code: def readline(self, fmt=None):
          
          prefix_size = self._fix()
  
          if fmt is None:
              content = self.read(prefix_size)
          else:
              fmt = self.endian + fmt
              fmt = _replace_star(fmt, prefix_size)
              content = struct.unpack(fmt, self.read(prefix_size))
  
          try:
              suffix_size = self._fix()
          except EOFError:
              # when endian is invalid and prefix_size > total file size
              suffix_size = -1
  
          if prefix_size != suffix_size:
              raise IOError(_FIX_ERROR)
  
          return content
docs: Return next unformatted "line". If format is given, unpack content,
          otherwise return byte string.