session of flask

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

app = Flask(__name__)
app.secret_key = b'_5#y2L"F4Q8z\n\xec]/'

@app.route('/')
def index():
    if 'username' in session:
        username = session['username']
        return 'Logged in as {}'.format(username)
    return 'You are not logged in'

@app.route('/login/<username>')
def login(username):
    session['username'] = username
    return redirect(url_for('index'))

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

if __name__ == '__main__':
    app.run(debug=True)