How to use the cryptostore.aggregator.util.book_flatten function in cryptostore

if start and end:
            data = [[key, self.conn.xrange(key, min=start, max=end)]]
        else:
            data = self.conn.xread({key: '0-0' if key not in self.last_id else self.last_id[key]})

        if len(data) == 0 or len(data[0][1]) == 0:
            return []

        LOG.info("%s: Read %d messages from Redis", key, len(data[0][1]))
        ret = []

        for update_id, update in data[0][1]:
            if dtype in {L2_BOOK, L3_BOOK}:
                update = json.loads(update['data'])
                update = book_flatten(update, update['timestamp'], update['receipt_timestamp'], update['delta'])
                for u in update:
                    for k in ('size', 'amount', 'price', 'timestamp', 'receipt_timestamp'):
                        if k in u:
                            u[k] = float(u[k])
                ret.extend(update)
            elif dtype in {TRADES, TICKER, OPEN_INTEREST}:
                for k in ('size', 'amount', 'price', 'timestamp', 'receipt_timestamp', 'bid', 'ask', 'open_interest'):
                    if k in update:
                        update[k] = float(update[k])
                ret.append(update)
            elif dtype == FUNDING:
                for k in update:
                    try:
                        update[k] = float(update[k])
                    except ValueError:
                        # ignore strings

def read(self, exchange, dtype, pair):
        key = f'{dtype}-{exchange}-{pair}'
        data = self._conn(key).consume(1000000, timeout=0.5)
        LOG.info("%s: Read %d messages from Kafka", key, len(data))
        ret = []
        for message in data:
            self.ids[key] = message
            update = json.loads(message.value().decode('utf8'))

            if dtype in {L2_BOOK, L3_BOOK}:
                update = book_flatten(update, update['timestamp'], update['delta'])
                ret.extend(update)
            if dtype in {TRADES, TICKER, FUNDING, OPEN_INTEREST}:
                ret.append(update)
        return ret