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

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

Answer 2

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 3

No there is not tasks support in Flask, but you can use flask-celery or simply run your function in separate thread(greenlet).

Answer 4

For some reason, Antony's code wasn't working for me. I didn't get any error messages or anything, but the test_job function wouldn't run.

I was able to get it working by installing Flask-APScheduler and then using the following code, which is a blend of Antony's code and the example from this Techcoil article.

from flask import Flask
from flask_apscheduler import APScheduler

app = Flask(__name__)

def test_job():
    print('I am working...')

scheduler = APScheduler()
scheduler.init_app(app)
scheduler.start()
scheduler.add_job(id='test-job', func=test_job, trigger='interval', seconds=1)