I'm trying to set up flash message to advise user that register was successful, for instance.

I followed this documentation and a few variants, but still it does not show.

I have also found this questions on SO, that suggests that the issue is with the SERVER_NAME configuration.

This should be very straight forward, so please check my code below.



from cs50 import SQL
from flask import Flask, flash, redirect, render_template, request,     session, url_for
from flask_session import Session
from tempfile import mkdtemp
from werkzeug.exceptions import default_exceptions
from werkzeug.security import check_password_hash, generate_password_hash

from helpers import apology, login_required, lookup, usd
from inspect import getmembers
from pprint import pprint

# Configure application
app = Flask(__name__)

# this seems to be the only acceptable parameter, otherwise will 404
app.config['SERVER_NAME'] = 'ide50-myname.cs50.io:8080'

@app.route("/register", methods=["GET", "POST"])
def register():
""" Register user """

# User reached route via POST (as by submitting a form via POST)
if request.method == "POST":
    if not result:
        return apology("Username already taken", 403)

    if result:
        flash("Register succesful")
        # here I tried with plain redirect, without using url_for, with no luck
        # it will show the message if used with render_template, but that does not work well for the app
        return redirect(url_for('login'))

# User reached route via GET (as by clicking a link or via redirect)
    return render_template("register.html")

2 Answers 2


In flask, flash messages are stored in the session cookie. The first thing that login does is session.clear(), which destroys the cookie, and with it, the message.

I would never recommend removing the session.clear() for any number of reasons (it's in the supplied code, it could be a security hole, it might not pass check50).

Notice how the staff solution goes right into the app after you register, not asking for login credentials. Also notice that helpers.login_required checks that session['user_id'] is set. login sets it. You could do something similar in register.

  • Well, I will log the user in automatically after register. Sounds like the only decent solution given the resources. Thanks
    – BernardA
    Commented Apr 9, 2018 at 20:26

I ran into a similar issue with flash() and redirect() too: I was trying to redirect back to /login while displaying a flash error message if the user did not enter a username or wrong password and etc.

In application.py, under @app.route("/login"), I made the following modifications:

    if session["user_id"]:    

This clears the session only if a user is currently logged in, and my flash now work after redirecting. However, I'm not certain if this change causes security issues.

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