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 (unless you’re using the cookie based backend).
Sessions are implemented via a piece of middleware.
To enable session functionality, do the following:
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.
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.
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.
For better performance, you may want to use a cache-based 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 cache documentation 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:
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.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.
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.
To use cookies-based sessions, set the SESSION_ENGINE
setting to
"django.contrib.sessions.backends.signed_cookies"
. The session data will be
stored using Django’s tools for cryptographic signing
and the SECRET_KEY
setting.
Note
It’s recommended to leave the SESSION_COOKIE_HTTPONLY
setting
True
to prevent tampering of the stored data from JavaScript.
Warning
The session data is signed but not encrypted
When using the cookies backend the session data can be read by the client.
A MAC (Message Authentication Code) is used to protect the data against changes by the client, so that the session data will be invalidated when being tampered with. The same invalidation happens if the client storing the cookie (e.g. your user’s browser) can’t store all of the session cookie and drops data. Even though Django compresses the data, it’s still entirely possible to exceed the common limit of 4096 bytes per cookie.
No freshness guarantee
Note also that while the MAC can guarantee the authenticity of the data
(that it was generated by your site, and not someone else), and the
integrity of the data (that it is all there and correct), it cannot
guarantee freshness i.e. that you are being sent back the last thing you
sent to the client. This means that for some uses of session data, the
cookie backend might open you up to replay attacks. Cookies will only be
detected as ‘stale’ if they are older than your
SESSION_COOKIE_AGE
.
Performance
Finally, the size of a cookie can have an impact on the speed of your site.
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.
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).
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.
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.
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:
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.value
is a datetime
or timedelta
object, the
session will expire at that specific date/time.value
is 0
, the user’s session cookie will expire
when the user’s Web browser is closed.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
.
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.
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.
request.session
. This
is more of a convention than a hard-and-fast rule.request.session
with a new object, and don’t access or
set its attributes. Use it like a Python dictionary.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 the
flush()
method of request.session
.
We are using this example as a demonstration of how to work with session
objects, not as a full logout()
implementation.
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')
An API is available to manipulate session data outside of a view:
>>> from django.contrib.sessions.backends.db import SessionStore
>>> import datetime
>>> 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 session_key
isn’t provided, one will be generated automatically:
>>> from django.contrib.sessions.backends.db import SessionStore
>>> s = SessionStore()
>>> s.save()
>>> s.session_key
'2b1189a188b44ad18c35e113ac6ceead'
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}
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.
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.
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.
A few Django settings give you control over session behavior:
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'
'django.contrib.sessions.backends.signed_cookies'
See configuring the session engine for more details.
Default: /tmp/
If you’re using file-based session storage, this sets the directory in which Django will store session data.
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.
Default: True
Whether to use HTTPOnly flag on the session cookie. If this is set to
True
, client-side JavaScript will not to be able to access the
session cookie.
HTTPOnly is a flag included in a Set-Cookie HTTP response header. It is not part of the RFC 2109 standard for cookies, and it isn’t honored consistently by all browsers. However, when it is honored, it can be a useful way to mitigate the risk of client side script accessing the protected cookie data.
Default: 'sessionid'
The name of the cookie to use for sessions. This can be whatever you want.
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.
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.
Default: False
Whether to expire the session when the user closes his or her browser. See “Browser-length sessions vs. persistent sessions” above.
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.
pickle
module for more information.django_session
.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.
Jul 07, 2017