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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _get_tweets(self, tweets_key, max_cnt_tweets):
 """Get at most `max_cnt_tweets` tweets from the Redis list `tweets_key`. Parameters tweets_key: str The key of the Redis list which stores the tweets. max_cnt_tweets: int The maximum number of tweets included in the returned list. If it is set to -1, then all the available tweets will be included. Returns ------- tweets A list of tweets """

        tweets = []
        if max_cnt_tweets == 0:
            return tweets
        elif max_cnt_tweets == -1:
            # Return all the tweets in the timeline.
            last_tweet_index = -1
        else:
            # Return at most max_cnt_tweets tweets.
            last_tweet_index = max_cnt_tweets - 1

        # Get the post IDs of the tweets.
        post_ids = self._rc.lrange(tweets_key, 0, last_tweet_index)

        if not post_ids:
            return tweets

        with self._rc.pipeline() as pipe:
            # Get the tweets with their user IDs and UNIX timestamps.
            pipe.multi()
            for post_id in post_ids:
                post_id_key = pytwis_constants.TWEET_KEY_FORMAT.format(post_id)
                pipe.hgetall(post_id_key)
            tweets = pipe.execute()

            # Get the userid-to-username mappings for all the user IDs associated with the tweets.
            userid_set = {tweet[pytwis_constants.TWEET_USERID_KEY] for tweet in tweets}
            userid_list = []
            pipe.multi()
            for userid in userid_set:
                userid_list.append(userid)
                userid_key = pytwis_constants.USER_PROFILE_KEY_FORMAT.format(userid)
                pipe.hget(userid_key, pytwis_constants.USERNAME_KEY)
            username_list = pipe.execute()

        userid_to_username = {userid: username for userid, username in\
                              zip(userid_list, username_list)}

        # Add the username for the user ID of each tweet.
        for tweet in tweets:
            tweet[pytwis_constants.USERNAME_KEY] = \
                userid_to_username[tweet[pytwis_constants.TWEET_USERID_KEY]]

        return tweets