def METHOD_NAME(self):
    base_fields = super(Customer, self).METHOD_NAME()
    customer_fields = ['first_name', 'last_name', 'customer_reference', 'consolidated_billing',
                       'payment_due_days', 'sales_tax_number', 'sales_tax_percent']
    fields_dict = {field: getattr(self, field, '') for field in
                   customer_fields}
    base_fields.update(fields_dict)
    return base_fields