I have a Flask web hosting with no access to cron command.

How can I execute some Python function every hour?

You can use BackgroundScheduler() from APScheduler package (v3.5.3):

import time
import atexit

from apscheduler.schedulers.background import BackgroundScheduler


def print_date_time():
    print(time.strftime("%A, %d. %B %Y %I:%M:%S %p"))


scheduler = BackgroundScheduler()
scheduler.add_job(func=print_date_time, trigger="interval", seconds=60)
scheduler.start()

# Shut down the scheduler when exiting the app
atexit.register(lambda: scheduler.shutdown())

Note that two of these schedulers will be launched when Flask is in debug mode. For more information, check out this question.

I’m a little bit new with the concept of application schedulers, but what I found here for APScheduler v3.3.1 , it’s something a little bit different. I believe that for the newest versions, the package structure, class names, etc., have changed, so I’m putting here a fresh solution which I made recently, integrated with a basic Flask application:

#!/usr/bin/python3
""" Demonstrating Flask, using APScheduler. """

from apscheduler.schedulers.background import BackgroundScheduler
from flask import Flask

def sensor():
    """ Function for test purposes. """
    print("Scheduler is alive!")

sched = BackgroundScheduler(daemon=True)
sched.add_job(sensor,'interval',minutes=60)
sched.start()

app = Flask(__name__)

@app.route("/home")
def home():
    """ Function for test purposes. """
    return "Welcome Home :) !"

if __name__ == "__main__":
    app.run()

I’m also leaving this Gist here, if anyone have interest on updates for this example.

Here are some references, for future readings:

You could make use of APScheduler in your Flask application and run your jobs via its interface:

import atexit

# v2.x version - see https://stackoverflow.com/a/38501429/135978
# for the 3.x version
from apscheduler.scheduler import Scheduler
from flask import Flask

app = Flask(__name__)

cron = Scheduler(daemon=True)
# Explicitly kick off the background thread
cron.start()

@cron.interval_schedule(hours=1)
def job_function():
    # Do your work here


# Shutdown your cron thread if the web process is stopped
atexit.register(lambda: cron.shutdown(wait=False))

if __name__ == '__main__':
    app.run()

I’ve tried using flask instead of a simple apscheduler what you need to install is

pip3 install flask_apscheduler

Below is the sample of my code:

from flask import Flask
from flask_apscheduler import APScheduler

app = Flask(__name__)
scheduler = APScheduler()

def scheduleTask():
    print("This test runs every 3 seconds")

if __name__ == '__main__':
    scheduler.add_job(id = 'Scheduled Task', func=scheduleTask, trigger="interval", seconds=3)
    scheduler.start()
    app.run(host="0.0.0.0")

For a simple solution, you could add a route such as

@app.route("/cron/do_the_thing", methods=['POST'])
def do_the_thing():
    logging.info("Did the thing")
    return "OK", 200

Then add a unix cron job that POSTs to this endpoint periodically. For example to run it once a minute, in terminal type crontab -e and add this line:

* * * * * /opt/local/bin/curl -X POST https://YOUR_APP/cron/do_the_thing

(Note that the path to curl has to be complete, as when the job runs it won’t have your PATH. You can find out the full path to curl on your system by which curl)

I like this in that it’s easy to test the job manually, it has no extra dependencies and as there isn’t anything special going on it is easy to understand.

Security

If you’d like to password protect your cron job, you can pip install Flask-BasicAuth, and then add the credentials to your app configuration:

app = Flask(__name__)
app.config['BASIC_AUTH_REALM'] = 'realm'
app.config['BASIC_AUTH_USERNAME'] = 'falken'
app.config['BASIC_AUTH_PASSWORD'] = 'joshua'

To password protect the job endpoint:

from flask_basicauth import BasicAuth
basic_auth = BasicAuth(app)

@app.route("/cron/do_the_thing", methods=['POST'])
@basic_auth.required
def do_the_thing():
    logging.info("Did the thing a bit more securely")
    return "OK", 200

Then to call it from your cron job:

* * * * * /opt/local/bin/curl -X POST https://falken:[email protected]_APP/cron/do_the_thing

