This function returns a list of all sentiments of the tweets of a
        specified user.

def user_sentiments(
        self,
        username = None
        ):
        """
        This function returns a list of all sentiments of the tweets of a
        specified user.
        """
        try:
           return [tweet.sentiment for tweet in self if tweet.username == username]
        except:
            log.error("error -- possibly no username specified")
            return None