Example of how to offload a blocking task in tornado using the threading module in python3

tornado_threads.py

from tornado.httpserver import HTTPServer
from tornado.ioloop import IOLoop
from tornado.web import Application, RequestHandler
from tornado.gen import coroutine, Task
import threading
from time import sleep


def do_block(callback, args):
    def _callback():
        n,  = args
        print("oh")
        sleep(n)
        print("hay")
        IOLoop.instance().add_callback(lambda: callback(n))
    t = threading.Thread(target=_callback)
    t.start()


class HandlerA(RequestHandler):
    @coroutine
    def get(self):
        foo = yield Task(do_block, args=(3, ))


class HandlerB(RequestHandler):
    def get(self):
        self.write("oh hay")
        self.finish()


HTTPServer(Application([
    ("/a", HandlerA),
    ("/b", HandlerB),
], debug=True)).listen(9999)


IOLoop.instance().start()