(string):
      for char in string:
          if not char.isdigit():
              return False
      return True