losing registered handlers in a socketio instance when calling init_app

[rienafairefr]

I was prototyping something out of flask-socketio a client-server setup with an remote external process and I was surprised by something in the initialisation, managed to build a minimally reproducible code below.

server code

from flask import Flask
from flask_socketio import SocketIO

# with this, it doesn't work: the handle_message is not called on messages
socketio = SocketIO(message_queue="redis://")
# change to this, and the handlers are called:
# socketio = SocketIO()

@socketio.on('message')
def handle_message(message):
    print(f"handle_message {message}")


if __name__ == '__main__':
    app = Flask(__name__)
    socketio.init_app(app)
    socketio.run(app)

An even more mimimal example, without socketio.run():

from flask import Flask
from flask_socketio import SocketIO

print('with message_queue')
socketio = SocketIO(message_queue="redis://")
print(f'1 handlers: {socketio.server.handlers} {socketio.handlers}')
socketio.on_event('message', print)
print(f'2 handlers: {socketio.server.handlers} {socketio.handlers}')
app = Flask(__name__)
socketio.init_app(app)
print(f'3 handlers: {socketio.server.handlers} {socketio.handlers}')

print('without message_queue')
socketio = SocketIO()
print(f'1 handlers: {socketio.server} {socketio.handlers}')
socketio.on_event('message', print)
print(f'2 handlers: {socketio.server} {socketio.handlers}')
app = Flask(__name__)
socketio.init_app(app)
print(f'3 handlers: {socketio.server.handlers} {socketio.handlers}')

which gives this log when runs:

1 handlers: {} []
2 handlers: {'/': {'message': <function print at 0x7b59edb20720>}} []
3 handlers: {} []
without message_queue
1 handlers: None []
2 handlers: None [('message', <function print at 0x7b59edb67c40>, '/')]
3 handlers: {'/': {'message': <function print at 0x7b59edb67c40>}} [('message', <function print at 0x7b59edb67c40>, '/')]

In the first case, well, the handler is registered, init_app is called once in the __init__ of socketio.SocketIO
but then self.server is initialized in again init_app (in my actual app code the init_app was called in a create_app factory, as I usually do for flask-* apps ), and the handlers are lost in the ether.
In the second case, the handlers are registered on the handlers attribute of the socketio instance instead of self.server.handlers, so they are re-added to the socketio.server in init_app

I think it might be called a bug in flask-socketio, but I'm not sure (this is more of a edge case when both using message_queue and an app factory. All the code paths I looked at are 5 to 10 years old so this might be more a case of "I didn't read the docs enough"

related discussions (I think) : the symptom is similar (handler seemingly defined but not called)
#2136
#2136

[miguelgrinberg]

This has been asked and discussed many times.

Flask-SocketIO is a Flask extension. You have two options to configure it:

Option 1: Pass your configuration when you call the SocketIO constructor, and don't call the init_app() method.
Option 2: Create the SocketIO class with no arguments, then pass your configuration when you call init_app().

There is no supported option in which you pass some arguments in the constructor and some others in init_app().