title: session.pop() does not clear cookies
body: <p>I am new to Flask framework and am playing around with it to learn it better. I am following <a href="https://github.com/realpython/discover-flask" rel="nofollow">this</a> tutorial along my way. </p>
  
  <p>As per the <a href="https://www.youtube.com/watch?v=BnBjhmspw4c" rel="nofollow">User Authentication</a> tutorial in the series, I am stuck with the below: </p>
  
  <p>In the tutorial, when the user logs out by hitting the <code>/logout</code> route, the first thing that happens is :</p>
  
  <pre><code>session.pop('logged_in', None)
  </code></pre>
  
  <p>Now as per the video mentioned above, the moment user hits the <code>/logout</code> route the cookie also gets deleted from the browser. </p>
  
  <p>Now 2 questions here:</p>
  
  <ol>
  <li>In my case, with the exact same setup as the tutorial, although the session might be getting invalidated from the server end, the cookie does NOT get deleted/changed in any way from the browser after the <code>/logout</code> route is hit. Is there something wrong that am doing?  </li>
  <li><code>session.pop(...)</code> => how/why exactly will it delete something from the front end, the browser. It can only control things on the server, isn't it ?</li>
  </ol>
  
  <p>For your reference below is my code (taken from the tutorial itself)</p>
  
  <pre><code># import the Flask class from the flask module
  from flask import Flask, render_template, redirect, url_for, request, session, flash
  
  
  # create the application object
  app = Flask(__name__)
  
  app.secret_key = 'my precious'
  
  
  # use decorators to link the function to a url
  @app.route('/')
  def home():
      return "Hello, World!"  # return a string
      #return render_template(index.html)
  
  
  @app.route('/welcome')
  def welcome():
      return render_template('welcome.html')  # render a template
  
  
  # route for handling the login page logic
  @app.route('/login', methods=['GET', 'POST'])
  def login():
      error = None
      if request.method == 'POST':
          if request.form['username'] != 'admin' or request.form['password'] != 'admin':
              error = 'Invalid Credentials. Please try again.'
          else:
              session['logged_in'] = True
              flash('You were just logged in')
              return redirect(url_for('home'))
      return render_template('login.html', error=error)
  
  
  @app.route('/logout')
  def logout():
      session.pop('logged_in', None)
      flash('You were just logged out')
      return redirect(url_for('welcome'))
  
  
  # start the server with the 'run()' method
  if __name__ == '__main__':
      app.run(debug=True)
  </code></pre>