+1 I used Tornado in the past because it had an async event loop before it was native to Python. So much simpler than having to set up wsgi servers etc with the added benefit of easily-configured background tasks (like having cron jobs built in to your application). This is really good when you're building an application that you want other people to be able to install and run easily.

It seems that Tornado is now offering the choice of its own event loop or the asyncio event loop. I built something recently in aiohttp because it felt like it had been built on asyncio from the ground up but will explore Tornado again.