root/django/tags/releases/0.95/docs/authentication.txt

=============================
User authentication in Django
=============================

Django comes with a user authentication system. It handles user accounts,
groups, permissions and cookie-based user sessions. This document explains how
things work.

Overview
========

The auth system consists of:

    * Users
    * Permissions: Binary (yes/no) flags designating whether a user may perform
      a certain task.
    * Groups: A generic way of applying labels and permissions to more than one
      user.
    * Messages: A simple way to queue messages for given users.

Installation
============

Authentication support is bundled as a Django application in
``django.contrib.auth``. To install it, do the following:

    1. Put ``'django.contrib.auth'`` in your ``INSTALLED_APPS`` setting.
    2. Run the command ``manage.py syncdb``.

Note that the default ``settings.py`` file created by
``django-admin.py startproject`` includes ``'django.contrib.auth'`` in
``INSTALLED_APPS`` for convenience. If your ``INSTALLED_APPS`` already contains
``'django.contrib.auth'``, feel free to run ``manage.py syncdb`` again; you
can run that command as many times as you'd like, and each time it'll only
install what's needed.

The ``syncdb`` command creates the necessary database tables, creates
permission objects for all installed apps that need 'em, and prompts you to
create a superuser account the first time you run it.

Once you've taken those steps, that's it.

Users
=====

Users are represented by a standard Django model, which lives in
`django/contrib/auth/models.py`_.

.. _django/contrib/auth/models.py: http://code.djangoproject.com/browser/django/trunk/django/contrib/auth/models.py

API reference
-------------

Fields
~~~~~~

``User`` objects have the following fields:

    * ``username`` -- Required. 30 characters or fewer. Alphanumeric characters
      only (letters, digits and underscores).
    * ``first_name`` -- Optional. 30 characters or fewer.
    * ``last_name`` -- Optional. 30 characters or fewer.
    * ``email`` -- Optional. E-mail address.
    * ``password`` -- Required. A hash of, and metadata about, the password.
      (Django doesn't store the raw password.) Raw passwords can be arbitrarily
      long and can contain any character. See the "Passwords" section below.
    * ``is_staff`` -- Boolean. Designates whether this user can access the
      admin site.
    * ``is_active`` -- Boolean. Designates whether this user can log into the
      Django admin. Set this to ``False`` instead of deleting accounts.
    * ``is_superuser`` -- Boolean. Designates that this user has all permissions
      without explicitly assigning them.
    * ``last_login`` -- A datetime of the user's last login. Is set to the
      current date/time by default.
    * ``date_joined`` -- A datetime designating when the account was created.
      Is set to the current date/time by default when the account is created.

Methods
~~~~~~~

``User`` objects have two many-to-many fields: ``groups`` and
``user_permissions``. ``User`` objects can access their related
objects in the same way as any other `Django model`_::

    myuser.objects.groups = [group_list]
    myuser.objects.groups.add(group, group,...)
    myuser.objects.groups.remove(group, group,...)
    myuser.objects.groups.clear()
    myuser.objects.permissions = [permission_list]
    myuser.objects.permissions.add(permission, permission, ...)
    myuser.objects.permissions.remove(permission, permission, ...]
    myuser.objects.permissions.clear()

In addition to those automatic API methods, ``User`` objects have the following
custom methods:

    * ``is_anonymous()`` -- Always returns ``False``. This is a way of
      differentiating ``User`` and ``AnonymousUser`` objects. Generally, you
      should prefer using ``is_authenticated()`` to this method.

    * ``is_authenticated()`` -- Always returns ``True``. This is a way to
      tell if the user has been authenticated.

    * ``get_full_name()`` -- Returns the ``first_name`` plus the ``last_name``,
      with a space in between.

    * ``set_password(raw_password)`` -- Sets the user's password to the given
      raw string, taking care of the password hashing. Doesn't save the
      ``User`` object.

    * ``check_password(raw_password)`` -- Returns ``True`` if the given raw
      string is the correct password for the user. (This takes care of the
      password hashing in making the comparison.)

    * ``get_group_permissions()`` -- Returns a list of permission strings that
      the user has, through his/her groups.

    * ``get_all_permissions()`` -- Returns a list of permission strings that
      the user has, both through group and user permissions.

    * ``has_perm(perm)`` -- Returns ``True`` if the user has the specified
      permission, where perm is in the format ``"package.codename"``.

    * ``has_perms(perm_list)`` -- Returns ``True`` if the user has each of the
      specified permissions, where each perm is in the format
      ``"package.codename"``.

