input: Write a function to check whether a string contains only digits. If the string contains any non-digit characters, return False. If the string is empty or contains only digits, return True.
  assert is_digit_only("0123")==True
  assert is_digit_only("012a3")==False
  assert is_digit_only("")==True
  assert is_digit_only("123.45")==False
  assert is_digit_only("789")==True
output: def is_digit_only(s):
      return s.isdigit()