How to use sessions¶

Enabling sessions¶

Sessions are implemented via a piece of middleware.

To enable session functionality, do the following:

• Edit the MIDDLEWARE_CLASSES setting and make sure it 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¶

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

class backends.base.SessionBase¶

This is the base class for all session objects. It 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')

pop(key)¶

Example: fav_color = request.session.pop('fav_color')

keys()¶

items()¶

setdefault()¶

clear()¶

It also has these methods:

flush()¶

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 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)¶

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()¶

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 SESSION_COOKIE_AGE.

This function accepts two optional keyword arguments:

• modification: last modification of the session, as a datetime object. Defaults to the current time.
• expiry: expiry information for the session, as a datetime object, an int() (in seconds), or None. Defaults to the value stored in the session by set_expiry(), if there is one, or None.

get_expiry_date()¶

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 SESSION_COOKIE_AGE seconds from now.

This function accepts the same keyword argumets as get_expiry_age().

get_expire_at_browser_close()¶

Returns either True or False, depending on whether the user’s session cookie will expire when the user’s Web browser is closed.

SessionBase.clear_expired()¶

Removes expired sessions from the session store. This class method is called by clearsessions.

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 = 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.")

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 the set_test_cookie() method of request.session in a view, and call 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¶

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

>>> from django.contrib.sessions.backends.db import SessionStore
>>> import datetime
>>> s = SessionStore()
>>> s['last_login'] = datetime.datetime(2005, 8, 20, 13, 35, 10)
>>> s.save()
>>> s.session_key
'2b1189a188b44ad18c35e113ac6ceead'

>>> s = SessionStore(session_key='2b1189a188b44ad18c35e113ac6ceead')
>>> s['last_login']
datetime.datetime(2005, 8, 20, 13, 35, 0)

In order to prevent session fixation attacks, sessions keys that don’t exist are regenerated:

>>> from django.contrib.sessions.backends.db import SessionStore
>>> s = SessionStore(session_key='no-such-session-here')
>>> s.save()
>>> s.session_key
'ff882814010ccbc3c870523934fee5a2'

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. When set to 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.

This setting is a global default and can be overwritten at a per-session level by explicitly calling the set_expiry() method of request.session as described above in using sessions in views.

Clearing the session store¶

As users create new sessions on your website, session data can accumulate in your session store. If you’re using the database backend, the django_session database table will grow. If you’re using the file backend, your temporary directory will contain an increasing number of files.

To understand this problem, consider what happens with the database backend. 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. A similar process happens with the file backend.

Django does not provide automatic purging of expired sessions. Therefore, it’s your job to purge expired sessions on a regular basis. Django provides a clean-up management command for this purpose: clearsessions. It’s recommended to call this command on a regular basis, for example as a daily cron job.

Note that the cache backend isn’t vulnerable to this problem, because caches automatically delete stale data. Neither is the cookie backend, because the session data is stored by the users’ browsers.

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.