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Tornado Web Framework Cheatsheet

A comprehensive cheat sheet for the Tornado web framework, covering its core concepts, modules, and usage patterns for building asynchronous web applications in Python.

Core Concepts & Application Structure

Tornado Application

The tornado.web.Application class is the central component. It maps request handlers to URL patterns.

Example:

import tornado.ioloop
import tornado.web

class MainHandler(tornado.web.RequestHandler):
    def get(self):
        self.write("Hello, world")


def make_app():
    return tornado.web.Application([
        (r"/", MainHandler),
    ])

if __name__ == "__main__":
    app = make_app()
    app.listen(8888)
    tornado.ioloop.IOLoop.current().start()

Request Handlers

tornado.web.RequestHandler

Base class for request handlers. Override HTTP method functions (e.g., get(), post()).

self.write(chunk)

Writes a chunk of output to the client. Can be called multiple times.

self.render(template_name, **kwargs)

Renders a template using the given context.

self.get_argument(name, default=None, strip=True)

Returns the value of the argument with the given name.

self.set_header(name, value)

Sets an HTTP header.

URL Routing

URL patterns are defined as a list of tuples: (r"/path", HandlerClass, dict(kwargs), name)

  • r"/path": Regular expression to match the URL.
  • HandlerClass: The request handler class to use.
  • kwargs: A dictionary of keyword arguments to pass to the handler’s initialize method.
  • name: Name of the route, can be used with reverse_url()

Example:

app = tornado.web.Application([
    (r"/", MainHandler),
    (r"/user/([a-zA-Z0-9]+)", UserHandler, dict(database=db)),
])

Asynchronous Operations and IOLoop

IOLoop Basics

The tornado.ioloop.IOLoop is the core of Tornado’s asynchronous execution.

IOLoop.current() - Returns the current IOLoop instance.
IOLoop.start() - Starts the IOLoop, blocking until stop() is called.
IOLoop.stop() - Stops the IOLoop.

Example:

 tornado.ioloop.IOLoop.current().start()

Asynchronous HTTP Client

tornado.httpclient.AsyncHTTPClient

Non-blocking HTTP client for making asynchronous requests.

fetch(request, callback=None, raise_error=True)

Performs an HTTP request. Returns a Future if callback is None, otherwise invokes the callback with the HTTPResponse object.

Example:

 
from tornado.httpclient import AsyncHTTPClient

async def fetch_url(url):
    http_client = AsyncHTTPClient()
    response = await http_client.fetch(url)
    print(response.body.decode())

 tornado.ioloop.IOLoop.current().run_sync(lambda: fetch_url("http://www.example.com"))

Futures and Coroutines

Tornado uses Futures to represent the eventual result of an asynchronous operation.
@tornado.gen.coroutine decorator allows you to write asynchronous code using yield (pre-Python 3.5) or async/await (Python 3.5+).

Example (using async/await):

import tornado.gen
from tornado.httpclient import AsyncHTTPClient

async def get_example_html():
    http_client = AsyncHTTPClient()
    response = await http_client.fetch("http://www.example.com")
    return response.body.decode()

Templates and UI Modules

Template Rendering

Tornado uses its own template language, which is similar to Django’s.
Templates are rendered using self.render(template_name, **kwargs) in a request handler.

Example:

class MyHandler(tornado.web.RequestHandler):
    def get(self):
        items = ["Item 1", "Item 2", "Item 3"]
        self.render("mytemplate.html", items=items)

Template files are typically located in a templates directory.

Template Syntax

{{ ... }}

Escapes and outputs the result.

{% ... %}

Template directives (e.g., if, for).

{# ... #}

Comments.

{{! ... }}

Unescaped output.

UI Modules

UI Modules are reusable components that render parts of a page. They can include CSS and JavaScript.

Example:

class MyModule(tornado.web.UIModule):
    def render(self, *args, **kwargs):
        return "<div>Hello from MyModule</div>"

# In template:
{# module MyModule() #}

Security and Deployment

Security Considerations

Always sanitize user input to prevent Cross-Site Scripting (XSS) and SQL Injection attacks.
Use HTTPS to encrypt communication between the client and server.
Protect against Cross-Site Request Forgery (CSRF) attacks by using xsrf_form_html() in your templates and enabling xsrf_cookies in your application settings.

Authentication

tornado.web.authenticated

Decorator to require authentication for a handler. Redirects to login_url if the user is not authenticated.

get_current_user()

Override this method in your handler to determine the current user. It should return a user object or None.

Example:

 
class BaseHandler(tornado.web.RequestHandler):
    def get_current_user(self):
        user_id = self.get_secure_cookie("user_id")
        if user_id:
            return self.db.get("SELECT * FROM users WHERE id = %s", int(user_id))
        return None

class MainHandler(BaseHandler):
    @tornado.web.authenticated
    def get(self):
        self.render("main.html", user=self.current_user)

Deployment

Tornado applications can be deployed using various methods:

  • Standalone: Using python app.py (suitable for development).
  • Reverse Proxy: Using a reverse proxy like Nginx or Apache (recommended for production).
  • Process Manager: Using a process manager like Supervisor or systemd to ensure the application restarts automatically if it crashes.

For production, use a reverse proxy to handle SSL termination, static file serving, and load balancing.
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