Frequently Asked Questions

Why doesn’t the scheduler run my jobs?

This could be caused by a number of things. The two most common issues are:

  1. Running the scheduler inside a uWSGI worker process while threads have not been enabled (see the next section for this)
  2. Running a BackgroundScheduler and then letting the execution reach the end of the script

To demonstrate the latter case, a script like this will not work:

from apscheduler.schedulers.background import BackgroundScheduler

def myjob():
    print('hello')

scheduler = BackgroundScheduler()
scheduler.start()
scheduler.add_job(myjob, 'cron', hour=0)

The script above will exit right after calling add_job() so the scheduler will not have a chance to run the scheduled job.

If you’re having any other issue, then enabling debug logging as instructed in the Troubleshooting section should shed some light into the problem.

Why am I getting a ValueError?

If you’re receiving an error like the following:

ValueError: This Job cannot be serialized since the reference to its callable (<bound method xxxxxxxx.on_crn_field_submission
of <__main__.xxxxxxx object at xxxxxxxxxxxxx>>) could not be determined. Consider giving a textual reference (module:function
name) instead.

This means that the function you are attempting to schedule has one of the following problems:

  • It is a lambda function (e.g. lambda x: x + 1)
  • It is a bound method (function tied to a particular instance of some class)
  • It is a nested function (function inside another function)
  • You are trying to schedule a function that is not tied to any actual module (such as a function defined in the REPL, hence __main__ as the module name)

In these cases, it is impossible for the scheduler to determine a “lookup path” to find that specific function instance in situations where, for example, the scheduler process is restarted, or a process pool worker is being sent the related job object.

Common workarounds for these problems include:

  • Converting a lambda to a regular function
  • Moving a nested function to the module level or to class level as either a class method or a static method
  • In case of a bound method, passing the unbound version (YourClass.method_name) as the target function to add_job() with the class instance as the first argument (so it gets passed as the self argument)

How can I use APScheduler with uWSGI?

uWSGI employs some tricks which disable the Global Interpreter Lock and with it, the use of threads which are vital to the operation of APScheduler. To fix this, you need to re-enable the GIL using the --enable-threads switch. See the uWSGI documentation for more details.

Also, assuming that you will run more than one worker process (as you typically would in production), you should also read the next section.

How do I share a single job store among one or more worker processes?

Short answer: You can’t.

Long answer: Sharing a persistent job store among two or more processes will lead to incorrect scheduler behavior like duplicate execution or the scheduler missing jobs, etc. This is because APScheduler does not currently have any interprocess synchronization and signalling scheme that would enable the scheduler to be notified when a job has been added, modified or removed from a job store.

Workaround: Run the scheduler in a dedicated process and connect to it via some sort of remote access mechanism like RPyC, gRPC or an HTTP server. The source repository contains an example of a RPyC based service that is accessed by a client.

How do I use APScheduler in a web application?

First read through the previous section.

If you’re running Django, you may want to check out django_apscheduler. Note, however, that this is a third party library and APScheduler developers are not responsible for it.

Likewise, there is an unofficial extension called Flask-APScheduler which may or may not be useful when running APScheduler with Flask.

For Pyramid users, the pyramid_scheduler library may potentially be helpful.

Other than that, you pretty much run APScheduler normally, usually using BackgroundScheduler. If you’re running an asynchronous web framework like aiohttp, you probably want to use a different scheduler in order to take some advantage of the asynchronous nature of the framework.

Is there a graphical user interface for APScheduler?

No graphical interface is provided by the library itself. However, there are some third party implementations, but APScheduler developers are not responsible for them. Here is a potentially incomplete list: