Flask-socketio context for Flask-SQLAlchemy

Question

I am creating a live reporting application using Flask, Flask-Socketio and Flask-SQLAlchemy. My current design creates a background thread on connection which querys an API and inserts into the applications data. However, when running this, I get the error

RuntimeError: No application found. Either work inside a view function or push an application context.

flask_react_app.py:

from threading import Lock
from flask import Blueprint, render_template
from .model import Stock
from . import socketio

main = Blueprint('main', __name__)
thread_lock = Lock()
thread = None


@main.route('/')
def index():
    """Serve client-side application."""
    return render_template('index.html', async_mode=socketio.async_mode)


def generate_data():
    """
    returns the Stock object query set
    """
    return [i.serialize for i in Stock.query.all()]


def background_thread():
    """Example of how to send server generated events to clients."""
    while True:
        socketio.sleep(10)
        socketio.emit("my_response", generate_data())


@socketio.on("connect")
def test_connect():
    """
    Connect method which fires off thread to notify worker to get data.
    :return: emits initial data.
    """

    global thread
    with thread_lock:
        if thread is None:
            thread = socketio.start_background_task(target=background_thread)
    socketio.emit("my_response", generate_data())

I have two issues with this. Firstly the bug mentioned, secondly, there must be a better way!

score 6 · Accepted Answer · answered Mar 14 '18 at 06:46

Your problem is not related to Flask-SocketIO, but to the fact that to use Flask-SQLAlchemy in a background thread you need an application context for it.

Try the following:

def background_thread(app):
    """Example of how to send server generated events to clients."""
    with app.app_context():
        while True:
            socketio.sleep(10)
            socketio.emit("my_response", generate_data())

Then in the place where you start the background thread pass the application instance as an argument:

thread = socketio.start_background_task(target=background_thread, args=(current_app._get_current_object(),))

Brilliant! Thought it was something to do with the app context! Sorry for the mislabelled error. — , Mar 14 '18 at 09:22

score 3 · Answer 2 · answered Mar 15 '18 at 00:32

Miguel's answer is right but if both keywords arguments are passed to the function, then the next exception is raised:

TypeError: background_thread() got an unexpected keyword argument 'args'

So you have to pass only

thread = socketio.start_background_task(background_thread, (current_app._get_current_object()))

Flask-socketio context for Flask-SQLAlchemy

2 Answers2

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