I have the following model:

class SrdRevisionWording(ModelDiff):
    srd_revision = models.ForeignKey(SrdRevision, on_delete=models.CASCADE)
    word = models.CharField(max_length=50, null=False)
    designation = models.CharField(max_length=200, null=False)
    is_list = models.BooleanField(null=False, default=False)
    parent_element = models.IntegerField(null=True) # ID of the Wording element in the previous SRD Revision
    __original_instance = None
    with_signal = True

    def get_revision_history_message(self, created=False, deleted=False, updated_field=None, check_history=False):
        if created:
            return 'Word "{}" added'.format(self.word)

        if deleted:
            return 'Word "{}" deleted'.format(self.word)

        match updated_field:
            case 'word':
                return 'Word {} changed to {}'.format(self.__original_instance.word, self.word)
            case 'designation' | 'is_list':
                return 'Word {} modified'.format(self.word)

The attribute __original_instance is a custom model field (not in database).
This attribute is instantiated during the pre_save signal :

@receiver(pre_save, sender=SrdRevisionDocument)
@receiver(pre_save, sender=SrdRevisionWording)
def model_pre_save_handler(sender, instance, **kwargs):
    print('pre_save')
    if instance.srd_revision.revision != 'A':
        try:
            previous_srd_revision_state = sender.objects.get(pk=instance.parent_element)
        except sender.DoesNotExist:
            instance.__original_instance = None
        else:
            instance.__original_instance = previous_srd_revision_state
        print(instance.__original_instance)

The print(instance.__original_instance) show me well the object SrdRevisionWording object (21).
I can also print the object in the post_save signal :

@receiver(post_save, sender=SrdRevisionDocument)
@receiver(post_save, sender=SrdRevisionWording)
def handle_revision_history_element_changed(sender, instance, **kwargs):
    print('signal')
    print(instance.__original_instance)
    if instance.with_signal is not False:
       message = instance.get_revision_history_message(created=True, check_history=True)

The issue occurs on the last line when calling instance.get_revision_history_message().
I got the following error in the models.py file:

return 'Word {} changed to {}'.format(self.__original_instance.word, self.word)
                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'word'

I think it’s more on OOP misunderstanding from myself. But can someone explain me why the __original_instance seems not to be instantiate/accessible for the Model method here ?