@property
    def invoice_id(self):
        number = (settings.INVOICE_ID_LENGTH - len(str(self.invoice_no))) * "0" + str(self.invoice_no)
        return '%(prefix)s%(year)s%(unique_id)s' % {'prefix': settings.INVOICE_ID_PREFIX,
                                                    'year': self.date.strftime("%y"),
                                                    'unique_id': number}
Unique invoice ID
dataset/ETHPy150Open bread-and-pepper/django-brookie/brookie/models.py/Invoice.invoice_id