    * ``has_module_perms(package_name)`` -- Returns ``True`` if the user has
      any permissions in the given package (the Django app label).

    * ``get_and_delete_messages()`` -- Returns a list of ``Message`` objects in
      the user's queue and deletes the messages from the queue.

    * ``email_user(subject, message, from_email=None)`` -- Sends an e-mail to
      the user. If ``from_email`` is ``None``, Django uses the
      `DEFAULT_FROM_EMAIL`_ setting.

    * ``get_profile()`` -- Returns a site-specific profile for this user.
      Raises ``django.contrib.auth.models.SiteProfileNotAvailable`` if the current site
      doesn't allow profiles.

.. _Django model: http://www.djangoproject.com/documentation/model_api/
.. _DEFAULT_FROM_EMAIL: http://www.djangoproject.com/documentation/settings/#default-from-email

Manager functions
~~~~~~~~~~~~~~~~~

The ``User`` model has a custom manager that has the following helper functions:

    * ``create_user(username, email, password)`` -- Creates, saves and returns
      a ``User``. The ``username``, ``email`` and ``password`` are set as
      given, and the ``User`` gets ``is_active=True``.

      See _`Creating users` for example usage.

    * ``make_random_password(length=10, allowed_chars='abcdefghjkmnpqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ23456789')``
      Returns a random password with the given length and given string of
      allowed characters. (Note that the default value of ``allowed_chars``
      doesn't contain ``"I"`` or letters that look like it, to avoid user
      confusion.

Basic usage
-----------

Creating users
~~~~~~~~~~~~~~

The most basic way to create users is to use the ``create_user`` helper
function that comes with Django::

    >>> from django.contrib.auth.models import User
    >>> user = User.objects.create_user('john', 'lennon@thebeatles.com', 'johnpassword')

    # At this point, user is a User object ready to be saved
    # to the database. You can continue to change its attributes
    # if you want to change other fields.
    >>> user.is_staff = True
    >>> user.save()

Changing passwords
~~~~~~~~~~~~~~~~~~

Change a password with ``set_password()``::

    >>> from django.contrib.auth.models import User
    >>> u = User.objects.get(username__exact='john')
    >>> u.set_password('new password')
    >>> u.save()

Don't set the ``password`` attribute directly unless you know what you're
doing. This is explained in the next section.

Passwords
---------

The ``password`` attribute of a ``User`` object is a string in this format::

    hashtype$salt$hash

That's hashtype, salt and hash, separated by the dollar-sign character.

Hashtype is either ``sha1`` (default) or ``md5`` -- the algorithm used to
perform a one-way hash of the password. Salt is a random string used to salt
the raw password to create the hash.

For example::

    sha1$a1976$a36cc8cbf81742a8fb52e221aaeab48ed7f58ab4

The ``User.set_password()`` and ``User.check_password()`` functions handle
the setting and checking of these values behind the scenes.

Previous Django versions, such as 0.90, used simple MD5 hashes without password
salts. For backwards compatibility, those are still supported; they'll be
converted automatically to the new style the first time ``check_password()``
works correctly for a given user.

Anonymous users
---------------

``django.contrib.auth.models.AnonymousUser`` is a class that implements
the ``django.contrib.auth.models.User`` interface, with these differences:

    * ``id`` is always ``None``.
    * ``is_anonymous()`` returns ``True`` instead of ``False``.
    * ``is_authenticated()`` returns ``False`` instead of ``True``.
    * ``has_perm()`` always returns ``False``.
    * ``set_password()``, ``check_password()``, ``save()``, ``delete()``,
      ``set_groups()`` and ``set_permissions()`` raise ``NotImplementedError``.

In practice, you probably won't need to use ``AnonymousUser`` objects on your
own, but they're used by Web requests, as explained in the next section.

Creating superusers
-------------------

``manage.py syncdb`` prompts you to create a superuser the first time you run
it after adding ``'django.contrib.auth'`` to your ``INSTALLED_APPS``. But if
you need to create a superuser after that via the command line, you can use the
``create_superuser.py`` utility. Just run this command::

    python /path/to/django/contrib/auth/create_superuser.py

Make sure to substitute ``/path/to/`` with the path to the Django codebase on
your filesystem.

Authentication in Web requests
==============================

Until now, this document has dealt with the low-level APIs for manipulating
authentication-related objects. On a higher level, Django can hook this
authentication framework into its system of `request objects`_.

First, install the ``SessionMiddleware`` and ``AuthenticationMiddleware``
middlewares by adding them to your ``MIDDLEWARE_CLASSES`` setting. See the
`session documentation`_ for more information.

Once you have those middlewares installed, you'll be able to access
``request.user`` in views. ``request.user`` will give you a ``User`` object
representing the currently logged-in user. If a user isn't currently logged in,
``request.user`` will be set to an instance of ``AnonymousUser`` (see the
previous section). You can tell them apart with ``is_authenticated()``, like so::

    if request.user.is_authenticated():
        # Do something for authenticated users.
    else:
        # Do something for anonymous users.

.. _request objects: http://www.djangoproject.com/documentation/request_response/#httprequest-objects
.. _session documentation: http://www.djangoproject.com/documentation/sessions/

How to log a user in
--------------------

Django provides two functions in ``django.contrib.auth``: ``authenticate()``
and ``login()``.

To authenticate a given username and password, use ``authenticate()``. It
takes two keyword arguments, ``username`` and ``password``, and it returns
a ``User`` object if the password is valid for the given username. If the
password is invalid, ``authenticate()`` returns ``None``. Example::

    from django.contrib.auth import authenticate
    user = authenticate(username='john', password='secret')
    if user is not None:
        print "You provided a correct username and password!"
    else:
        print "Your username and password were incorrect."

To log a user in, in a view, use ``login()``. It takes an ``HttpRequest``
object and a ``User`` object. ``login()`` saves the user's ID in the session,
using Django's session framework, so, as mentioned above, you'll need to make
sure to have the session middleware installed.

This example shows how you might use both ``authenticate()`` and ``login()``::

    from django.contrib.auth import authenticate, login

    def my_view(request):
        username = request.POST['username']
        password = request.POST['password']
        user = authenticate(username=username, password=password)
        if user is not None:
            login(request, user)
            # Redirect to a success page.
        else:
            # Return an error message.

How to log a user out
---------------------

To log out a user who has been logged in via ``django.contrib.auth.login()``,
use ``django.contrib.auth.logout()`` within your view. It takes an
``HttpRequest`` object and has no return value. Example::

    from django.contrib.auth import logout

    def logout_view(request):
        logout(request)
        # Redirect to a success page.

Note that ``logout()`` doesn't throw any errors if the user wasn't logged in.

Limiting access to logged-in users
----------------------------------

The raw way
~~~~~~~~~~~

