===================

How to use sessions

===================

.. module:: django.contrib.sessions

   :synopsis: Provides session management for Django projects.

Django provides full support for anonymous sessions. The session framework lets

you store and retrieve arbitrary data on a per-site-visitor basis. It stores

data on the server side and abstracts the sending and receiving of cookies.

Cookies contain a session ID -- not the data itself.

Enabling sessions

=================

Sessions are implemented via a piece of :doc:`middleware </ref/middleware>`.

To enable session functionality, do the following:

    * Edit the ``MIDDLEWARE_CLASSES`` setting and make sure

      ``MIDDLEWARE_CLASSES`` contains ``'django.contrib.sessions.middleware.SessionMiddleware'``.

      The default ``settings.py`` created by ``django-admin.py startproject`` has

      ``SessionMiddleware`` activated.

If you don't want to use sessions, you might as well remove the

``SessionMiddleware`` line from ``MIDDLEWARE_CLASSES`` and ``'django.contrib.sessions'``

from your ``INSTALLED_APPS``. It'll save you a small bit of overhead.

Configuring the session engine

==============================

.. versionadded:: 1.0

By default, Django stores sessions in your database (using the model

``django.contrib.sessions.models.Session``). Though this is convenient, in

some setups it's faster to store session data elsewhere, so Django can be

configured to store session data on your filesystem or in your cache.

Using database-backed sessions

------------------------------

If you want to use a database-backed session, you need to add

``'django.contrib.sessions'`` to your ``INSTALLED_APPS`` setting.

Once you have configured your installation, run ``manage.py syncdb``

to install the single database table that stores session data.

Using cached sessions

---------------------

For better performance, you may want to use a cache-based session backend.

.. versionchanged:: 1.1

   Django 1.0 did not include the ``cached_db`` session backend.

To store session data using Django's cache system, you'll first need to make

sure you've configured your cache; see the :doc:`cache documentation

</topics/cache>` for details.

.. warning::

    You should only use cache-based sessions if you're using the Memcached

    cache backend. The local-memory cache backend doesn't retain data long

    enough to be a good choice, and it'll be faster to use file or database

    sessions directly instead of sending everything through the file or

    database cache backends.

Once your cache is configured, you've got two choices for how to store data in

the cache:

    * Set :setting:`SESSION_ENGINE` to

      ``"django.contrib.sessions.backends.cache"`` for a simple caching session

      store. Session data will be stored directly your cache. However, session

      data may not be persistent: cached data can be evicted if the cache fills

      up or if the cache server is restarted.

    * For persistent, cached data, set :setting:`SESSION_ENGINE` to

      ``"django.contrib.sessions.backends.cached_db"``. This uses a

      write-through cache -- every write to the cache will also be written to

      the database. Session reads only use the database if the data is not

      already in the cache.

Both session stores are quite fast, but the simple cache is faster because it

disregards persistence. In most cases, the ``cached_db`` backend will be fast

enough, but if you need that last bit of performance, and are willing to let

session data be expunged from time to time, the ``cache`` backend is for you.

If you use the ``cached_db`` session backend, you also need to follow the

configuration instructions for the `using database-backed sessions`_.

Using file-based sessions

-------------------------

To use file-based sessions, set the ``SESSION_ENGINE`` setting to

``"django.contrib.sessions.backends.file"``.

You might also want to set the ``SESSION_FILE_PATH`` setting (which defaults

to output from ``tempfile.gettempdir()``, most likely ``/tmp``) to control

where Django stores session files. Be sure to check that your Web server has

permissions to read and write to this location.

Using sessions in views

=======================

When ``SessionMiddleware`` is activated, each ``HttpRequest`` object -- the

first argument to any Django view function -- will have a ``session``

attribute, which is a dictionary-like object. You can read it and write to it.

A session object has the following standard dictionary methods:

    * ``__getitem__(key)``

      Example: ``fav_color = request.session['fav_color']``

    * ``__setitem__(key, value)``

      Example: ``request.session['fav_color'] = 'blue'``

    * ``__delitem__(key)``

      Example: ``del request.session['fav_color']``. This raises ``KeyError``

      if the given ``key`` isn't already in the session.

