class MongoClient(pymongo.common.BaseObject)
Connection to MongoDB.
Method resolution order:
MongoClient
pymongo.common.BaseObject
__builtin__.object
class Connection(pymongo.mongo_client.MongoClient)
Connection to MongoDB.
Method resolution order:
Connection
pymongo.mongo_client.MongoClient
pymongo.common.BaseObject
__builtin__.object
我们先看一下源码，从这两个类的继承来看，connection是继承了MongoClient的，建议使用MongoClient而不是使用Connection。（也就是说，MongoClient可以使用方法Connection都可以使用）
from pymongo import MongoClient
client = MongoClient('192.168.40.87', 27037)
db_name = 'TCL_Useraction'
db = client[db_name]
collection_useraction = db['useraction']
这里是通过字典的方式访问数据库和集合，同时你也可以通过.（点号）的方式访问
做实验时，发现Python用Connection()连接时，修改操作速度非常快，而用MongoClient()建立的连接，操作速度慢很多。
from pymongo import MongoClient,Connection
import time
db = Connection('192.168.1.101', 27017).performance_test
#client = MongoClient('192.168.1.101', 27017)
#db = client.performance_test
db.drop_collection("updates")
collection = db.updates
collection.insert({"x": 1})
collection.find_one()
start = time.time()
for i in range(100000):
collection.update({}, {"$push" : {"x" : 1}})
...
>python test_mongo_conn.py
8.43799996376
>python test_mongo_client.py
62.5780000687
用Connection() 8秒，MongoClient()则花了62秒，这性能相差也太多了。
很是疑惑，于是查了pymongo的文档，原来两者有个选项的默认行为不同：
class pymongo.connection.Connection([host='localhost'[,port=27017[,max_pool_size=10[,network_timeout=None[, document_class=dict[, tz_aware=False[, **kwargs]]]]]]])
Write Concern options:
safe: Connection disables acknowledgement of write operations. Use safe=True to enable write acknowledgement.
w: (integer or string) If this is a replica set, write operations will block until they have been replicated to the specified number or tagged set of servers. w=<int> always includes the replica set primary (e.g. w=3 means write to the primary and wait until replicated to two secondaries). Implies safe=True.
wtimeout: (integer) Used in conjunction with w. Specify a value in milliseconds to control how long to wait for write propagation to complete. If replication does not complete in the given timeframe, a timeout exception is raised. Implies safe=True.
j: If True block until write operations have been committed to the journal. Ignored if the server is running without journaling. Implies safe=True.
fsync: If True force the database to fsync all files before returning. When used with j the server awaits the next group commit before returning. Implies safe=True.
safe选项决定操作是“瞬时完成”与“安全操作”，connection()默认是safe=False,即瞬时完成，不等服务器回应，而MongoClient()默认是safe=True，即安全操作，等服务器确认后才继续下一步操作。
所以一个8秒，一个62秒，这个差距实际上是“瞬时完成”与“安全操作”两者的性能差别。
当将Connection() 和MongoClient()建立连接时指定相同的safe参数，两者的性能表现是一样的。
client = MongoClient('192.168.1.101', 27017,safe=False)
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