The simple, raw way to limit access to pages is to check
``request.user.is_authenticated()`` and either redirect to a login page::

    from django.http import HttpResponseRedirect

    def my_view(request):
        if not request.user.is_authenticated():
            return HttpResponseRedirect('/login/?next=%s' % request.path)
        # ...

...or display an error message::

    def my_view(request):
        if not request.user.is_authenticated():
            return render_to_response('myapp/login_error.html')
        # ...

The login_required decorator
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

As a shortcut, you can use the convenient ``login_required`` decorator::

    from django.contrib.auth.decorators import login_required

    def my_view(request):
        # ...
    my_view = login_required(my_view)

Here's an equivalent example, using the more compact decorator syntax
introduced in Python 2.4::

    from django.contrib.auth.decorators import login_required

    @login_required
    def my_view(request):
        # ...

``login_required`` does the following:

    * If the user isn't logged in, redirect to ``/accounts/login/``, passing
      the current absolute URL in the query string as ``next``. For example:
      ``/accounts/login/?next=/polls/3/``.
    * If the user is logged in, execute the view normally. The view code is
      free to assume the user is logged in.

Note that you'll need to map the appropriate Django view to ``/accounts/login/``.
To do this, add the following line to your URLconf::

    (r'^accounts/login/$', 'django.contrib.auth.views.login'),

Here's what ``django.contrib.auth.views.login`` does::

    * If called via ``GET``, it displays a login form that POSTs to the same
      URL. More on this in a bit.

    * If called via ``POST``, it tries to log the user in. If login is
      successful, the view redirects to the URL specified in ``next``. If
      ``next`` isn't provided, it redirects to ``/accounts/profile/`` (which is
      currently hard-coded). If login isn't successful, it redisplays the login
      form.

It's your responsibility to provide the login form in a template called
``registration/login.html`` by default. This template gets passed three
template context variables:

    * ``form``: A ``FormWrapper`` object representing the login form. See the
      `forms documentation`_ for more on ``FormWrapper`` objects.
    * ``next``: The URL to redirect to after successful login. This may contain
      a query string, too.
    * ``site_name``: The name of the current ``Site``, according to the
      ``SITE_ID`` setting. See the `site framework docs`_.

