Sessions in JustPy


JustPy supports secure, server based sessions.

Sessions are supported by default. If you want to disable them, add the line:


to the configuration file, justpy.env

In the configuration file. You can also set the name of the cookie JustPy will use by setting the value of SESSION_COOKIE_NAME. The default value is 'jp_token'. The max age of the cookie in seconds is set using COOKIE_MAX_AGE, default is 7 days.

Your justpy.env file could include for example:

SESSION_COOKIE_NAME = "my_cookie_name"
SECRET_KEY = "my_very_secret_key_that_only_I_know"

When a request is received and sessions are enabled, JustPy checks if a session cookie already exits. If not, a new unique session id is generated (using uuid4).

The value of the JustPy cookie is signed using the python package itsdangerous. In order to do so, a secret key needs to be provided. Set the secret key by setting the value of SECRET_KEY in the configuration file justpy.env. JustPy comes with a default value ("$$$my_secret_string$$$") but be sure to change this for production.

The advantage of signing the cookie is that cookie tampering can be detected. If JustPy detects that the cookie was tampered with, a Bad Cookie response is returned to the browser.


If you get a Bad Cookie response while testing you site, it may mean that you changed your secret key and your cookie is signed with the previous one. Just erase the cookie in your browser and try again.

If the cookie is validated, the session inserts the session id into the request object as request.session_id.


The JustPy cookie ONLY holds the signed session id. There is no other information in the cookie.


The JustPy cookie is an HttpOnly cookie and is inaccessible to JavaScript on the page.

Simple Example

The session id can be used as an index or key to store information about a session. In the example below we define a dictionary called session_dict and hold the information there. If you need the session data to persist between server restarts, you would use a permanent data store like a a shelve or database to hold session data.

In this example, for each session, a tally is kept of the number of visits and the number of clicks. JustPy provides the session id as part of the information that event handlers receive. If the standard msg argument is used, then the session id can be found in msg.session_id.

import justpy as jp

session_dict = {}

def session_test(request):
   wp = jp.WebPage()
   if request.session_id not in session_dict:
        session_dict[request.session_id] = {'visits': 0, 'events': 0}
   session_data = session_dict[request.session_id]
   session_data['visits'] += 1
   jp.Div(text=f'My session id: {request.session_id}', classes='m-2 p-1 text-xl', a=wp)
   visits_div = jp.Div(text=f'Number of visits: {session_data["visits"]}', classes='m-2 p-1 text-xl', a=wp)
   b = jp.Button(text=f'Number of Click Events: {session_data["events"]}', classes='m-1 bg-blue-500 hover:bg-blue-700 text-white font-bold py-2 px-4 rounded-full', a=wp)
   b.visits_div = visits_div

   def my_click(self, msg):
       session_data = session_dict[msg.session_id]
       session_data['events'] += 1
       self.text =f'Number of Click Events: {session_data["events"]}'
       self.visits_div.text = f'Number of visits: {session_data["visits"]}'

   b.on('click', my_click)
   return wp


Run the program above. Close, open and reload browser tabs and click the button. The session information persists.

Login Example

The example below shows how you could implement a very simple login/logout mechanism.

import justpy as jp
import asyncio

login_form_html = """
<div class="w-full max-w-xs">
  <form class="bg-white shadow-md rounded px-8 pt-6 pb-8 mb-4">
    <div class="mb-4">
      <label class="block text-gray-700 text-sm font-bold mb-2" for="username">
      <input class="shadow appearance-none border rounded w-full py-2 px-3 text-gray-700 leading-tight focus:outline-none focus:shadow-outline"  type="text" placeholder="Username" name="user_name">
    <div class="mb-6">
      <label class="block text-gray-700 text-sm font-bold mb-2" for="password">
      <input class="shadow appearance-none border border-red-500 rounded w-full py-2 px-3 text-gray-700 mb-3 leading-tight focus:outline-none focus:shadow-outline" name="password" type="password" placeholder="******************">
      <p class="text-red-500 text-xs italic">The password is 'password'</p>
    <div class="flex items-center justify-between">
      <button class="bg-blue-500 hover:bg-blue-700 text-white font-bold py-2 px-4 rounded focus:outline-none focus:shadow-outline" type="button" name="sign_in_btn">
        Sign In
  <p class="text-center text-gray-500 text-xs">
    Example from

alert_html = """
<div role="alert" class="m-1 p-1 w-1/3">
  <div class="bg-red-500 text-white font-bold rounded-t px-4 py-2">
    Password is incorrect
  <div class="border border-t-0 border-red-400 rounded-b bg-red-100 px-4 py-3 text-red-700">
    <p>Please enter password again.</p>

users = {}

async def login_test(request):
    wp = jp.WebPage()
    session_id = request.session_id
    if session_id in users:
        if users[session_id]['logged_in']:
            jp.Div(a=wp, text=f'Your session id is: {session_id}', classes='m-1 p-1 text-xl ')
            jp.Div(a=wp, text=f'You are already logged in...', classes='m-1 p-1 text-2l')
            log_out_btn = jp.Button(text='Logout', classes=jp.Styles.button_bordered + ' m-1 p-1', a=wp)
            log_out_btn.s_id = session_id

            def log_out(self, msg):
                users[self.s_id]['logged_in'] = False
       = '/login_test'

            log_out_btn.on('click', log_out)

            logged_in = True
            logged_in = False
        users[session_id] = {}
        users[session_id]['logged_in'] = False
        logged_in = False
    if not logged_in:
        return await login_page(request)  # Return different page if not logged in
    return wp

async def login_page(request):
        if users[request.session_id]['logged_in']:
            return await login_test(request)
    wp = jp.WebPage()
    wp.display_url = 'login_page'  # Sets the url to display in browser without reloading page
    jp.Div(text='Please login', a=wp, classes='m-2 p-2 w-1/4 text-xl font-semibold')
    login_form = jp.parse_html(login_form_html, a=wp, classes='m-2 p-2 w-1/4')
    alert = jp.parse_html(alert_html, show=False, a=wp)
    session_div = jp.Div(text='Session goes here', classes='m-1 p-1 text-xl', a=wp)
    sign_in_btn = login_form.name_dict['sign_in_btn']
    sign_in_btn.user_name = login_form.name_dict['user_name']
    sign_in_btn.session_id = request.session_id
    sign_in_btn.alert = alert

    async def sign_in_click(self, msg):
        if login_form.name_dict['password'].value == 'password':
            session_div.text = request.session_id + ' logged in successfully'
   = False
            return await login_successful(wp, request.session_id)
            session_div.text = request.session_id + ' login not successful'
   = True

    sign_in_btn.on('click', sign_in_click)

    return wp

async def login_successful(wp, s_id):
    users[s_id]['logged_in'] = True
    wp.display_url = 'login_successful'
    jp.Div(text='Login successful. You are now logged in', classes='m-1 p-1 text-2xl', a=wp)
    await wp.update()
    await asyncio.sleep(3)
    wp.redirect = '/login_test'