Flask: index() returns render_template(...) . Why does return index() not work?

Using Flask. This must be a basic gotcha I just don't know about.

I have two templates, index.html and index8.html

index() sets url to one of these two strings, based on a session parameter, and returns render_template(url, data={...})

Then I've got a toggleMode() which sets the session parameter (from a POST), and then calls return index()

It does seem to run the index() code... it prints out index.html or index8.html correctly, which is passed to render_template, and returned, and returned... but it never uses index8.html.

If you change html template files, do you need a redirect? Is that it?

Why does Flask not change templates, in this situation? I'm printing the template name, and it is 'index8.html' passed in to render_template. But it still renders as though I had passed in 'index.html'. (Or, rather, it doesn't do a new render, at all, despite Flask returning a 200 reply to toggleMode())

EDIT:

Ok minimal example...

@app.route('/')
def index():
   
    try:
        mode = session['MODE'] == "Four"
    except:
        session['MODE'] = "Four"
        mode = True

    
    print ('index.html' if mode else 'index8.html')

    url4 = 'index.html'
    url8 = 'index8.html'

    return render_template(url4 if mode else url8, data={...})

@app.route('/toggleMode', methods=['POST'])
def toggle_mode():
    data = request.get_json()
    print (data)
    session['MODE'] = data[0]['mode']
    print(session['MODE'])
    return index()

I get the correct index.html or index8.html printed out, but toggling the mode always renders with index.html

source https://stackoverflow.com/questions/72453184/flask-index-returns-render-template-why-does-return-index-not-work