How do I create a web interface to a simple python script?

Have you considered Flask? Like Tornado, it is both a “micro-framework” and a simple web server, so it has everything you need right out of the box. http://flask.pocoo.org/

This example (right off the homepage) pretty much sums up how simple the code can be:

from flask import Flask
app = Flask(__name__)

@app.route("https://stackoverflow.com/")
def hello():
    return "Hello World!"

if __name__ == "__main__":
    app.run()