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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 async def evaluate(self):
 """Evaluate the query observer. :param return_emitted: True if the emitted diffs should be returned (testing only) """


        @database_sync_to_async
        def remove_subscribers():
            models.Observer.subscribers.through.objects.filter(
                observer_id=self.id
            ).delete()

        @database_sync_to_async
        def get_subscriber_sessions():
            return list(
                models.Observer.subscribers.through.objects.filter(observer_id=self.id)
                .distinct('subscriber_id')
                .values_list('subscriber_id', flat=True)
            )

        try:
            settings = get_queryobserver_settings()

            start = time.time()
            # Evaluate the observer
            added, changed, removed = await database_sync_to_async(self._evaluate)()
            duration = time.time() - start

            # Log slow observers.
            if duration > settings['warnings']['max_processing_time']:
                self._warning("Slow observed viewset", duration=duration)

            # Remove subscribers of really slow observers.
            if duration > settings['errors']['max_processing_time']:
                logger.error(
                    "Removing subscribers to extremely slow observed viewset ({})".format(
                        self._get_logging_id()
                    ),
                    extra=self._get_logging_extra(duration=duration),
                )
                await remove_subscribers()

            if self._meta.change_detection == Options.CHANGE_DETECTION_POLL:
                # Register poller.
                await get_channel_layer().send(
                    CHANNEL_MAIN,
                    {
                        'type': TYPE_POLL,
                        'observer': self.id,
                        'interval': self._meta.poll_interval,
                    },
                )

            message = {
                'type': TYPE_ITEM_UPDATE,
                'observer': self.id,
                'primary_key': self._meta.primary_key,
                'added': added,
                'changed': changed,
                'removed': removed,
            }

            # Only generate notifications in case there were any changes.
            if added or changed or removed:
                for session_id in await get_subscriber_sessions():
                    await get_channel_layer().group_send(
                        GROUP_SESSIONS.format(session_id=session_id), message
                    )

        except Exception:
            logger.exception(
                "Error while evaluating observer ({})".format(self._get_logging_id()),
                extra=self._get_logging_extra(),
            )