def ProductsForm(category, products):

    ''' Produces an appropriate _ProductsForm subclass for the given render

    type. '''

    # Each Category.RENDER_TYPE value has a subclass here.

    RENDER_TYPES = {

    }

    # Produce a subclass of _ProductsForm which we can alter the base_fields on

    class ProductsForm(RENDER_TYPES[category.render_type]):

        pass

from django.db.models import Sum

register = template.Library()

ProductAndQuantity = namedtuple("ProductAndQuantity", ["product", "quantity"])

@register.assignment_tag(takes_context=True)

def available_categories(context):

    ''' Returns all of the available product categories '''

    return rego.Category.objects.all()

@register.assignment_tag(takes_context=True)

def invoices(context):

    ''' Returns all of the invoices that this user has. '''

    return rego.Invoice.objects.filter(cart__user=context.request.user)

@register.assignment_tag(takes_context=True)

def items_pending(context):

    ''' Returns all of the items that this user has in their current cart,

    and is awaiting payment. '''

    all_items = rego.ProductItem.objects.filter(

        cart__user=context.request.user,

        cart__active=True,

    )

    return all_items

@register.assignment_tag(takes_context=True)

def items_purchased(context):

    ''' Returns all of the items that this user has purchased '''

    all_items = rego.ProductItem.objects.filter(

        cart__user=context.request.user,

    handled = False if products_form.errors else True

    discounts = discount.available_discounts(request.user, [], products)

    return products_form, discounts, handled

@transaction.atomic

def set_quantities_from_products_form(products_form, current_cart):

    for product_id, quantity, field_name in products_form.product_quantities():

        product = rego.Product.objects.get(pk=product_id)

        try:

            current_cart.set_quantity(product, quantity, batched=True)

        except ValidationError as ve:

            products_form.add_error(field_name, ve)

    if products_form.errors:

        raise ValidationError("Cannot add that stuff")

    current_cart.end_batch()

def handle_voucher(request, prefix):

    ''' Handles a voucher form in the given request. Returns the voucher

    form instance, and whether the voucher code was handled. '''

    voucher_form = forms.VoucherForm(request.POST or None, prefix=prefix)

    current_cart = CartController.for_user(request.user)

            handled = True

    else:

        handled = False

    return (voucher_form, handled)

@login_required

def checkout(request):

    ''' Runs checkout for the current cart of items, ideally generating an

    invoice. '''

    current_cart = CartController.for_user(request.user)