How to communicate from a Python client to Ruby on Rails application?
Ruby on Rails 5 supports API only app, if you are using this app for APIs only, else just create a normal app and expose API endpoints that render json outputs. Use Active Record for mapping your MySql database and jbuilder for json views (both of which are available by default when you create new app). You will find lots of tutorials if you google Use Ruby on Rails as API app.
Good luck with it.. :)
I am to develop an app, and I have to choose between Ruby on Rails or Python+Django. So far I want to do it with Ruby on Rails, because i feel more comfortable doing what I have to do on it. 
But there is a problem. There would be a client app written on Python that has to communicate with mine. Cause that app should be able to communicate with mine.
First: I think if it is a matter of communicating with the MySQL database there wouldn't be an issue, cause the Python app is able to query to MySQL server with proper authentication right?
Second: and more important question: If I have a Ruby written API, to ease the queries, Could the Python app be able to invoke functions in that API and get the results? If it is possible, How could I achieve that?