How to automatically fill out an auto increment number in form field?

Hello,

I have a model ...

class PurchaseOrder(models.Model):

    po_number = models.IntegerField(unique=True)

and a function ...

def get_po_number(self, *args, **kwargs):

    if not self.id:

        last_po = PurchaseOrder.objects.order_by('po_number').last()

        if last_po:

            last_po_num = last_po.po_number[-2:]

            new_po_num = int(last_po_num) + 1

        else:

            new_po_num = '0001'

        self.po_number = new_po_num

        return self.po_number

what I'm trying to do is use the get_po_number() function to get a default value for the po_number field in the model. However when I set the default in the model like so... 

class PurchaseOrder(models.Model):

    po_number = models.IntegerField(unique=True, default=get_po_number(self))

it says "NameError: name 'self' is not defined"

What am I doing wrong? Thanks!

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