sqlalchemy模拟side_effect引发异常

Question

我正在尝试模拟一个引发异常的函数，但是它不起作用。

class TestGetPos(unittest.TestCase):

    @patch('db.apis._create_engine')     
    def test_get_all_pos(self, mock_sqlalchemy_engine):

        mock_sqlalchemy_engine.return_value.execute.return_value.fetchall.return_value = [1,2,3]                
        assert get_all_pos(None) == [1,2,3]        

        mock_sqlalchemy_engine.return_value.execute('testqry').side_effect = DBAPIError
        print(get_all_pos(None))
        with self.assertRaises(DBAPIError):
            print(get_all_pos(None))




import sqlalchemy

def _create_engine(app):    
    impac_engine = sqlalchemy.create_engine(        
        app.config['DB'],
        poolclass=sqlalchemy.pool.NullPool  # this setting enables NOT to use Pooling, preventing from timeout issues.
    )
    return impac_engine

def get_all_pos(app):
    engine = _create_engine(app)
    qry = """SELECT DISTINCT id, name FROM po_t ORDER BY name ASC"""
    try:
        cursor = engine.execute(qry)
        rows = cursor.fetchall()
        return rows
    except Exception as re:
        raise re