I agree with Yuri.

`Task.set_exception()` (let's assume it works) is very dangerous: if cancellation exception doesn't bubble up from coroutine code there is a very high chance to get broken invariants and not-released resources.

The same situation is possible with classic threads: killing a thread without unwinding a call stack leads to locked mutexes etc.

Regarding distinguishing explicit cancellation from timeout exhausting: it can be done with current asyncio design by using a flag. Take a look on async_timeout context manager, __aexit__() implementation: https://github.com/aio-libs/async-timeout/blob/master/async_timeout/__init__.py#L88-L97