Tornado run task in a thread pool.

tornado_thread.py

""" Run a task in a background thread.
python3 -m pip install tornado --user

python3 tornado_thread.py
curl http://localhost:8888/2

See `run_on_executor` documentation:
https://www.tornadoweb.org/en/stable/concurrent.html#tornado.concurrent.run_on_executor

See for another approach:
- https://www.tornadoweb.org/en/stable/faq.html#why-isn-t-this-example-with-time-sleep-running-in-parallel

```python

    class ThreadPoolHandler(RequestHandler):
        async def get(self):
            for i in range(5):
                print(i)
                await IOLoop.current().run_in_executor(None, time.sleep, 1)

```

"""
import tornado.ioloop
import tornado.web

from concurrent.futures import ThreadPoolExecutor
from tornado.concurrent import run_on_executor

class MainHandler(tornado.web.RequestHandler):
    executor = ThreadPoolExecutor(max_workers=4)

    @run_on_executor
    def background_task(self, num):
        import time
        time.sleep(int(num))
        return num

    @tornado.gen.coroutine
    def get(self, num):
        res = yield self.background_task(num)
        self.write(res)

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

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