If I run this code:

    import asyncio as a

    @a.coroutine
    def coro1():
        yield from a.ensure_future(coro2())
        print("Still here")
        yield from a.sleep(1)
        print("Still here 2")

    @a.coroutine
    def coro2():
        yield from a.sleep(1)
        res = task.cancel()
        print("Canceled task:", res)

    loop = a.get_event_loop()
    task = a.ensure_future(coro1())
    loop.run_until_complete(task)

I expect the task to stop shortly after a call to cancel(). It should surely stop when I try to sleep(). But it doesn't. On my machine this prints:

    Canceled task: True
    Still here
    Still here 2

So, cancel() returns True, but the task doesn't seem to be canceled.