pip install Flask

from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello_world():
    return 'Hello World!'

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

pip install web.py

import web

urls = (
    '/', 'index'
)

class index:
    def GET(self):
        return "Hello, world!"

if __name__ == "__main__":
    app = web.application(urls, globals())
    app.run()

pip install tornado

from tornado.web import Application
from tornado.web import RequestHandler
from tornado.ioloop import IOLoop


class IndexHandler(RequestHandler):
    def get(self):
        self.write("hello world")


handlers = [
    ("/", IndexHandler),
]

if __name__ == '__main__':
    app = Application(handlers)
    app.listen(8080)  # http://localhost:8080/
    IOLoop.current().start()