Flask app.py file to show how to render a cookie. To see entire Flask code, go to https://github.com/fike/flask-pytest-cookie

app.py

from flask import Flask, render_template
from flask import make_response
from flask import request
import random
from datetime import datetime
import socket

from flask.globals import request
app = Flask(__name__)

def resp_html(rangenumbers, bgcolor, hostname):
    resp = make_response(
            render_template(
                'index.html', 
                the_date=datetime.now(),
                numbers=rangenumbers,
                css_bg=bgcolor,
                hostname=hostname))
    
    return resp

@app.route("/")
def index():
    rangenumbers = range(1,13)
    bgcolor = random.choice( ['orange', 'green', 'purple'] )
    cookie_color = request.cookies.get('color')
    hostname = socket.gethostname()

    if cookie_color is not None:
        bgcolor = cookie_color
        response = resp_html(rangenumbers, bgcolor, hostname)
    else:
        response = response = resp_html(rangenumbers, bgcolor, hostname) 
        response.set_cookie('color', bgcolor)  
    
    return response

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

style.css

body{
    width: 90%;
    margin: auto;
    font-family: "Ubuntu";
}

.green {
    background-color: greenyellow;
 }

.orange {
    background-color: orange;
}

.purple {
    background-color: mediumpurple;
}

test_app.py

import re

def test_cookie(app, client):
    del app
    response = client.get("/")
    assert response.status_code == 200
    cookie_header = response.headers['Set-Cookie']
    cookie_color = re.match('^color=(ligthgray|orange|purple)', cookie_header)
    cookie_match = str(cookie_color.group(1))
    print(cookie_match)
    assert cookie_match in str(['ligthgray', 'orange', 'purple'])