> Yes, I agree with you about my weird way of debugging. But anyway, changing API with adding ability to pass actual cause would be welcome.

I'm not opposed to the idea, btw. If we do decide to add an argument to 'cancel', we probably should do the same for concurrent.futures.

Another possibility would be to allow cancellation via Future.set_exception:

  task.set_exception(asyncio.CancelledError('message'))