def clear(
            self  # type: ORMTask
    ):
        """Delete all objects created by this task.

        Iterate over `self.object_classes` and delete all objects of the listed classes.
        """
        # mark this task as incomplete
        self.mark_incomplete()

        # delete objects
        for object_class in self.object_classes:
            self.session.query(object_class).delete()

        self.close_session()