code: def login():
      
      if request.method == "POST":
          username = request.form["username"]
          password = request.form["password"]
          error = None
          user = User.query.filter_by(username=username).first()
  
          if user is None:
              error = "Incorrect username."
          elif not user.check_password(password):
              error = "Incorrect password."
  
          if error is None:
              # store the user id in a new session and return to the index
              session.clear()
              session["user_id"] = user.id
              return redirect(url_for("index"))
  
          flash(error)
  
      return render_template("auth/login.html")
docs: Log in a registered user by adding the user id to the session.