Avoid infinite signals when saving an object instance in Django's pre_save or post_save

models.py

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

# models definition here ...

@receiver(post_save, sender=MyModel)
def handler_that_saves_a_mymodel_instance(sender, instance, **kwargs):
    # without this check the save() below causes infinite post_save signals
    if not getattr(instance, 'processed', False):
        instance.some_field = complex_calculation()
        instance.processed = True
        instance.save()