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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:
 def enabled(self):
 """Returns True if this notification is enabled based on the value of Notification model instance. Note: Notification names/display_names are persisted in the "Notification" model where each mode instance can be flagged as enabled or not, and are selected/subscribed to by each user in their user profile. See also: `site_notifications.update_notification_list` """

        if not self._notification_enabled:
            self._notification_enabled = self.notification_model.enabled
        return self._notification_enabled