[Python] How to test Retry in Celery application in Python?

I'm trying to test if the application is retrying.

@celery.task(bind=False, default_retry_delay=30)
def convert_video(gif_url, webhook):
// doing something
VideoManager().convert(gif_url)
return
except Exception as exc:
raise convert_video.retry(exc=exc)


And I'm mocking the test

@patch('src.video_manager.VideoManager.convert')
@patch('requests.post')
def test_retry_failed_task(self, mock_video_manager, mock_requests):
mock_video_manager.return_value= {'webm':'file.webm', 'mp4':'file.mp4', 'ogv' : 'file.ogv', 'snapshot':'snapshot.png'}
mock_video_manager.side_effect = Exception('some error')

server.convert_video.retry = MagicMock()

server.convert_video('gif_url', 'http://www.company.com/webhook?attachment_id=1234')
server.convert_video.retry.assert_called_with(ANY)


And I'm getting this error

TypeError: exceptions must be old-style classes or derived from BaseException, not MagicMock
​

Which is obvious but I don't know how to do it otherwise to test if the method is being called.

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