问题：
/site-packages/pymongo/topology.py:150: UserWarning: MongoClient opened before fork. Create MongoClient only after forking. See PyMongo's documentation for details: http://api.mongodb.org/python/current/faq.html#is-pymongo-fork-safe
"MongoClient opened before fork. Create MongoClient only "
官方文档说明：http://api.mongodb.com/python/current/faq.html#multiprocessing
大致意思是使用多进程中创建MongoDB链接时，要在每个进程中单独创建连接。