Run actions on Tornado main loop, after it starts - python-3.x

Question

I'm creating a python3 tornado web server that may listen to an MQTT broker and whenever listens a new message from it, broadcasts it to the connected browsers, through web sockets. However, seems that Tornado doesn't like calls to its API from a thread different to IOLoop.current() and I can't figure out another solution...
I've already tried to write some code. I've put the whole MQTT client (in this case called PMCU client), on a separated thread which loops and listens to MQTT notifications.
def on_pmcu_data(data):
for websocket_client in websocket_clients:
print("Sending websocket message")
websocket_client.write_message(data) # Here it stuck!
print("Sent")
class WebSocketHandler(tornado.websocket.WebSocketHandler):
def open(self):
websocket_clients.append(self)
def on_close(self):
websocket_clients.remove(self)
def make_app():
return tornado.web.Application([
(r'/ws', WebSocketHandler)
])
if __name__ == "__main__":
main_loop = IOLoop().current()
pmcu_client = PMCUClient(on_pmcu_data)
threading.Thread(target=lambda: pmcu_client.listen("5.4.3.2")).start()
app = make_app()
app.listen(8080)
main_loop.start()
However as I said, seems that calls to Tornado API outside the IOLoop.current() blocks: the code above only prints Sending websocket message.
My intent is to run websocket_client.write_message(data) on IOLoop.current() event loop. But seems that the function IOLoop.current().spawn_callback(lambda: websocket_client.write_message(data)) not works after IOLoop.current() has started. How could I achieve that?
I know that I have a huge misunderstanding of IOLoop, asyncio, on which it depends, and python3 async.

Answer 1

on_pmcu_data is being called in a separate thread but the websocket is controlled by Tornado's event loop. You can't write to a websocket from a thread unless you have access to the event loop.
You'll need to ask the IOLoop to write the data to websockets.
Solution 1:
For simple cases, if you don't want to change much in the code, you can do this:
if __name__ == "__main__":
main_loop = IOLoop().current()
on_pmcu_data_callback = lambda data: main_loop.add_callback(on_pmcu_data, data)
pmcu_client = PMCUClient(on_pmcu_data_callback)
...
This should solve your problem.
Solution 2:
For more elaborate cases, you can pass the main_loop to PMCUClient class and then use add_callback (or spawn_callback) to run on_pmcu_data.
Example:
if __name__ == "__main__":
main_loop = IOLoop().current()
pmcu_client = PMCUClient(on_pmcu_data, main_loop) # also pass the main loop
...
Then in PMCUCLient class:
class PMCUClient:
def __init__(self, on_pmcu_data, main_loop):
...
self.main_loop = main_loop
def lister(...):
...
self.main_loop.add_callback(self.on_pmcu_data, data)