Session[‘username‘] = ‘admin‘

session.pop('username', None)

@app.route('/')
def index():
   if 'username' in session:
      username = session['username']
         return 'Logged in as ' + username + '<br>' + \
         "<b><a href = '/logout'>click here to log out</a></b>"
   return "You are not logged in <br><a href = '/login'></b>" + \
      "click here to log in</b></a>"

@app.route('/login', methods = ['GET', 'POST'])
def login():
   if request.method == 'POST':
      session['username'] = request.form['username']
      return redirect(url_for('index'))
   return '''

   <form action = "" method = "post">
      <p><input type = text name = username/></p>
      <p<<input type = submit value = Login/></p>
   </form>

   '''

@app.route('/logout')
def logout():
   # remove the username from the session if it is there
   session.pop('username', None)
   return redirect(url_for('index'))

from flask import Flask, session, redirect, url_for, escape, request
app = Flask(__name__)
app.secret_key = 'any random string‘