    * ``__contains__(key)``

      Example: ``'fav_color' in request.session``

    * ``get(key, default=None)``

      Example: ``fav_color = request.session.get('fav_color', 'red')``

    * ``keys()``

    * ``items()``

    * ``setdefault()``

    * ``clear()``

.. versionadded:: 1.0

   ``setdefault()`` and ``clear()`` are new in this version.

It also has these methods:

    * ``flush()``

      .. versionadded:: 1.0

      Delete the current session data from the session and regenerate the

      session key value that is sent back to the user in the cookie. This is

      used if you want to ensure that the previous session data can't be

      accessed again from the user's browser (for example, the

      :func:`django.contrib.auth.logout()` function calls it).

    * ``set_test_cookie()``

      Sets a test cookie to determine whether the user's browser supports

      cookies. Due to the way cookies work, you won't be able to test this

      until the user's next page request. See `Setting test cookies`_ below for

      more information.

    * ``test_cookie_worked()``

      Returns either ``True`` or ``False``, depending on whether the user's

      browser accepted the test cookie. Due to the way cookies work, you'll

      have to call ``set_test_cookie()`` on a previous, separate page request.

      See `Setting test cookies`_ below for more information.

    * ``delete_test_cookie()``

      Deletes the test cookie. Use this to clean up after yourself.

    * ``set_expiry(value)``

      .. versionadded:: 1.0

      Sets the expiration time for the session. You can pass a number of

      different values:

            * If ``value`` is an integer, the session will expire after that

              many seconds of inactivity. For example, calling

              ``request.session.set_expiry(300)`` would make the session expire

              in 5 minutes.

            * If ``value`` is a ``datetime`` or ``timedelta`` object, the

              session will expire at that specific date/time.

            * If ``value`` is ``0``, the user's session cookie will expire

              when the user's Web browser is closed.

            * If ``value`` is ``None``, the session reverts to using the global

              session expiry policy.

      Reading a session is not considered activity for expiration

      purposes. Session expiration is computed from the last time the

      session was *modified*.

    * ``get_expiry_age()``

      .. versionadded:: 1.0

      Returns the number of seconds until this session expires. For sessions

      with no custom expiration (or those set to expire at browser close), this

      will equal ``settings.SESSION_COOKIE_AGE``.

    * ``get_expiry_date()``

      .. versionadded:: 1.0

      Returns the date this session will expire. For sessions with no custom

      expiration (or those set to expire at browser close), this will equal the

      date ``settings.SESSION_COOKIE_AGE`` seconds from now.

    * ``get_expire_at_browser_close()``

      .. versionadded:: 1.0

      Returns either ``True`` or ``False``, depending on whether the user's

      session cookie will expire when the user's Web browser is closed.

You can edit ``request.session`` at any point in your view. You can edit it

multiple times.

Session object guidelines

-------------------------

    * Use normal Python strings as dictionary keys on ``request.session``. This

      is more of a convention than a hard-and-fast rule.

    * Session dictionary keys that begin with an underscore are reserved for

      internal use by Django.

    * Don't override ``request.session`` with a new object, and don't access or

      set its attributes. Use it like a Python dictionary.

Examples

--------

This simplistic view sets a ``has_commented`` variable to ``True`` after a user

posts a comment. It doesn't let a user post a comment more than once::

    def post_comment(request, new_comment):

        if request.session.get('has_commented', False):

            return HttpResponse("You've already commented.")

        c = comments.Comment(comment=new_comment)

        c.save()

        request.session['has_commented'] = True

        return HttpResponse('Thanks for your comment!')

This simplistic view logs in a "member" of the site::

    def login(request):

        m = Member.objects.get(username=request.POST['username'])

        if m.password == request.POST['password']:

            request.session['member_id'] = m.id

            return HttpResponse("You're logged in.")

        else:

            return HttpResponse("Your username and password didn't match.")

...And this one logs a member out, according to ``login()`` above::

    def logout(request):

        try:

            del request.session['member_id']

        except KeyError:

            pass

        return HttpResponse("You're logged out.")

