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+++ b/thirdparty/google_appengine/lib/django/docs/sessions.txt Tue Aug 26 21:49:54 2008 +0000
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+===================
+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
+=================
+
+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.
+
+.. _middleware: ../middleware/
+
+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.
+
+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.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
+===========================
+
+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.
+
+Clearing the session table
+==========================
+
+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 in ``django/bin/daily_cleanup.py``.
+That script deletes any session in the session table whose ``expire_date`` is
+in the past -- but your application may have different requirements.
+
+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: ``'sessionid'``
+
+The name of the cookie to use for sessions. This can be whatever you want.
+
+SESSION_COOKIE_SECURE
+---------------------
+
+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.
+
+SESSION_EXPIRE_AT_BROWSER_CLOSE
+-------------------------------
+
+Default: ``False``
+
+Whether to expire the session when the user closes his or her browser. See
+"Browser-length sessions vs. persistent sessions" above.
+
+SESSION_SAVE_EVERY_REQUEST
+--------------------------
+
+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.
+
+.. _Django settings: ../settings/
+
+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.
+
+.. _`the pickle module`: http://www.python.org/doc/current/lib/module-pickle.html
+
+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.