Why is run called twice in the Django dev server?

Question

I want to make the Django development server do something before it starts running. To do this, I created a new app, added it to the top of INSTALLED_APPS, and then created a management/commands/runserver.py file in the app with the following code:

from django.contrib.staticfiles.management.commands.runserver import Command as RunserverCommand
class Command(RunserverCommand):
    def run(self, *args, **options):
        self.stdout.write('About to start running on ' + self.addr)
        super(Command, self).run(*args, **options)

(The thing I actually want to do is more complicated than writing one line to stdout, of course, but this is the simplest example that demonstrates the problem. The reason I override run, rather than handle or some other method, is because I need self.addr to already be set when this code runs.)

When I run ./manage.py runserver, the line "About to start running on 127.0.0.1" appears not once, but twice before the server starts running. Why is this happening and what can be done about it?

Answer 1

The local development server runs a separate process for the auto-reloader. You can turn off the auto-reload process by passing the --noreload flag.

python manage.py runserver --noreload

Answer 2

The auto-reloader process turned out to be the culprit; turns out the autoreload process gets the same arguments, and goes through the same initialization process, as the original. The solution was to have the pre-server code execute only if it's not running in the process spawned by the autoreloader, which can be detected through an environment variable:

import os
from django.contrib.staticfiles.management.commands.runserver import Command as RunserverCommand
class Command(RunserverCommand):
    def run(self, *args, **options):
        if os.environ.get('RUN_MAIN') != 'true':
            self.stdout.write('About to start running on ' + self.addr)
        super(Command, self).run(*args, **options)