signals do not work in a standalone script?

in my models.py:

from django.db import models
from django.dispatch import receiver

class MyModel(models.Model):
    slug = models.SlugField()

@receiver(models.signals.pre_save, sender=MyModel)
def on_pre_save(sender, **kwargs):
    print 'on_pre_save'

when I create a model instance from a command line when running manage.py shell. The signal gets dispatched and I see the output.

But when I use my model in a standalone script, the si开发者_如何学运维gnal does not seem to get dispatched because I see no output. why?

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My script's initialization logic was outdated. After I converted this script into a manage.py custom command, signals started working.