def initial_data(cls, product_quantities):
        ''' Prepares initial data for an instance of this form.
        product_quantities is a sequence of (product,quantity) tuples '''

        f = [
            {
                _ItemQuantityProductsForm.CHOICE_FIELD: product.id,
                _ItemQuantityProductsForm.QUANTITY_FIELD: quantity,
            }
            for product, quantity in product_quantities
            if quantity > 0
        ]
        return f
Prepares initial data for an instance of this form.
        product_quantities is a sequence of (product,quantity) tuples