login.py

@app.route("/")
@user_login_needed
def index():
    return render_template("index.html")
    
return user_login_needed(index) # no extra parameters passed since index does not take any parameters


@app.route("/login", methods=["GET", "POST"])
def login():
    if request.method == "GET":
        return render_template("login.html")
    elif request.method == "POST":
        user_name = request.form['username']
        password = request.form['password']
        if is_user_valid(user_name, password):
            session['user_logged_in'] = True 
            error_message = "User {} successfuly logged in.".format(user_name)
            flash(error_message)
            print(error_message)
            return redirect(url_for("index"))
        else:
            print("Invalid user.")
            flash("Invalid user/password!")
            return redirect(url_for("login"))
    else:
        error_message = "Invalid request method:{}".format(request.method)
        print(error_message)
        flash(error_message)
        return redirect(url_for("login"))

    
def is_user_valid(user_name, password):
    return user_name == "john" and password == "1234"
    

@app.route("/logout", methods=["GET"])
def logout():
    session['user_logged_in'] = False
    print("User logout successful.")
    flash("Logout successful") 
    return redirect(url_for("login"))