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.

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.

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!')

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.")

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.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_object(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.

Settings

A few Django settings give you control over session behavior:

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.

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.