Python MongoDB-在Mongo的不同集合中存储推文时出错

我使用的是Python2.7，我可以使用下面的程序将tweets存储在CSV文件中，但我希望tweets存储在Mongo数据库中不同用户名的不同集合中

我有一个包含用户名的列表，我想将每个用户的tweet存储在同一个数据库中的单独集合中

下面是我提取推文并将其存储在CSV中的代码

# Load the required Libraries
import tweepy #https://github.com/tweepy/tweepy
import csv
import sys

#Twitter API credentials

consumer_key = "fRw12aumIqkAWD6PP5ZHk7vva"
consumer_secret = "K9K0yL2pwngp3JXEdMGWUOEB7AaGWswXcq72WveRvnD4ZSphNQ"
access_key = "771287280438968320-XnbtNtBt40cs6gUOk6F9bjgmUABM0qG"
access_secret = "afUppGRqcRi2p9fzLhVdYQXkfMEm72xduaWD6uNs3HhKg"

def get_all_tweets(screen_name):

    #authorize twitter, initialize tweepy
    auth = tweepy.OAuthHandler(consumer_key, consumer_secret)
    auth.set_access_token(access_key, access_secret)
    api = tweepy.API(auth)

    #initialize a list to hold all the tweepy Tweets
    alltweets = []    

    #make initial request for most recent tweets (200 is the maximum allowed count)
    new_tweets = api.user_timeline(screen_name = screen_name,count=200)

    #save most recent tweets
    alltweets.extend(new_tweets)

    #save the id of the oldest tweet less one
    oldest = alltweets[-1].id - 1

    #keep grabbing tweets until there are no tweets left to grab
    while len(new_tweets) > 0:
        print ("getting tweets before %s" % (oldest))

        #all subsiquent requests use the max_id param to prevent duplicates
        new_tweets = api.user_timeline(screen_name = screen_name,count=200,max_id=oldest)

        #save most recent tweets
        alltweets.extend(new_tweets)

        #update the id of the oldest tweet less one
        oldest = alltweets[-1].id - 1

        print ("...%s tweets downloaded so far" % (len(alltweets)))

    #transform the tweepy tweets into a 2D array that will populate the csv    
    user_tweet = [[tweet.id_str, tweet.created_at, tweet.text.encode("utf-8")] for tweet in alltweets]


    connection = MongoClient()
    db = connection.Stage2_DataBase

    db.screen_name.insert(user_tweet)

    pass

我想像这样储存推文

# usernames is the list of my users
for i in usernames:
    get_all_tweets(i)

错误：-->54
TypeError:列表索引必须是整数，而不是str