apscheduler in Flask executes twice [duplicate]

In debug mode, Flask's reloader will load the flask app twice (How to stop Flask from initialising twice in Debug Mode?). I'm not sure why this is, but it causes apscheduler's jobs to be scheduled twice. A quick print "loaded scheduler" right before sched.start() confirms this.

There are a couple ways around this, as mentioned in the linked answer. The one I found that worked best is just to disable the reloader like so:

app.run(use_reloader=False)

It means I have to reload my app manually as I develop it, but it's a small price to pay to get apscheduler working.

When using the reloader, there are the master and child processes. Your scheduler thread runs in both. You need to prevent the scheduler from running in the master process

if not app.debug or os.environ.get('WERKZEUG_RUN_MAIN') == 'true':  sched = Scheduler()  sched.add_interval_job(test_scheduler, seconds=5)  sched.start()

You can start the scheduler in Flask's before_first_request() decorator, which "registers a function to be run before the first request to this instance of the application".

import timeimport atexitfrom apscheduler.schedulers.background import BackgroundSchedulerdef print_date_time():    print(time.strftime("%A, %d. %B %Y %I:%M:%S %p"))@app.before_first_requestdef init_scheduler():    scheduler = BackgroundScheduler()    scheduler.add_job(func=print_date_time, trigger="interval", seconds=3)    scheduler.start()    # Shut down the scheduler when exiting the app    atexit.register(lambda: scheduler.shutdown())

Note that before_first_request() will always be called again with the first request after server reload.