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## Django documentation

# 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

Session functionality is enabled by default.

You can turn session functionality on and off by editing the `MIDDLEWARE_CLASSES` setting. To activate sessions, make sure `MIDDLEWARE_CLASSES` contains `"django.middleware.sessions.SessionMiddleware"`.

If you're dealing with an admin site, make sure the `SessionMiddleware` line appears before the `AdminUserRequired` line. (The middleware classes are applied in order, and the admin middleware requires that the session middleware come first.)

If you don't want to use sessions, you might as well remove the `SessionMiddleware` line from `MIDDLEWARE_CLASSES`. It'll save you a small bit of overhead.

## Using sessions in views

Each `HttpRequest` object -- the first argument to any Django view function -- has 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.
> - `get(key, default=None)` Example: `fav_color = request.session.get('fav_color', 'red')`

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.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')

## Using sessions out of views

Internally, each session is just a normal Django model. The `Session` model is defined in `django/models/core.py`. Because it's a normal model, you can access sessions using the normal Django database API:

    >>> from django.models.core import sessions
    >>> s = sessions.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'

Only available in Django development version. 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.

## 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: `'hotclub'`

The name of the cookie to use for sessions. This can be whatever you want.

`'hotclub'` is a reference to the Hot Club of France, the band Django Reinhardt played in.

### SESSION_SAVE_EVERY_REQUEST

Default: `False`

Only available in Django development version.

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.

## 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 `core_sessions` .
> - 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.

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