Cart Checkers

Cart checkers are functions that determine whether or not a cart can be checked out in its current state. If a cart is ready to be checked out, and all cart checkers pass, it is said to be complete.

If you think of the entire cart as being like a form, cart checkers are like validators. (We don’t actually call them that, because Django REST Framework validators perform a separate function within Lorikeet; ensuring that individual instances of LineItem, DeliveryAddress and so on are valid.)

Cart checkers are run in two places. One is in the GET /_cart/ endpoint, where any checkers that fail are listed in incomplete_reasons, so the client user interface can show details. The other is in the POST /_cart/checkout/ endpoint, where any checkers that fail will prevent checkout from happening, resulting in a 422 response with a reason of "incomplete".

Writing a Cart Checker

Cart checkers are functions that accept a Cart instance as an argument; they should either raise a IncompleteCartError if the cart is not ready to be checked out, or return successfully if it is.

Here’s one that’s built in to Lorikeet:

def payment_method_required(cart):
    """Checks that a payment method is set on the cart."""

    if cart.payment_method is None:
        raise IncompleteCartError(code='not_set',
                                  message='A payment method is required.',

If your cart checker identifies multiple different reasons the cart can’t be checked out, it should instead raise a IncompleteCartErrorSet, which can be passed a list of IncompleteCartError instances.

Once you’ve written your cart checker, add it to the LORIKEET_CART_COMPLETE_CHECKERS setting.


The default value for LORIKEET_CART_COMPLETE_CHECKERS contains important built-in checkers that you probably don’t want to disable, because they prevent things like going through checkout with an empty cart. If you override this setting, make sure you include them!

Built-in Cart Checkers

The built-in cart checkers are documented in the Cart Checkers section of the API documentation.

Handling an Incomplete Cart on the Client


Document the API side of things