Issue when running schedule with Flask
flask, python, scheduled-tasks, task
Solution
When you run the development server with the reloader (the default when `debug=True`), the module executes twice, causing two instances of `t`. You can verify this by adding `print(id(t))`.
The simplest way around this is to pass `use_reloader=False` to `app.run`. You can see this answer for an alternative solution that allows you to use the reloader.
Problem
I need to run a certain task periodically on my Flask application. I decided to use a simple library - Schedule (https://github.com/dbader/schedule) for doing this. I am running the task scheduler on a separate thread from the main application thread. Here's the relevant code snippet. ``` import schedule import time from flask import Flask, request from threading import Thread app = Flask(__name__) start_time = time.time() def run_every_10_seconds(): print("Running periodic task!") print "Elapsed time: " + str(time.time() - start_time) def run_schedule(): while 1: schedule.run_pending() time.sleep(1) @app.route('/', methods=['GET']) def index(): return '<html>test</html>' if __name__ == '__main__': schedule.every(10).seconds.do(run_every_10_seconds) t = Thread(target=run_schedule) t.start() print "Start time: " + str(start_time) app.run(debug=True, host='0.0.0.0', port=5000) ``` When I run this, I'd like 'Running periodic task!' to print every 10 seconds. However, this is the output I get. ``` * Running on http://0.0.0.0:5000/ * Restarting with reloader Start time: 1417002869.99 Running periodic task! Elapsed time: 10.0128278732 Running periodic task! Elapsed time: 10.0126948357 Running periodic task! Elapsed time: 20.0249710083 Running periodic task! Elapsed time: 20.0247309208 Running periodic task! Elapsed time: 30.0371530056 Running periodic task! Elapsed time: 30.0369319916 ``` Clearly, for some reason, the task seems to be executing twice every 10 seconds, instead of once. However, if I run merely the task scheduler alone instead of running it alongside Flask (by simply commenting the app.run() line), it runs properly. ``` Start time: 1417003801.52 Running periodic task! Elapsed time: 10.0126750469 Running periodic task! Elapsed time: 20.0246500969 Running periodic task! Elapsed time: 30.0366458893 ``` What could be the reason behind this? Is there a problem with the way the tasks are queued when running multiple threads? It still doesn't explain why two tasks are being scheduled at a time when only one should be.