Learn TORNADO with Real Code Examples

import tornado.ioloop
import tornado.web

count = 0

class CounterHandler(tornado.web.RequestHandler):
    async def get(self):
        self.write({'count': count})

class IncrementHandler(tornado.web.RequestHandler):
    async def post(self):
        global count
        count += 1
        self.write({'count': count})

class DecrementHandler(tornado.web.RequestHandler):
    async def post(self):
        global count
        count -= 1
        self.write({'count': count})

class ResetHandler(tornado.web.RequestHandler):
    async def post(self):
        global count
        count = 0
        self.write({'count': count})

app = tornado.web.Application([
    (r'/counter', CounterHandler),
    (r'/counter/increment', IncrementHandler),
    (r'/counter/decrement', DecrementHandler),
    (r'/counter/reset', ResetHandler)
])

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

Demonstrates a simple Tornado REST API with a counter using async handlers and in-memory state.

import tornado.ioloop
import tornado.web

class HelloHandler(tornado.web.RequestHandler):
    async def get(self):
        self.write({'message': 'Hello World'})

app = tornado.web.Application([
    (r'/', HelloHandler)
])

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

A minimal Tornado API returning Hello World.

import tornado.ioloop
import tornado.web
import json

class EchoHandler(tornado.web.RequestHandler):
    async def post(self):
        data = json.loads(self.request.body)
        self.write(data)

app = tornado.web.Application([
    (r'/echo', EchoHandler)
])

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

A POST endpoint that echoes JSON data.

import tornado.ioloop
import tornado.web

class GreetHandler(tornado.web.RequestHandler):
    async def get(self):
        name = self.get_argument('name', 'Guest')
        self.write({'message': f'Hello {name}'})

app = tornado.web.Application([
    (r'/greet', GreetHandler)
])

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

Using query parameters to greet users.

import tornado.ioloop
import tornado.web

class UserHandler(tornado.web.RequestHandler):
    async def get(self, user_id):
        self.write({'id': int(user_id), 'name': f'User {user_id}'})

app = tornado.web.Application([
    (r'/users/(\d+)', UserHandler)
])

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

Returns user info based on URL parameter.

import tornado.ioloop
import tornado.web

class LogMiddleware(tornado.web.RequestHandler):
    async def prepare(self):
        print(f'{self.request.method} {self.request.uri}')

class IndexHandler(LogMiddleware):
    async def get(self):
        self.write({'message': 'Check console for logs'})

app = tornado.web.Application([
    (r'/', IndexHandler)
])

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

Simple middleware to log requests in Tornado.

import tornado.ioloop
import tornado.web
import asyncio

class AsyncHandler(tornado.web.RequestHandler):
    async def get(self):
        await asyncio.sleep(1)
        self.write({'message': 'Async response'})

app = tornado.web.Application([
    (r'/async', AsyncHandler)
])

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

An async route returning data after a delay.

import tornado.ioloop
import tornado.web

class DefaultHandler(tornado.web.RequestHandler):
    def prepare(self):
        self.set_status(404)
        self.write({'error': 'Not Found'})

app = tornado.web.Application([], default_handler_class=DefaultHandler)

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

Custom 404 error response.

import tornado.ioloop
import tornado.web
import json

count = 0

class LogHandler(tornado.web.RequestHandler):
    async def prepare(self):
        print(f'{self.request.method} {self.request.uri}')

class EchoHandler(LogHandler):
    async def post(self):
        data = json.loads(self.request.body)
        self.write(data)

class CounterHandler(LogHandler):
    async def get(self):
        self.write({'count': count})
    async def post(self, action):
        global count
        if action == 'increment': count += 1
        elif action == 'decrement': count -= 1
        elif action == 'reset': count = 0
        self.write({'count': count})

app = tornado.web.Application([
    (r'/echo', EchoHandler),
    (r'/counter', CounterHandler),
    (r'/counter/(increment|decrement|reset)', CounterHandler)
])

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

Logging, JSON echo, and counter in a single Tornado app.