===================
How to use sessions
===================

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 middleware_ and a Django model.

To enable session functionality, do these two things:

    * 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.

    * Add ``'django.contrib.sessions'`` to your ``INSTALLED_APPS`` setting, and
      run ``manage.py syncdb`` to install the single database table that stores
      session data.

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.

.. _middleware: ../middleware/

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.

It implements 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()``

It also has these three methods:

    * ``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.

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 = members.get_object(username__exact=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.")

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

Internally, 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'

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.

Clearing the session table
==========================

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 in ``django/bin/daily_cleanup.py``.
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 `Django settings`_ give you control over session behavior:

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"`` 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_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://www.python.org/doc/current/lib/module-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.
