Yield screen_name, string tuples, where the string is the
    concatenation of all tweets of this user.

def extract_tweets(json_file):
    """ Yield screen_name, string tuples, where the string is the
    concatenation of all tweets of this user. """
    for screen_name, tweet_iter in groupby(parse_json(json_file), lambda x: x[0]):
        tweets = [t[1] for t in tweet_iter]
        yield screen_name, ' '.join(tweets)