Why does "The session is unavailable because no secret key was set" appear?

Question

Flask

There is such an error when trying to log in
__init__.py

from flask import Flask
...
from flask_session import Session

app = Flask(__name__)

app.config['SECRET_KEY'] = 'gfgfgghghgfhgfhgfhgfhfgghghghghghg'
Session(app)

routes.py

from flask import render_template, session, url_for, redirect, request, Blueprint
...
from mywebsite.users.forms import (LoginForm, RegisterForm)
from mywebsite import db, bcrypt, app

users = Blueprint('users', "__name__")

@users.route('/login', methods=['GET', 'POST'])
def login():
  loginForm = LoginForm()
  if request.method == 'POST':
                # очищается, если уже хранит пользователя
    session.pop('user_id', None)

When I drop flask_session no such error appears

Answer 1

Nicoda didn't use session. I usually use flask_login,
but the logic suggests that session.pop() should be done not for login, but for logout
, even the comment hints that the user is being taken out here, and they are not
inaccurately copy-pasted .

if request.method == 'POST':
    session['user_id'] = request.form['username']
    return redirect(url_for('index'))