Get best answer to a question.

        :param query: A question to get an answer
        :type query: :class:`str`

        :returns: An answer to a question
        :rtype: :class:`str`

        :raises: :class:`NoAnswerError` when can not found answer to a question

def get_best_answer(self, query):
        """Get best answer to a question.

        :param query: A question to get an answer
        :type query: :class:`str`

        :returns: An answer to a question
        :rtype: :class:`str`

        :raises: :class:`NoAnswerError` when can not found answer to a question

        """
        query = to_unicode(query)
        session = self.Session()

        grams = self._get_grams(session, query)
        if not grams:
            raise NoAnswerError('Can not found answer')

        documents = set([doc for gram in grams for doc in gram.documents])

        self._recalc_idfs(session, grams)

        idfs = dict((gram.gram, gram.idf) for gram in grams)

        docs = dict(
            (doc.answer, _cosine_measure(idfs, self._get_tf_idfs(doc)))
            for doc in documents)
        docs = dict((key, val) for (key, val) in docs.items() if val)

        session.commit()

        try:
            max_ratio = max(docs.values())
            answers = [answer for answer in docs.keys()
                       if docs.get(answer) == max_ratio]

            answer = random.choice(answers)
            logger.debug('{0} -> {1} ({2})'.format(query, answer, max_ratio))
            return (answer, max_ratio)
        except ValueError:
            raise NoAnswerError('Can not found answer')
        finally:
            session.commit()