Flask: How to render a template AND save request.headers info?

from flask import Flask
import uuid

...

uuid_to_email = {}

...

@app.route('/')
def index():
    user_id = str(uuid.uuid4())
    uuid_to_email[user_id] = request.headers.get('X-MS-CLIENT-PRINCIPAL-NAME')
    return render_template('index.html', uuid=user_id) # where it is implied that you would then use the uuid in the client-side code to story it and pass it back to the endpoints you want to do that with