How to use the tasklib.task.Task.NotSaved function in tasklib

def test_set_unsaved_task_as_dependency(self):
        # Adds only one dependency to task with no dependencies
        t = Task(self.tw, description='test task')
        dependency = Task(self.tw, description='needs to be done first')

        # We only save the parent task, dependency task is unsaved
        t.save()
        t['depends'] = set([dependency])

        self.assertRaises(Task.NotSaved, t.save)

def start(self):
        if not self.saved:
            raise Task.NotSaved(
                'Task needs to be saved before it can be started',
            )

        # Refresh, and raise exception if task is already completed/deleted
        self.refresh(only_fields=['status'])

        if self.completed:
            raise Task.CompletedTask('Cannot start a completed task')
        elif self.deleted:
            raise Task.DeletedTask('Deleted task cannot be started')
        elif self.active:
            raise Task.ActiveTask('Task is already active')

        self.backend.start_task(self)

        # Refresh the status again, so that we have updated info stored

def delete(self):
        if not self.saved:
            raise Task.NotSaved(
                'Task needs to be saved before it can be deleted',
            )

        # Refresh the status, and raise exception if the task is deleted
        self.refresh(only_fields=['status'])

        if self.deleted:
            raise Task.DeletedTask('Task was already deleted')

        self.backend.delete_task(self)

        # Refresh the status again, so that we have updated info stored
        self.refresh(only_fields=['status', 'start', 'end'])

def stop(self):
        if not self.saved:
            raise Task.NotSaved(
                'Task needs to be saved before it can be stopped',
            )

        # Refresh, and raise exception if task is already completed/deleted
        self.refresh(only_fields=['status'])

        if not self.active:
            raise Task.InactiveTask('Cannot stop an inactive task')

        self.backend.stop_task(self)

        # Refresh the status again, so that we have updated info stored
        self.refresh(only_fields=['status', 'start'])

def serialize_depends(self, cur_dependencies):
        # Check that all the tasks are saved
        for task in (cur_dependencies or set()):
            if not task.saved:
                raise Task.NotSaved(
                    'Task \'%s\' needs to be saved before '
                    'it can be set as dependency.' % task,
                )

        return super(Task, self).serialize_depends(cur_dependencies)

def remove_annotation(self, annotation):
        if not self.saved:
            raise Task.NotSaved('Task needs to be saved to remove annotation')

        if isinstance(annotation, TaskAnnotation):
            annotation = annotation['description']

        self.backend.denotate_task(self, annotation)
        self.refresh(only_fields=['annotations'])

def add_annotation(self, annotation):
        if not self.saved:
            raise Task.NotSaved('Task needs to be saved to add annotation')

        self.backend.annotate_task(self, annotation)
        self.refresh(only_fields=['annotations'])

def refresh(self, only_fields=None, after_save=False):
        # Raise error when trying to refresh a task that has not been saved
        if not self.saved:
            raise Task.NotSaved('Task needs to be saved to be refreshed')

        new_data = self.backend.refresh_task(self, after_save=after_save)

        if only_fields:
            to_update = dict(
                [(k, new_data.get(k)) for k in only_fields],
            )
            self._update_data(to_update, update_original=True)
        else:
            self._load_data(new_data)

def done(self):
        if not self.saved:
            raise Task.NotSaved(
                'Task needs to be saved before it can be completed',
            )

        # Refresh, and raise exception if task is already completed/deleted
        self.refresh(only_fields=['status'])

        if self.completed:
            raise Task.CompletedTask('Cannot complete a completed task')
        elif self.deleted:
            raise Task.DeletedTask('Deleted task cannot be completed')

        self.backend.complete_task(self)

        # Refresh the status again, so that we have updated info stored
        self.refresh(only_fields=['status', 'start', 'end'])