> - modify tests to set the event loop to the newly created event loop, instead of setting it to None

I'm not sure that this particular change is a great idea. I kind of liked that unittests ensure that loop is passed everywhere explicitly in asyncio.


> - modify get_event_loop() to always raise a RuntimeError if the thread has no event loop. Before an AssertionError was not raised if python runs with -O option

+1.


> - modify BaseEventLoop._assert_is_current_event_loop() to fail if the thread has an event loop

Hm, I think I don't understand why this function doesn't to anything when there is no loop in the thread... Let's fix it. In your patch, you should also replace 'except AsserionError' with 'except RuntimeError'.


> Does it sound reasonable to raise an exception if an event loop is used from the wrong thread?

I think we should in debug mode at least.


> Getting thread ID or current thread may be expensive.

Victor, can you benchmark this?  I'm pretty sure that Guido is right about this, but sometimes syscalls are pretty fast.  I doubt that a lot of people know about debug mode, so if it has a negligible cost I'd do the check every time.