I have the bulk of my web application in React (front-end) and Node (server), and am trying to use Python for certain computations. My intent is to send data from my Node application to a Python web service in JSON format, do the calculations in my Python web service, and send the data back to my Node application. 
Flask looks like a good option, but I do not intend to have any front-end usage for my Python web service. Would appreciate any thoughts on how to do this.
In terms of thoughts:
1) You can build a REST interface to your python code using Flask. Make REST calls from your nodejs. 
2) You have to decide if your client will wait synchronously for the result. If it takes a relatively long time you can use a web hook as a callback for the result.
Python web service with React/Node Application