ops.py 文件源码

def downgrade_ops(self):
        """An instance of :class:`.DowngradeOps`.

        .. seealso::

            :attr:`.MigrationScript.downgrade_ops_list`
        """
        if len(self._downgrade_ops) > 1:
            raise ValueError(
                "This MigrationScript instance has a multiple-entry "
                "list for DowngradeOps; please use the "
                "downgrade_ops_list attribute.")
        elif not self._downgrade_ops:
            return None
        else:
            return self._downgrade_ops[0]