从SQLAlchemy对象构建词典列表的最快方法

Question

我有一段代码将SQLAlchemy对象处理成Python字典。在下面的代码中，agg，containers和sales都是SQLAlchemy对象，其结构如下：

agg -- (one to many) --> containers -- (one to many) --> sales

当前，此循环的每次迭代大约需要0.1秒。反正有优化的方法吗？

_json = {}
_json['period_sales'] = []

for container in agg.containers:
    s = time.time()
    for sale in container.sales:
        _json['period_sales'].append({
            'start_date': None,
            'end_date': None,
            'sale': sale.purchases
        })
    print('{}'.format(time.time() - s))