def file_size(self):
        """:return: size of file we manager"""
        if self._file_size is None:
            if isinstance(self._path_or_fd, string_types()):
                self._file_size = os.stat(self._path_or_fd).st_size
            else:
                self._file_size = os.fstat(self._path_or_fd).st_size
            # END handle path type
        # END update file size
        return self._file_size