带有pymongo和$ ne:null的SyntaxError

这是我的代码:

#! /usr/bin/python

import os
from pymongo.connection import Connection
from pymongo.master_slave_connection import MasterSlaveConnection


database = 'toto'
collection = 'logs'

master = Connection(host="X.X.X.X", port=27017)
slave1 = Connection(host="X.X.X.X", port=27017)
con = MasterSlaveConnection(master, slaves=[slave1, master])

db = getattr(con,database)

#host_name.append("getattr(db,collection).distinct( 'host_name' )")
#print host_name[1]

hosts = db.logs.distinct( 'host_name' )

services = db.logs.distinct("service_description" , { "service_description" : { $ne : null } } )

#print hosts
print services

我收到了这个错误:

File "./rapport.py", line 23
    services = db.logs.distinct("service_description" , { "service_description" : { $ne : null } } )
                                                                                    ^
SyntaxError: invalid syntax

为什么我不能"$ne : null"在我的代码中使用？我不明白,因为当我"db.logs.distinct("service_description" , { "service_description" : { $ne : null } } )"直接在mongodb中执行此查询时,它可以工作.

我也试过这个,但它不起作用:

services = db.logs.distinct("service_description", { "service_description" : { "$ne" : None } } )

谢谢你的帮助.