Django2.0手册: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 (unless you’re
using the cookie based backend).

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 database-backed sessions¶

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 migrate
to install the single database table that stores session data.

Using cached sessions¶

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. Additionally, the local-memory cache backend is
NOT multi-process safe, therefore probably not a good choice for production
environments.

If you have multiple caches defined in CACHES, Django will use the
default cache. To use another cache, set SESSION_CACHE_ALIAS to the
name of that cache.

Once your cache is configured, you’ve got two choices for how to store data in
the cache:

  • Set SESSION_ENGINE to
    "django.contrib.sessions.backends.cache" for a simple caching session
    store. Session data will be stored directly in 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.
  • For persistent, cached data, set 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.

Using file-based 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.

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).

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:

  • 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. Note that datetime
    and timedelta values are only serializable if you are using the
    PickleSerializer.
  • 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 arguments 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.

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.

Session serialization¶

By default, Django serializes session data using JSON. You can use the
SESSION_SERIALIZER setting to customize the session serialization
format. Even with the caveats described in Write your own serializer, we highly
recommend sticking with JSON serialization especially if you are using the
cookie backend
.

For example, here’s an attack scenario if you use pickle to serialize
session data. If you’re using the signed cookie session backend and SECRET_KEY is known by an attacker
(there isn’t an inherent vulnerability in Django that would cause it to leak),
the attacker could insert a string into their session which, when unpickled,
executes arbitrary code on the server. The technique for doing so is simple and
easily available on the internet. Although the cookie session storage signs the
cookie-stored data to prevent tampering, a SECRET_KEY leak
immediately escalates to a remote code execution vulnerability.

Bundled serializers

class serializers.JSONSerializer

A wrapper around the JSON serializer from django.core.signing. Can
only serialize basic data types.

In addition, as JSON supports only string keys, note that using non-string
keys in request.session won’t work as expected:

>>> # initial assignment
>>> request.session[0] = 'bar'
>>> # subsequent requests following serialization & deserialization
>>> # of session data
>>> request.session[0]  # KeyError
>>> request.session['0']
'bar'

Similarly, data that can’t be encoded in JSON, such as non-UTF8 bytes like
'\xd9' (which raises UnicodeDecodeError), can’t be stored.

See the Write your own serializer section for more details on limitations
of JSON serialization.

class serializers.PickleSerializer

Supports arbitrary Python objects, but, as described above, can lead to a
remote code execution vulnerability if SECRET_KEY becomes known
by an attacker.

Write your own serializer

Note that unlike PickleSerializer,
the JSONSerializer cannot handle
arbitrary Python data types. As is often the case, there is a trade-off between
convenience and security. If you wish to store more advanced data types
including datetime and Decimal in JSON backed sessions, you will need
to write a custom serializer (or convert such values to a JSON serializable
object before storing them in request.session). While serializing these
values is fairly straightforward
(DjangoJSONEncoder may be helpful),
writing a decoder that can reliably get back the same thing that you put in is
more fragile. For example, you run the risk of returning a datetime that
was actually a string that just happened to be in the same format chosen for
datetimes).

Your serializer class must implement two methods,
dumps(self, obj) and loads(self, data), to serialize and deserialize
the dictionary of session data, respectively.

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.

示例¶

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.

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:

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¶

Note

The examples in this section import the SessionStore object directly
from the django.contrib.sessions.backends.db backend. In your own code,
you should consider importing SessionStore from the session engine
designated by SESSION_ENGINE, as below:

>>> 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 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.

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.

Note

Some browsers (Chrome, for example) provide settings that allow users to
continue browsing sessions after closing and re-opening the browser. In
some cases, this can interfere with the
SESSION_EXPIRE_AT_BROWSER_CLOSE setting and prevent sessions
from expiring on browser close. Please be aware of this while testing
Django applications which have the
SESSION_EXPIRE_AT_BROWSER_CLOSE setting enabled.

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.

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 attackers 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 or any
    picklable Python object when using
    PickleSerializer. 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 SessionStore object¶

When working with sessions internally, Django uses a session store object from
the corresponding session engine. By convention, the session store object class
is named SessionStore and is located in the module designated by
SESSION_ENGINE.

All SessionStore classes available in Django inherit from
SessionBase and implement data manipulation methods,
namely:

In order to build a custom session engine or to customize an existing one, you
may create a new class inheriting from SessionBase or
any other existing SessionStore class.

Extending most of the session engines is quite straightforward, but doing so
with database-backed session engines generally requires some extra effort (see
the next section for details).

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

抽象基本会话模型。

session_key

主键。字段本身可能包含多达40个字符。当前实现生成一个32个字符的字符串(一个随机的数字序列和小写的ascii字母)。

session_data

包含编码和序列化会话字典的字符串。

expire_date

指定会话何时到期的日期时间。

但是,过期的会话对用户不可用,但在运行 clearsessions 管理命令之前,它们仍可能存储在数据库中。

classmethod get_session_store_class()

返回要与此会话模型一起使用的会话存储类。

get_decoded()

返回解码的会话数据。

解码由会话存储类执行。

还可以通过子类 BaseSessionManager 自定义模型管理器。

class base_session.BaseSessionManager
encode(session_dict)

返回序列化并编码为字符串的给定会话字典。

编码由绑定到模型类的会话存储类执行。

save(session_key, session_dict, expire_date)

为提供的会话密钥保存会话数据,或在数据为空时删除会话。

通过重写以下描述的方法和属性,实现了 SessionStore 类的定制:

class backends.db.SessionStore

实现数据库支持的会话存储。

classmethod get_model_class()

如果需要的话,重写此方法以返回自定义会话模型。

create_model_instance(data)

返回会话模型对象的新实例,该实例表示当前会话状态。

重写此方法提供了在将会话模型数据保存到数据库之前修改它的能力。

class backends.cached_db.SessionStore

实现缓存数据库支持的会话存储。

cache_key_prefix

添加到会话键中以生成缓存键字符串的前缀。

例如¶

下面的示例显示了一个自定义数据库支持的会话引擎,它包括一个用于存储帐户id的附加数据库列(从而提供了一个选项,用于查询数据库中帐户的所有活动会话):

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

如果要从Django的内置` cached_db` 会话存储迁移到基于“cached_db“ 的自定义存储,则应重写缓存键前缀,以防止名称空间冲突:

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

    # ...

URL中的会话ID¶

Django会话框架完全是基于cookie的。 正如PHP所做的那样,它不会回退到将会话ID放置在URL中作为最后的手段。 这是一个有意设计的决定。 这种行为不仅使URL变得很难看,而且使您的站点容易受到会话ID的盗用。