Tornado: Starting server in separate thread gives... RuntimeError: There is no current event loop in thread 'Thread-4'

Created on 13 Mar 2018 · 3Comments · Source: tornadoweb/tornado

In the 5.0.0 the following code no longer works and throws the error shown below. Is there still a way to start a tornado server in separate thread? For context, I'm using this for the (a self driving car project)[https://github.com/wroscoe/donkey] its web controller. This still works in v4.5.3.

import tornado.web

class WebServer(tornado.web.Application):

    def __init__(self):
        handlers = [ (r"/test", TestHandler), ]
        settings = {'debug': True}
        super().__init__(handlers, **settings)

    def run(self, port=8886):
        self.listen(port)
        tornado.ioloop.IOLoop.instance().start()

class TestHandler(tornado.web.RequestHandler):
    def get(self):
        self.write("test success")

ws = WebServer()


from threading import Thread
t = Thread(target=ws.run, args=())
t.daemon = True
t.start()

Exception in thread Thread-4:
Traceback (most recent call last):
  File "/home/wroscoe/miniconda3/envs/donkey/lib/python3.6/threading.py", line 916, in _bootstrap_inner
    self.run()
  File "/home/wroscoe/miniconda3/envs/donkey/lib/python3.6/threading.py", line 864, in run
    self._target(*self._args, **self._kwargs)
  File "<ipython-input-1-65816933cb20>", line 12, in run
    super().__init__(self.handlers, **self.settings)
  File "/home/wroscoe/miniconda3/envs/donkey/lib/python3.6/site-packages/tornado/web.py", line 1961, in __init__
    autoreload.start()
  File "/home/wroscoe/miniconda3/envs/donkey/lib/python3.6/site-packages/tornado/autoreload.py", line 118, in start
    io_loop = ioloop.IOLoop.current()
  File "/home/wroscoe/miniconda3/envs/donkey/lib/python3.6/site-packages/tornado/ioloop.py", line 283, in current
    loop = asyncio.get_event_loop()
  File "/home/wroscoe/miniconda3/envs/donkey/lib/python3.6/asyncio/events.py", line 694, in get_event_loop
    return get_event_loop_policy().get_event_loop()
  File "/home/wroscoe/miniconda3/envs/donkey/lib/python3.6/asyncio/events.py", line 602, in get_event_loop
    % threading.current_thread().name)
RuntimeError: There is no current event loop in thread 'Thread-4'.

Source

wroscoe

👍2

Most helpful comment

As it says, there is no current event loop in a new thread. So you have to create it.

This code works for me:

import asyncio
import tornado.web

class WebServer(tornado.web.Application):

    def __init__(self):
        handlers = [ (r"/test", TestHandler), ]
        settings = {'debug': True}
        super().__init__(handlers, **settings)

    def run(self, port=8886):
        self.listen(port)
        tornado.ioloop.IOLoop.instance().start()

class TestHandler(tornado.web.RequestHandler):
    def get(self):
        self.write("test success")


ws = WebServer()


def start_server():
    asyncio.set_event_loop(asyncio.new_event_loop())
    ws.run()


from threading import Thread
t = Thread(target=start_server, args=())
t.daemon = True
t.start()

t.join()

MrP4p3r on 13 Mar 2018

👍18 ❤10 😄3

All 3 comments

As it says, there is no current event loop in a new thread. So you have to create it.

This code works for me:

import asyncio
import tornado.web

class WebServer(tornado.web.Application):

    def __init__(self):
        handlers = [ (r"/test", TestHandler), ]
        settings = {'debug': True}
        super().__init__(handlers, **settings)

    def run(self, port=8886):
        self.listen(port)
        tornado.ioloop.IOLoop.instance().start()

class TestHandler(tornado.web.RequestHandler):
    def get(self):
        self.write("test success")


ws = WebServer()


def start_server():
    asyncio.set_event_loop(asyncio.new_event_loop())
    ws.run()


from threading import Thread
t = Thread(target=start_server, args=())
t.daemon = True
t.start()

t.join()

MrP4p3r on 13 Mar 2018

👍18 ❤10 😄3

Yeah, this is asyncio's policy which Tornado has inherited in 5.0. You must either create an event loop explicitly for each thread or use AnyThreadEventLoopPolicy.