Construct a list for address label.

        Non-empty premises elements are appended to the address label in the
        order of organisation_name, department_name, po_box_number (which
        must be prepended with 'PO Box', sub_building_name, building_name,
        building_number, then the rest of the elements except for Town and
        Postcode because we want them in their own fields. This isn't strict
        address label but we're probably loading them into a database.

def generate_address_label(self):
        """Construct a list for address label.

        Non-empty premises elements are appended to the address label in the
        order of organisation_name, department_name, po_box_number (which
        must be prepended with 'PO Box', sub_building_name, building_name,
        building_number, then the rest of the elements except for Town and
        Postcode because we want them in their own fields. This isn't strict
        address label but we're probably loading them into a database.
        """
        if self.organisation_name:
            self.address_label.append(self.organisation_name)
        if self.department_name:
            self.address_label.append(self.department_name)
        if self.po_box_number:
            self.address_label.append('PO Box ' + self.po_box_number)

        elements = [
                self.sub_building_name,
                self.building_name,
                self.building_number,
                self.dependent_thoroughfare,
                self.thoroughfare,
                self.double_dependent_locality,
                self.dependent_locality,
        ]

        for element in elements:
            if element:
                self._append_to_label(element)

        # pad label to length of 7 if not already
        if len(self.address_label) < 7:
            for i in range(7 - len(self.address_label)):
                self.address_label.append('')

        # finally, add post town
        self.address_label[5] = self.post_town

        return ", ".join([f for f in self.address_label if f])