How can I detect whether a variable exists in Flask session?

Question

I would like to detect if a session['logged_in'] key exists, which means my user has already logged in.

For Example:

if (session['logged_in'] != None):
    if session['logged_in'] == True:
        return redirect(url_for('hello'))

However, if the key 'logged_in' doesnt exist it therefore generates an error. As a session object is like a dictionary I thought I would be able to use the had_key() method, but this doesn't seem to work either. Is there an easy way to detect if a session contains data without generating an error?

Answer 1

In general, accessing session items is as simple as using a dictionary.

You could use has_key() (there is no had_key() method), but it would be better to use in or get(). An example flask app that accesses session items:

from flask import Flask, session, request, redirect, url_for

app = Flask(__name__)

@app.route('/')
def index():
    if session.get('logged_in') == True:
        return 'You are logged in'
    return 'You are not logged in'

@app.route('/login')
def login():
    session['logged_in'] = True
    return redirect(url_for('index'))

@app.route('/logout')
def logout():
    session.pop('logged_in', None)
    return redirect(url_for('index'))

if __name__ == '__main__':
    app.secret_key = 'ssssshhhhh'
    app.run()

Answer 2

You can use something like this to check if a key exists in the session dict:

if session.get('logged_in'):
    if session['logged_in']:
        return redirect(url_for('hello'))

Answer 3

if session.get('logged_in') is not None:
    # do something for logged in user
    return render_template('success.html')
else:
    # do something for non-logged in user
    return render_template('index.html')

Give that a try, it should work.

Sorry for the belated answer, but was struggling upon the same things myself and this is the basic formula.

In my scenario, I have a users table in a mysql database, by which when a user logs in or creates an account, a session variable is created and set to that user's id.

If this user now navigates to root ('/'), the session variable is detected and the success.html is served.

If no value for session['logged_in'] is found, than index.html is served, where they can create an account or login.