The standard ``django.contrib.auth.logout()`` function actually does a bit

more than this to prevent inadvertent data leakage. It calls

``request.session.flush()``. We are using this example as a demonstration of

how to work with session objects, not as a full ``logout()`` implementation.

Setting test cookies

====================

As a convenience, Django provides an easy way to test whether the user's

browser accepts cookies. Just call ``request.session.set_test_cookie()`` in a

view, and call ``request.session.test_cookie_worked()`` in a subsequent view --

not in the same view call.

This awkward split between ``set_test_cookie()`` and ``test_cookie_worked()``

is necessary due to the way cookies work. When you set a cookie, you can't

actually tell whether a browser accepted it until the browser's next request.

It's good practice to use ``delete_test_cookie()`` to clean up after yourself.

Do this after you've verified that the test cookie worked.

Here's a typical usage example::

    def login(request):

        if request.method == 'POST':

            if request.session.test_cookie_worked():

                request.session.delete_test_cookie()

                return HttpResponse("You're logged in.")

            else:

                return HttpResponse("Please enable cookies and try again.")

        request.session.set_test_cookie()

        return render_to_response('foo/login_form.html')

Using sessions out of views

===========================

.. versionadded:: 1.0

An API is available to manipulate session data outside of a view::

    >>> from django.contrib.sessions.backends.db import SessionStore

    >>> s = SessionStore(session_key='2b1189a188b44ad18c35e113ac6ceead')

    >>> s['last_login'] = datetime.datetime(2005, 8, 20, 13, 35, 10)

    >>> s['last_login']

    datetime.datetime(2005, 8, 20, 13, 35, 0)

    >>> s.save()

If you're using the ``django.contrib.sessions.backends.db`` backend, each

session is just a normal Django model. The ``Session`` model is defined in

``django/contrib/sessions/models.py``. Because it's a normal model, you can

access sessions using the normal Django database API::

    >>> from django.contrib.sessions.models import Session

    >>> s = Session.objects.get(pk='2b1189a188b44ad18c35e113ac6ceead')

    >>> s.expire_date

    datetime.datetime(2005, 8, 20, 13, 35, 12)

Note that you'll need to call ``get_decoded()`` to get the session dictionary.

This is necessary because the dictionary is stored in an encoded format::

    >>> s.session_data

    'KGRwMQpTJ19hdXRoX3VzZXJfaWQnCnAyCkkxCnMuMTExY2ZjODI2Yj...'

    >>> s.get_decoded()

    {'user_id': 42}

When sessions are saved

=======================

By default, Django only saves to the session database when the session has been

modified -- that is if any of its dictionary values have been assigned or

deleted::

    # Session is modified.

    request.session['foo'] = 'bar'

    # Session is modified.

    del request.session['foo']

    # Session is modified.

    request.session['foo'] = {}

    # Gotcha: Session is NOT modified, because this alters

    # request.session['foo'] instead of request.session.

    request.session['foo']['bar'] = 'baz'

In the last case of the above example, we can tell the session object

explicitly that it has been modified by setting the ``modified`` attribute on

the session object::

    request.session.modified = True

To change this default behavior, set the ``SESSION_SAVE_EVERY_REQUEST`` setting

to ``True``. If ``SESSION_SAVE_EVERY_REQUEST`` is ``True``, Django will save

the session to the database on every single request.

Note that the session cookie is only sent when a session has been created or

modified. If ``SESSION_SAVE_EVERY_REQUEST`` is ``True``, the session cookie

will be sent on every request.

Similarly, the ``expires`` part of a session cookie is updated each time the

session cookie is sent.

Browser-length sessions vs. persistent sessions

===============================================

You can control whether the session framework uses browser-length sessions vs.

persistent sessions with the ``SESSION_EXPIRE_AT_BROWSER_CLOSE`` setting.

By default, ``SESSION_EXPIRE_AT_BROWSER_CLOSE`` is set to ``False``, which

means session cookies will be stored in users' browsers for as long as

``SESSION_COOKIE_AGE``. Use this if you don't want people to have to log in

every time they open a browser.