You could try using APScheduler’s BackgroundScheduler to integrate interval job into your Flask app. Below is the example that uses blueprint and app factory (init.py) :

from datetime import datetime

# import BackgroundScheduler
from apscheduler.schedulers.background import BackgroundScheduler
from flask import Flask

from webapp.models.main import db 
from webapp.controllers.main import main_blueprint    

# define the job
def hello_job():
    print('Hello Job! The time is: %s' % datetime.now())

def create_app(object_name):
    app = Flask(__name__)
    app.config.from_object(object_name)
    db.init_app(app)
    app.register_blueprint(main_blueprint)
    # init BackgroundScheduler job
    scheduler = BackgroundScheduler()
    # in your case you could change seconds to hours
    scheduler.add_job(hello_job, trigger="interval", seconds=3)
    scheduler.start()

    try:
        # To keep the main thread alive
        return app
    except:
        # shutdown if app occurs except 
        scheduler.shutdown()

Hope it helps 🙂

Ref :

  1. https://github.com/agronholm/apscheduler/blob/master/examples/schedulers/background.py

Another alternative might be to use Flask-APScheduler which plays nicely with Flask, e.g.:

  • Loads scheduler configuration from Flask configuration,
  • Loads job definitions from Flask configuration

More information here:

https://pypi.python.org/pypi/Flask-APScheduler

A complete example using schedule and multiprocessing, with on and off control and parameter to run_job()
the return codes are simplified and interval is set to 10sec, change to every(2).hour.do()for 2hours. Schedule is quite impressive it does not drift and I’ve never seen it more than 100ms off when scheduling. Using multiprocessing instead of threading because it has a termination method.

#!/usr/bin/env python3

import schedule
import time
import datetime
import uuid

from flask import Flask, request
from multiprocessing import Process

app = Flask(__name__)
t = None
job_timer = None

def run_job(id):
    """ sample job with parameter """
    global job_timer
    print("timer job id={}".format(id))
    print("timer: {:.4f}sec".format(time.time() - job_timer))
    job_timer = time.time()

def run_schedule():
    """ infinite loop for schedule """
    global job_timer
    job_timer = time.time()
    while 1:
        schedule.run_pending()
        time.sleep(1)

@app.route('/timer/<string:status>')
def mytimer(status, nsec=10):
    global t, job_timer
    if status=='on' and not t:
        schedule.every(nsec).seconds.do(run_job, str(uuid.uuid4()))
        t = Process(target=run_schedule)
        t.start()
        return "timer on with interval:{}sec\n".format(nsec)
    elif status=='off' and t:
        if t:
            t.terminate()
            t = None
            schedule.clear()
        return "timer off\n"
    return "timer status not changed\n"

if __name__ == '__main__':
    app.run(host="0.0.0.0", port=5000)

You test this by just issuing:

$ curl http://127.0.0.1:5000/timer/on
timer on with interval:10sec
$ curl http://127.0.0.1:5000/timer/on
timer status not changed
$ curl http://127.0.0.1:5000/timer/off
timer off
$ curl http://127.0.0.1:5000/timer/off
timer status not changed

Every 10sec the timer is on it will issue a timer message to console:

127.0.0.1 - - [18/Sep/2018 21:20:14] "GET /timer/on HTTP/1.1" 200 -
timer job id=b64ed165-911f-4b47-beed-0d023ead0a33
timer: 10.0117sec
timer job id=b64ed165-911f-4b47-beed-0d023ead0a33
timer: 10.0102sec

You might want to use some queue mechanism with scheduler like RQ scheduler or something more heavy like Celery (most probably an overkill).

You may use flask-crontab module, which is quite easy.

Step 1: pip install flask-crontab

Step 2:

from flask import Flask
from flask_crontab import Crontab

app = Flask(__name__)
crontab = Crontab(app)

Step 3:

@crontab.job(minute="0", hour="6", day="*", month="*", day_of_week="*")
def my_scheduled_job():
    do_something()

Step 4: On cmd, hit

flask crontab add

Done. now simply run your flask application, and you can check your function will call at 6:00 every day.

You may take reference from Here (Official DOc).