Getting data from the session[] Python, Flask

Question

I'm trying to make simple cart system to the online shop using Python and framework - Flask. I'm getting data from form in products.html then writing it into the session. I have a problem with getting this data and returning it to the cart.html - I'm get just clear page instead of names of the products i added to the cart.

products.html

    {% for el in products %}
        <p>Name {{ el.product_name }}</p>
        <p>Description {{ el.product_description }}</p>
        <p>Image </p> <img width="200" height="200" src="data:;base64,{{ el.product_img }}">
        <p>Cost: {{ el.product_cost }} тенге.</p>

        <form method="post" action="/cart">
            <input type="hidden" name="cart_prod_name" value="{{ el.product_name }}">
            <input type="submit" value="Add to cart">
        </form>
    {% endfor %}

Python function cart() :

@app.route('/cart', methods=['POST', 'GET'])
def cart():
    if 'cart' not in session:
        session['cart'] = []

    if request.method == 'POST':
        cart_prod_name = request.form['cart_prod_name']
        session['cart'] += cart_prod_name
        return redirect('/cart')

    if request.method == 'GET':
        cart_products = session['cart']
        return render_template('cart.html', cart_products=cart_products)

cart.html:

{% for cart_product in cart_products %}
    <p>{{ cart_product.order_prod_name }}</p>
{% endfor %}

Answer 1

From flask.session docs:

Be advised that modifications on mutable structures are not picked up automatically, in that situation you have to explicitly set the attribute to True yourself.

your carts holder object is a mutable structure == list so you have to set after changes

session.modified = True

Answer 2

I sloved that problem. Exactly I needed to iterate cart_product in cart_roducts and output cart_product and don't call it to output order_prod_name .

Fixed cart.html :

{% for cart_product in cart_products %}
    <p>{{ cart_product }}</p>
{% endfor %}