How would I go about creating a web app using node.js (Express.js) that can access the python or ipython REPL command line? Essentially I would like to have something like the IPython Notebook, but integrated into a node.js web application. I was also thinking about using websockets for low-latency, two-way communication between the app and the python instance on the server.
Use ZMQ node binding, and talk directly to the kernel using same protocol than IPython, if you want IPython notebook for node you can create a node kernle (need to be updated for last protocol version). IPython webserver already have websocket and Emacs EIN alredy use it to speek to server through websocket.
[Edit]
Some clarification after comments. About the notebook, at first order when you open the notebook web app 3 component are involved.
(K) talk to (S) using ZMQ. (S) act as a bridge that doe ZMQ<->Websocket translation (C) Talk to S through websocket.
What goes on the wire on ZMQ and Websocket is language agnostic, so you don't care of the language of (K), (S) or (C). ZMQ and Websocket have a high change of not using the same transport, not using the same interface/ip, and not using the same port.