Run while loop concurrently with Flask server

Question

I'm updating some LEDs using python. I've been doing this like so:

from LEDs import *
myLEDs = LEDs()
done = False
while not done:
  myLEDs.iterate()

I wanted to use Flask to act as a bridge between some nice-looking ReactJS front-end I can run in my browser (to change the current pattern, etc) and the LED-controlling code in Python.

I have Flask working fine, can handle HTTP requests, etc. I'm wondering how I can set myLEDs.iterate() to continuously run (or run on a rapid schedule) concurrently with my flask app, while still being able to communicate with one another, like so:

myLEDs = LEDs()

@app.route('/changePattern',methods=['POST'])
def changePattern():
  n = request.json['num']
  myLEDs.setPattern(n)
  return jsonify(**locals())

if __name__ == '__main__':
  app.debug = True
  myLEDs.setToFrequentlyIterateAndStillTalkToFlask()
  app.run()

I came across celery, which seems like it would do the trick, but also seems like overkill for how simple my problem is.

Is using Flask overkill for simply wanting a UI to manage my python back-end code? Is there a simpler library than Celery to use for running something in the background?

Edit

This is part of a larger project to develop an app with a Node-Webkit front-end attached to a Python backend. I'm open to changing my approach to this app if it doesn't seem feasible.

Although I am not the most qualified to comment here: I think this might help you. https://stackoverflow.com/questions/25639221/how-to-run-recurring-task-in-the-python-flask-framework — Metzen, Mar 10 '15 at 22:31

score 18 · Answer 1 · edited May 23 '17 at 12:02

Use multiprocess to run the loop in a different process as the Flask HTTP requests:

import time
from flask import Flask, jsonify
from multiprocessing import Process, Value


app = Flask(__name__)


tasks = [
   {
      'id': 1,
      'title': u'Buy groceries',
      'description': u'Milk, Cheese, Pizza, Fruit, Tylenol', 
      'done': False
   },
   {
      'id': 2,
      'title': u'Learn Python',
      'description': u'Need to find a good Python tutorial on the web', 
      'done': False
   }
]


@app.route('/todo/api/v1.0/tasks', methods=['GET'])
def get_tasks():
   return jsonify({'tasks': tasks})


def record_loop(loop_on):
   while True:
      if loop_on.value == True:
         print("loop running")
      time.sleep(1)


if __name__ == "__main__":
   recording_on = Value('b', True)
   p = Process(target=record_loop, args=(recording_on,))
   p.start()  
   app.run(debug=True, use_reloader=False)
   p.join()

The tasks part is from here, multiprocessing code from me.
Note the "use_reloader=False" part. This is necessary to avoid running the loop twice. For the reason see here

The functionality can be tested by starting up the server with

python <your_name_for_the example>.py

and calling

curl -i http://localhost:5000/todo/api/v1.0/tasks

Thanks for this! I have a follow-up question. I'm trying to redirect to a different page from a concurrent process in a while-loop. But I'm having no luck using either redirect(url_for("register"),code=302) or render_template('register.html'). Also looked into RequestRedirect from werkzeug.routing, but also doesn't seem to work. Got rid of any errors by setting the 'SERVER_NAME' and adding 'with app.app_context():' before the redirects. But still nothing. — Jorgos, Sep 03 '18 at 07:46

Run while loop concurrently with Flask server

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