Hi Leen,
I think you already posted another question around the same problem.
If you build a web application with OutSystems, you have to host and run it form an OutSystems environment.
Normally backend and frontend is developed in OutSystems.
If you have another platform you use for your front-end, then in OutSystems you could still develop the backend and have develop one ore more API's to expose the backend functionality, that than can be consumed in your other development tool.
The other way around, if you want to use OutSystems as a front end, with another framework like Python Tornado as backend, then your Python solution needs to expose web services that OutSystems front end can consume.
Out of curiosity, why are you using OutSystems together with Python Tornado?
Regards,
Daniel