Flask-SocketIO: Background task is started twice in reloader mode

Question

I'm trying to use background tasks with flask-socketio and run into two problems:

when in reloader mode, the background task is started twice
when changing the code, flask reloads but the background tasks are not killed

My code:

from flask import Flask
from flask_socketio import SocketIO
import random

application = Flask(__name__)
socketio = SocketIO(application, async_mode='eventlet')
import eventlet
eventlet.monkey_patch()

def background_thread():
    print('starting backgound task')
    thread_id = random.randint(1,100)
    while True:
        print(f'background thread {thread_id}')
        socketio.sleep(1)

if __name__ == '__main__':
    print('--------------------')
    socketio.start_background_task(background_thread)
    socketio.run(application, debug=True, use_reloader=True)

Output:

python application.py
--------------------
starting backgound task
background thread 26
 * Restarting with stat
--------------------
starting backgound task
background thread 59
background thread 26
 * Detected change in '/Users/philipp/code/nifmap/application.py', reloading
background thread 26
background thread 59
background thread 26
background thread 59

When I then hit ^C then one background task is killed, but not the other. This happens only when I do debug=True. How can I start the background task so that it's only started once and then it's killed upon reload?

The same happens when using gevent, starting with async_mode='eventlet' and doing:

from gevent import monkey
monkey.patch_all()

Versions:

eventlet==0.33.0
Flask==2.0.2
Flask-SocketIO==5.1.1
Flask-WTF==0.15.1
greenlet==1.1.2
python-engineio==4.3.0
python-socketio==5.5.0
websocket-client==1.2.3
Werkzeug==2.0.2

Python 3.8.2

score 0 · Answer 1 · answered Dec 22 '21 at 13:35

After seeing many people having run into the same problem and reading this from the author of Flask-SocketIO:

It appears that the reloader does not play nice with eventlet

I gave up on auto reload and instead start Flask without the reloader

socketio.run(application, debug=True, use_reloader=True)

and use entr to detect file changes:

ls *.py | entr -r python application.py

Flask-SocketIO: Background task is started twice in reloader mode

1 Answers1