input: Write a function to determine if a given string contains only digits. If the string contains only digits, return True, 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("0a23")==False
  assert is_digit_only("")==True
  assert is_digit_only("123456")==True
  assert is_digit_only("123.456")==False
output: def is_digit_only(string):
      for char in string:
          if not char.isdigit():
              return False
      return True