input: Write a function to check whether a string contains only digits. Return True if all characters in the string are digits, otherwise return False. If the input 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("123abc")==False
  assert is_digit_only("")==True
output: def is_digit_only(string):
      for char in string:
          if not char.isdigit():
              return False
      return True