code: def fd(self):
          
          if isinstance(self._rlist.path_or_fd(), string_types()):
              raise ValueError("File descriptor queried although mapping was generated from path")
          # END handle type
          return self._rlist.path_or_fd()
docs: :return: file descriptor used to create the underlying mapping.
  
          **Note:** it is not required to be valid anymore
          :raise ValueError: if the mapping was not created by a file descriptor