How to perform periodic task with Flask in Python

Question

I've been using Flask to provide a simple web API for my k8055 USB interface board; fairly standard getters and putters, and Flask really made my life a lot easier.

But I want to be able to register changes of state as / near when whey happen.

For instance, if I have a button connected to the board, I can poll the api for that particular port. But if I wanted to have the outputs directly reflect the outputs, whether or not someone was talking to the api, I would have something like this.

while True:     board.read()     board.digital_outputs = board.digital_inputs     board.read()     time.sleep(1) 

And every second, the outputs would be updated to match the inputs.

Is there any way to do this kind of thing under Flask? I've done similar things in Twisted before but Flask is too handy for this particular application to give up on it just yet...

Thanks.

Answer 1

For my Flask application, I contemplated using the cron approach described by Pashka in his answer, the schedule library, and APScheduler.

I found APScheduler to be simple and serving the periodic task run purpose, so went ahead with APScheduler.

Example code:

from flask import Flask  from apscheduler.schedulers.background import BackgroundScheduler   app = Flask(__name__)  def test_job():     print('I am working...')  scheduler = BackgroundScheduler() job = scheduler.add_job(test_job, 'interval', minutes=1) scheduler.start() 

Answer 2

You could use cron for simple tasks.

Create a flask view for your task.

# a separate view for periodic task @app.route('/task') def task():     board.read()     board.digital_outputs = board.digital_inputs 

Then using cron, download from that url periodically

# cron task to run each minute 0-59 * * * * run_task.sh 

Where run_task.sh contents are

wget http://localhost/task 

Cron is unable to run more frequently than once a minute. If you need higher frequency, (say, each 5 seconds = 12 times per minute), you must do it in tun_task.sh in the following way

# loop 12 times with a delay for i in 1 2 3 4 5 6 7 8 9 10 11 12 do     # download url in background for not to affect delay interval much     wget -b http://localhost/task     sleep 5s done