Django snippets

class Bill(models.Model):
    
    def __number():
        """
        Automatic bill number generator.
        
        bill_emmited: returns a list with all the bill objects
                      with bill_type=False
        latest_object: from bill_emmited get the latest one
                       and convert it to string to manipulate
        first_number: returns the first number of the latest_object field
                      and adds 1
        If there's no bill objects, return first_number=1. The overall
        function returns either an incremented first_number slash year or
        one slash year.
        """
        current_year = date.today().strftime('%y')
        try:
            bill_emmited = Bill.objects.filter(bill_type__exact=False)
            latest_object = bill_emmited.latest('number').__str__()
            first_number = int(latest_object.split("/")[0]) + 1
        except Bill.DoesNotExist:
            first_number = 1
        return '%s/%s' % (first_number, current_year)
        
    number = models.CharField(_('Bill number'), max_length=10, unique=True, \
    default=__number)
    bill_type = models.BooleanField()