Enabling sessions¶

Sessions are implemented via a piece of middleware.

To enable session functionality, do the following:

• Edit the MIDDLEWARE setting and make sure it contains 'django.contrib.sessions.middleware.SessionMiddleware'. The default settings.py created by django-admin startproject has SessionMiddleware activated.

If you don’t want to use sessions, you might as well remove the SessionMiddleware line from MIDDLEWARE 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, default=__not_given)¶

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

keys()¶

items()¶

setdefault()¶

clear()¶

It also has these methods:

flush()¶

Deletes the current session data from the session and deletes the session 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.

get_session_cookie_age()¶

Returns the value of the setting SESSION_COOKIE_AGE. This can be overridden in a custom session backend.

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.

Note

This method is used by session backends to determine the session expiry age in seconds when saving the session. It is not really intended for usage outside of that context.

In particular, while it is possible to determine the remaining lifetime of a session just when you have the correct modification value and the expiry is set as a datetime object, where you do have the modification value, it is more straight-forward to calculate the expiry by-hand:

expires_at = modification + timedelta(seconds=settings.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.

This function accepts the same keyword arguments as get_expiry_age(), and similar notes on usage apply.

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.

clear_expired()¶

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

cycle_key()¶

Creates a new session key while retaining the current session data. django.contrib.auth.login() calls this method to mitigate against session fixation.

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.check_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 a way to test whether the user’s browser accepts cookies. 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:

from django.http import HttpResponse
from django.shortcuts import render


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(request, "foo/login_form.html")

Using sessions out of views¶

>>> from importlib import import_module
>>> from django.conf import settings
>>> SessionStore = import_module(settings.SESSION_ENGINE).SessionStore

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

>>> from django.contrib.sessions.backends.db import SessionStore
>>> s = SessionStore()
>>> # stored as seconds since epoch since datetimes are not serializable in JSON.
>>> s["last_login"] = 1376587691
>>> s.create()
>>> s.session_key
'2b1189a188b44ad18c35e113ac6ceead'
>>> s = SessionStore(session_key="2b1189a188b44ad18c35e113ac6ceead")
>>> s["last_login"]
1376587691

SessionStore.create() is designed to create a new session (i.e. one not loaded from the session store and with session_key=None). save() is designed to save an existing session (i.e. one loaded from the session store). Calling save() on a new session may also work but has a small chance of generating a session_key that collides with an existing one. create() calls save() and loops until an unused session_key is generated.

If you’re using the django.contrib.sessions.backends.db backend, each session is 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.

The session is not saved if the response’s status code is 500.

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 their 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:

• SESSION_CACHE_ALIAS

• SESSION_COOKIE_AGE

• SESSION_COOKIE_DOMAIN

• SESSION_COOKIE_HTTPONLY

• SESSION_COOKIE_NAME

• SESSION_COOKIE_PATH

• SESSION_COOKIE_SAMESITE

• SESSION_COOKIE_SECURE

• SESSION_ENGINE

• SESSION_EXPIRE_AT_BROWSER_CLOSE

• SESSION_FILE_PATH

• SESSION_SAVE_EVERY_REQUEST

• SESSION_SERIALIZER

Session security¶

Subdomains within a site are able to set cookies on the client for the whole domain. This makes session fixation possible if cookies are permitted from subdomains not controlled by trusted users.

For example, an attacker could log into good.example.com and get a valid session for their account. If the attacker has control over bad.example.com, they can use it to send their session key to you since a subdomain is permitted to set cookies on *.example.com. When you visit good.example.com, you’ll be logged in as the attacker and might inadvertently enter your sensitive personal data (e.g. credit card info) into the attacker’s account.

Another possible attack would be if good.example.com sets its SESSION_COOKIE_DOMAIN to "example.com" which would cause session cookies from that site to be sent to bad.example.com.

Technical details¶

• The session dictionary accepts any json serializable value when using JSONSerializer.

• 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.

Extending database-backed session engines¶

Creating a custom database-backed session engine built upon those included in Django (namely db and cached_db) may be done by inheriting AbstractBaseSession and either SessionStore class.

AbstractBaseSession and BaseSessionManager are importable from django.contrib.sessions.base_session so that they can be imported without including django.contrib.sessions in INSTALLED_APPS.

class base_session.AbstractBaseSession¶

The abstract base session model.

session_key¶

Primary key. The field itself may contain up to 40 characters. The current implementation generates a 32-character string (a random sequence of digits and lowercase ASCII letters).

session_data¶

A string containing an encoded and serialized session dictionary.

expire_date¶

A datetime designating when the session expires.

Expired sessions are not available to a user, however, they may still be stored in the database until the clearsessions management command is run.

classmethod get_session_store_class()¶

Returns a session store class to be used with this session model.

get_decoded()¶

Returns decoded session data.

Decoding is performed by the session store class.

You can also customize the model manager by subclassing BaseSessionManager:

class base_session.BaseSessionManager¶

encode(session_dict)¶

Returns the given session dictionary serialized and encoded as a string.

Encoding is performed by the session store class tied to a model class.

save(session_key, session_dict, expire_date)¶

Saves session data for a provided session key, or deletes the session in case the data is empty.

Customization of SessionStore classes is achieved by overriding methods and properties described below:

class backends.db.SessionStore¶

Implements database-backed session store.

classmethod get_model_class()¶

Override this method to return a custom session model if you need one.

create_model_instance(data)¶

Returns a new instance of the session model object, which represents the current session state.

Overriding this method provides the ability to modify session model data before it’s saved to database.

class backends.cached_db.SessionStore¶

Implements cached database-backed session store.

cache_key_prefix¶

A prefix added to a session key to build a cache key string.

from django.contrib.sessions.backends.db import SessionStore as DBStore
from django.contrib.sessions.base_session import AbstractBaseSession
from django.db import models


class CustomSession(AbstractBaseSession):
    account_id = models.IntegerField(null=True, db_index=True)

    @classmethod
    def get_session_store_class(cls):
        return SessionStore


class SessionStore(DBStore):
    @classmethod
    def get_model_class(cls):
        return CustomSession

    def create_model_instance(self, data):
        obj = super().create_model_instance(data)
        try:
            account_id = int(data.get("_auth_user_id"))
        except (ValueError, TypeError):
            account_id = None
        obj.account_id = account_id
        return obj

class SessionStore(CachedDBStore):
    cache_key_prefix = "mysessions.custom_cached_db_backend"

    # ...

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.