Communication between python script and Arduino sketch I'm trying to transfer data beetween the two scripts. I'm already using the YunMesseger library (https://github.com/plotly/YunMessenger). But it only make the 

Arduino -> Python

communication.
I would like to know how to make the reverse way.

I would like to point that I'm search for a solution that doesn't involve the use of the Bridge, because I don't want to have concurrency problems.
One option is to use spacebrew. This is a cloud-based service, however you can run your own server which is based on node.js, so it should work on the Yun.

Your Arduino can then send or receive data using yunSpacebrew, and you can connect it to python using pySpacebrew. This is a pure python library, so it ought to run fine on the Yun.