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Solve the following problem using Python, implementing the functions described below, one line at a time
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<USER_TASK:>
Description:
 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