Tornado app launch in PythonAnywhere failed with "Unhandled Exception"

Question

I tried code from a tutorial in the PythonAnywhere help-pages:

• Using tornado | PythonAnywhere help

In the information section, they explained that they need to see this code and paste it into the WSGI application file. After restarting it should indicate that the application is running.

Code

import tornado.web
import tornado.wsgi

class MainHandler(tornado.web.RequestHandler):
    def get(self):
        self.write("Hello from Tornado")

application = tornado.wsgi.WSGIApplication([
    (r"/", MainHandler),
])

I tried to launch the web application written in Tornado on the PythonAnywhere service, expecting that I would succeed.

Application not running, error shown

However, after following all the instructions, the web application did not launch. Instead I see this error:

There was an error loading your PythonAnywhere-hosted site. There may be a bug in your code.
Error code: Unhandled Exception"

Question

Can you explain why the application launch failed? Maybe with an example of how to properly run an application using Tornado.