from stories import story, argument, Success

class PurchaseProduct:

    @story
    @argument('user')
    @argument('product')
    @argument('shipment_details')
    def purchase(I):

        I.create_order
        I.calculate_price
        I.request_payment
        I.notify_user

    def create_order(self, ctx):

        return Success(order=Order.objects.create(user=ctx.user, product=ctx.product))

    def calculate_price(self, ctx):

        return Success(...