Problems with ajax and session variable in Web2Py

I'm developing an app in Web2Py that consists in a little e-commerce. Have a controller and page that the link is localhost:8000/topranchos/produto, with products, were topranchos is the app.

In the page produto there are a list of products like this:

The image is in this link

When the button "Adicionar ao carrinho" is clicked, the javascript function is executed:

<script>
    function adicionarCarrinho(prod, qtde) {
        quantidade = document.querySelector(qtde).value
        console.log(quantidade)
      if(quantidade > 0) {
          $.get("{{=URL(f="adicionarCarrinho")}}", {produto: prod, qtde: quantidade} )
            .done(function( data ) {
                console.log (data)
                var atual =document.querySelector(".badge-carrinho").innerHTML;
                document.querySelector(".badge-carrinho").innerHTML =
                parseInt(quantidade) + parseInt(atual);
                alert("Adicionado ao carrinho com sucesso");
                });
        }
        else alert("Selecione a quantidade de itens deste produto que você deseja");
    }
</script>

It's make a requisition to the action default/adicionarCarrinho:

def adicionarCarrinho():
if request.vars:
    session.carrinho.append(
    #{'produto':db(db.produto.id == request.vars['produto']).select(),
    {'produto':int(request.vars['produto']),
    'quantidade':int(request.vars['qtde'])}
    )
    print "----------"
    print session.carrinho
    return str("OK")

Where session.carrinho have a list that was declared on db.py model:

#carrinho
session.carrinho = []

On the terminal, the command print session.carrinho print the item received by the ajax request, but when I add other itens the list is empty. When I click on the page of carrinho, that shows the session.carrinho's informations, the var is empty.

How can I repair this? I tried use cookies of course Web2Py book, but I dummie on Web2Py and not has success yet :/

thank you!