If ``SESSION_EXPIRE_AT_BROWSER_CLOSE`` is set to ``True``, Django will use

browser-length cookies -- cookies that expire as soon as the user closes his or

her browser. Use this if you want people to have to log in every time they open

a browser.

.. versionadded:: 1.0

This setting is a global default and can be overwritten at a per-session level

by explicitly calling ``request.session.set_expiry()`` as described above in

`using sessions in views`_.

Clearing the session table

==========================

If you're using the database backend, note that session data can accumulate in

the ``django_session`` database table and Django does *not* provide automatic

purging. Therefore, it's your job to purge expired sessions on a regular basis.

To understand this problem, consider what happens when a user uses a session.

When a user logs in, Django adds a row to the ``django_session`` database

table. Django updates this row each time the session data changes. If the user

logs out manually, Django deletes the row. But if the user does *not* log out,

the row never gets deleted.

Django provides a sample clean-up script: ``django-admin.py cleanup``.

That script deletes any session in the session table whose ``expire_date`` is

in the past -- but your application may have different requirements.

Settings

========

A few :doc:`Django settings </ref/settings>` give you control over session behavior:

SESSION_ENGINE

--------------

.. versionadded:: 1.0

.. versionchanged:: 1.1

   The ``cached_db`` backend was added

Default: ``django.contrib.sessions.backends.db``

Controls where Django stores session data. Valid values are:

    * ``'django.contrib.sessions.backends.db'``

    * ``'django.contrib.sessions.backends.file'``

    * ``'django.contrib.sessions.backends.cache'``

    * ``'django.contrib.sessions.backends.cached_db'``

See `configuring the session engine`_ for more details.

SESSION_FILE_PATH

-----------------

.. versionadded:: 1.0

Default: ``/tmp/``

If you're using file-based session storage, this sets the directory in

which Django will store session data.

SESSION_COOKIE_AGE

------------------

Default: ``1209600`` (2 weeks, in seconds)

The age of session cookies, in seconds.

SESSION_COOKIE_DOMAIN

---------------------

Default: ``None``

The domain to use for session cookies. Set this to a string such as

``".lawrence.com"`` (note the leading dot!) for cross-domain cookies, or use

``None`` for a standard domain cookie.

SESSION_COOKIE_NAME

-------------------

Default: ``'sessionid'``

The name of the cookie to use for sessions. This can be whatever you want.

SESSION_COOKIE_PATH

-------------------

.. versionadded:: 1.0

Default: ``'/'``

The path set on the session cookie. This should either match the URL path of

your Django installation or be parent of that path.

This is useful if you have multiple Django instances running under the same

hostname. They can use different cookie paths, and each instance will only see

its own session cookie.

SESSION_COOKIE_SECURE

---------------------

Default: ``False``

Whether to use a secure cookie for the session cookie. If this is set to

``True``, the cookie will be marked as "secure," which means browsers may

ensure that the cookie is only sent under an HTTPS connection.

SESSION_EXPIRE_AT_BROWSER_CLOSE

-------------------------------

Default: ``False``

Whether to expire the session when the user closes his or her browser. See

"Browser-length sessions vs. persistent sessions" above.

SESSION_SAVE_EVERY_REQUEST

--------------------------

Default: ``False``

Whether to save the session data on every request. If this is ``False``

(default), then the session data will only be saved if it has been modified --

that is, if any of its dictionary values have been assigned or deleted.

.. _Django settings: ../settings/

Technical details

=================

    * The session dictionary should accept any pickleable Python object. See

      `the pickle module`_ for more information.

    * Session data is stored in a database table named ``django_session`` .

    * Django only sends a cookie if it needs to. If you don't set any session

      data, it won't send a session cookie.

.. _`the pickle module`: http://docs.python.org/library/pickle.html

Session IDs in URLs

===================

The Django sessions framework is entirely, and solely, cookie-based. It does

not fall back to putting session IDs in URLs as a last resort, as PHP does.

This is an intentional design decision. Not only does that behavior make URLs

ugly, it makes your site vulnerable to session-ID theft via the "Referer"

header.