事务隔离级别&悲观乐观锁

本文是对https://github.com/ScrappyZhang/python_web_Crawler_DA_ML_DL/blob/master/web%E5%85%A8%E6%A0%88%E5%BC%80%E5%8F%91/Django%E7%94%B5%E5%95%86%E7%BD%91%E7%AB%99%E5%BC%80%E5%8F%91%E5%AE%9E%E4%BE%8B/19_%E

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