code: def file_signature(file_name: str) -> Optional[Tuple]:
      
      try:
          st = os.stat(file_name)
      except FileNotFoundError:
          return None
      return stat.S_IFMT(st.st_mode), st.st_size, st.st_mtime
docs: Return an identity signature for file name
      :param file_name: name of file
      :return: mode, size, last modified time if file exists, otherwise none