Flask 中使用 PyMongo 报错 ServerSelectionTimeoutError: No servers found yet

在写接口的时候，因为当初的弱智，没有选择 Flask-PyMongo 等库在初始化 app 的时候一并初始化 mongo 示例，而是在每个接口里创建一个 mongoClient。。

mongo = MongoClient(mongo_uri, connect=False)

最近新业务里，需要一个会被频繁访问，且会进行数据库查询的接口。

 ```
 # 这是逻辑函数的开始
 device_id = query_data['deviceId']
 collection = mongo.app.user
 
 user_data = collection.find_one({"deviceId": device_id})
 # 后面还有很多但是没有必要所以省略了
 ```

然后发现该接口会偶尔出现下面的报错信息，但是其他会使用到 Mongo 的接口却没有出现过这个问题。报错信息如下：

 ```
 Traceback (most recent call last):
   File "/root/py27test/lib/python2.7/site-packages/flask/app.py", line 2292, in wsgi_app
     response = self.full_dispatch_request()
   File "/root/py27test/lib/python2.7/site-packages/flask/app.py", line 1815, in full_dispatch_request
     rv = self.handle_user_exception(e)
   File "/root/py27test/lib/python2.7/site-packages/flask/app.py", line 1718, in handle_user_exception
     reraise(exc_type, exc_value, tb)
   File "/root/py27test/lib/python2.7/site-packages/flask/app.py", line 1813, in full_dispatch_request
     rv = self.dispatch_request()
   File "/root/py27test/lib/python2.7/site-packages/flask/app.py", line 1799, in dispatch_request
     return self.view_functions[rule.endpoint](**req.view_args)
   # 这里是接口的部分
   File "./utils/v5.py", line 47, in decorated_function
     return f(*args, **kwargs)
   File "./utils/v5.py", line 1081, in user_function
     data, error_message = query_module.query(query_data, mongo)
   File "./fetcher/tools_v4.py", line 81, in user_query
     user_data = collection.find_one({"deviceId": device_id})
   # 接口的部分结束
   File "/root/py27test/lib/python2.7/site-packages/pymongo/collection.py", line 1262, in find_one
     for result in cursor.limit(-1):
   File "/root/py27test/lib/python2.7/site-packages/pymongo/cursor.py", line 1189, in next
     if len(self.__data) or self._refresh():
   File "/root/py27test/lib/python2.7/site-packages/pymongo/cursor.py", line 1087, in _refresh
     self.__session = self.__collection.database.client._ensure_session()
   File "/root/py27test/lib/python2.7/site-packages/pymongo/mongo_client.py", line 1558, in _ensure_session
     return self.__start_session(True, causal_consistency=False)
   File "/root/py27test/lib/python2.7/site-packages/pymongo/mongo_client.py", line 1511, in __start_session
     server_session = self._get_server_session()
   File "/root/py27test/lib/python2.7/site-packages/pymongo/mongo_client.py", line 1544, in _get_server_session
     return self._topology.get_server_session()
   File "/root/py27test/lib/python2.7/site-packages/pymongo/topology.py", line 427, in get_server_session
     None)
   File "/root/py27test/lib/python2.7/site-packages/pymongo/topology.py", line 199, in _select_servers_loop
     self._error_message(selector))
 ServerSelectionTimeoutError: No servers found yet
 
 ```

[汗]搜索了 stackoverflow 的解决方法一个是connect=False,另一个是在 fork 前等待两秒。似乎不太适用，猛男落泪，所以来求助一哈各位大佬。

Python：2.7.5 PyMongo：3.7.1