Flask Session Clears When Ever Redirecting

Question

my flask session clears whenever I redirect. Python code below. I am trying to add some cart items to the session, and then be able to add more items to the cart. The issue is, whenever I do: return redirect(url_for('buy')), the a get request is triggered asking for the page, and when the button is pressed, when the post request is send, the session is cleared.

from flask import *
from flask_pymongo import PyMongo
from flask_bootstrap import Bootstrap

app = Flask(__name__)

app.config['MONGO_URI'] = "mongodb://localhost:27017/one-stop-shop"
app.secret_key = 'asdfasdfasdf asdfa saf sdafw er wafe  wf waef wa we awe dsf sadl f;jaosidf hopwji'

Bootstrap(app)
mongo = PyMongo(app)


@app.route('/')
def home():
    return render_template('home.html')


@app.route('/add', methods=['GET', 'POST'])
def add():
    if request.method == 'GET':
        return render_template('add.html')
    elif request.method == 'POST':
        doc = {}
        for item in request.form:
            if item != 'add':
                doc[item] = request.form[item]
        mongo.db.products.insert_one(doc)
        return redirect('/')


@app.route('/buy', methods=['GET', 'POST'])
def buy():
    if request.method == 'GET':
        session['cart-items'] = {}
        items = list(mongo.db.products.find({}))
        return render_template('buy2.html', products=items)
    elif request.method == 'POST':
        try:
            print(session)
            items2 = session['cart-items']
        except KeyError:
            items2 = {}
        items = {}
        for name, value in request.form.items():
            items[name] = value
        print(items, 1)
        if items2 != {}:
            for key, value in items.items():
                if key in items2:
                    if items[key] + items2[key] > mongo.db.products.find({'_id': key}):
                        items[key] = mongo.db.products.find({'_id': key})['Quantity Limit']
                    else:
                        items[key] += items2[key]
        session['cart-items'] = items
        print(items, 2)
        print(items2, 3)
        print(session)
        print(session['cart-items'])
        return redirect(url_for('buy'))

if __name__ == '__main__':
    app.run(debug=True)

Any help is appreciated. BTW after clearing, when doing print(session), this is the output: <SecureCookieSession {'cart-items': {}}

Answer 1

Try setting session.modified = true before you redirect.

From the 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.