how to get 'n' tweets from twitter of any user

I m new in python. help me out. THE PROBLEM: i got all tweets of respective user. I want n [suppose(n=20)] no of latest tweet of respective user.how to fetch n no of tweets ?? below is my code:

def get_all_tweets(screen_name):


    auth = tweepy.OAuthHandler(consumer_key, consumer_secret)
    auth.set_access_token(access_key, access_secret)
    api = tweepy.API(auth)

    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)

    alltweets.extend(new_tweets)
    print len(alltweets)

    oldest = alltweets[-1].id - 1

    while len(new_tweets) > 0:
        print "getting tweets before %s" % (oldest)
        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))



    f = open('testfile.txt', 'w+')
    for tweet in alltweets:
        f.write(str(tweet.text))
        f.write('\n')
        f.close()
    tweets = (tweet.text)
    return tweets
    pass
# to encode the data
reload(sys)
sys.setdefaultencoding('utf-8')

if __name__ == '__main__':
    get_all_tweets("xyz")