I am a web-dev noob but I'll try my best to be clear about the problem, explain my approach, and what I've tried. I also include imports in-case those are causing the problem but I'm pretty sure I've isolated it to what I describe.

I am trying to use Flask-session to keep information private but some values are "lost". This code is trimmed down significantly for simplicity. The user lands on /send and we render the loading template. loading.js does fetch() on /deploy while it runs an animation, then() we go to /results when the deploy function is done.

loading.js

function navigate() {
    window.location.href = 'results';  // redirect to results page when done!
}


 // deploy the contract while the loading screen goes then navigate to results page
const data = fetch('deploy').then(navigate);
  
loopThroughMessages(messages);

main.py

from flask_session import Session

app = Flask(__name__, 
static_folder='static',
template_folder='templates')

# for the session, i.e passing values 
app.config["SESSION_PERMANENT"] = False
app.config["SESSION_TYPE"] = "filesystem"
app.config.from_object(__name__)
Session(app)


@app.route('/send')
def main():
    # take url parameters and do stuff with them
    return render_template('loading.html')


@app.route("/deploy")
def deploy_contract():
    session['contract_address'] = some_fnc()

    # fetch() requires that this function return a json
    return {} 


@app.route("/results")
def serve_results_page():
    # pull saved values from the session
    data = {'contract_key' : session['contract_address']
    } # calling session here causes the error, the contract_address key doesn't exist

    return render_template('results.html', data=data)

So contract_address is saved to the session but when we get to /results, the server has no way to associate that session with the client.

We want to keep our contract_address private so sending it to loading.js is not an option. I'm guessing that since http is stateless, I need to pass a cookie to and from my js and python files but I'm a bit lost on how to implement it. Are cookies unnecessary (because the server doesn't actually need to receive any data from my js files)? Should I be using redirects or something besides fetch()?

Hacky fixes, different approaches, and resources are all welcome. I feel like I'm close, like there's a simple way to use cookies that I'm overlooking.

I will be continuing to research and detail the approaches I'm considering Edit1: Looking at Flask's should_set_cookie method