web.py development has stagnated lately. It's a great framework that many other Python frameworks used as inspiration, such as the webapp framework on Google App Engine and Tornado (and even Flask). However, Flask is more modern, under more active development, and is extensively documented. It's simple to use like web.py, and I would argue it's one of the cleanest Python Web frameworks out there.

Tornado has a very similar feel to web.py, yet can do much more when you are ready for it. If you like web.py you might as well use tornado.

Tornado is a different animal -- it's asynchronous so when you write code for it you have to program using callbacks rather than in a traditional style.

My current project requires real-time/always-on connections so I started to develop it using Tornado and then decided to switch to the Quora model -- use a traditional Web framework like Flask for most things, and connect back to Tornado for the real-time stuff.

When I switched to this model, the development process sped up considerably. In addition to just being really-well designed, Flask has an amazing debugger that makes me more productive, and it's easier to write unittests for Flask because you can write them in a traditional way and don't have to contend with Tornado's IOLoop.

For real-time stuff, you could forgo Tornado all together and instead use gevent to deploy your Flask app (http://flask.pocoo.org/docs/deploying/others/#gevent), like some have done with Django and Pyramid, but I haven't tried this yet.

You don't need to write tornado in an async fashion. You can use it like you would Flask, but its got a more similar feel to web.py than Flask.