If you'd prefer not to call the template ``registration/login.html``, you can
pass the ``template_name`` parameter via the extra arguments to the view in
your URLconf. For example, this URLconf line would use ``myapp/login.html``
instead::

    (r'^accounts/login/$', 'django.contrib.auth.views.login', {'template_name': 'myapp/login.html'}),

Here's a sample ``registration/login.html`` template you can use as a starting
point. It assumes you have a ``base.html`` template that defines a ``content``
block::

    {% extends "base.html" %}

    {% block content %}

    {% if form.has_errors %}
    <p>Your username and password didn't match. Please try again.</p>
    {% endif %}

    <form method="post" action=".">
    <table>
    <tr><td><label for="id_username">Username:</label></td><td>{{ form.username }}</td></tr>
    <tr><td><label for="id_password">Password:</label></td><td>{{ form.password }}</td></tr>
    </table>

    <input type="submit" value="login" />
    <input type="hidden" name="next" value="{{ next }}" />
    </form>

    {% endblock %}

.. _forms documentation: http://www.djangoproject.com/documentation/forms/
.. _site framework docs: http://www.djangoproject.com/documentation/sites/

Limiting access to logged-in users that pass a test
---------------------------------------------------

To limit access based on certain permissions or some other test, you'd do
essentially the same thing as described in the previous section.

The simple way is to run your test on ``request.user`` in the view directly.
For example, this view checks to make sure the user is logged in and has the
permission ``polls.can_vote``::

    def my_view(request):
        if not (request.user.is_authenticated() and request.user.has_perm('polls.can_vote')):
            return HttpResponse("You can't vote in this poll.")
        # ...

As a shortcut, you can use the convenient ``user_passes_test`` decorator::

    from django.contrib.auth.decorators import user_passes_test

    def my_view(request):
        # ...
    my_view = user_passes_test(lambda u: u.has_perm('polls.can_vote'))(my_view)

Here's the same thing, using Python 2.4's decorator syntax::

    from django.contrib.auth.decorators import user_passes_test

    @user_passes_test(lambda u: u.has_perm('polls.can_vote'))
    def my_view(request):
        # ...

``user_passes_test`` takes a required argument: a callable that takes a
``User`` object and returns ``True`` if the user is allowed to view the page.
Note that ``user_passes_test`` does not automatically check that the ``User``
is not anonymous.

``user_passes_test()`` takes an optional ``login_url`` argument, which lets you
specify the URL for your login page (``/accounts/login/`` by default).

Example in Python 2.3 syntax::

    from django.contrib.auth.decorators import user_passes_test

    def my_view(request):
        # ...
    my_view = user_passes_test(lambda u: u.has_perm('polls.can_vote'), login_url='/login/')(my_view)

Example in Python 2.4 syntax::

    from django.contrib.auth.decorators import user_passes_test

    @user_passes_test(lambda u: u.has_perm('polls.can_vote'), login_url='/login/')
    def my_view(request):
        # ...

Limiting access to generic views
--------------------------------

To limit access to a `generic view`_, write a thin wrapper around the view,
and point your URLconf to your wrapper instead of the generic view itself.
For example::

    from django.views.generic.date_based import object_detail

    @login_required
    def limited_object_detail(*args, **kwargs):
        return object_detail(*args, **kwargs)

.. _generic view: http://www.djangoproject.com/documentation/generic_views/

Permissions
===========

Django comes with a simple permissions system. It provides a way to assign
permissions to specific users and groups of users.

It's used by the Django admin site, but you're welcome to use it in your own
code.

The Django admin site uses permissions as follows:

    * Access to view the "add" form and add an object is limited to users with
      the "add" permission for that type of object.
    * Access to view the change list, view the "change" form and change an
      object is limited to users with the "change" permission for that type of
      object.
    * Access to delete an object is limited to users with the "delete"
      permission for that type of object.

Permissions are set globally per type of object, not per specific object
instance. For example, it's possible to say "Mary may change news stories," but
it's not currently possible to say "Mary may change news stories, but only the
ones she created herself" or "Mary may only change news stories that have a
certain status, publication date or ID." The latter functionality is something
Django developers are currently discussing.

Default permissions
-------------------

Three basic permissions -- add, create and delete -- are automatically created
for each Django model that has a ``class Admin`` set. Behind the scenes, these
permissions are added to the ``auth_permission`` database table when you run
``manage.py syncdb``.

Note that if your model doesn't have ``class Admin`` set when you run
``syncdb``, the permissions won't be created. If you initialize your database
and add ``class Admin`` to models after the fact, you'll need to run
``manage.py syncdb`` again. It will create any missing permissions for
all of your installed apps.

Custom permissions
------------------

To create custom permissions for a given model object, use the ``permissions``
`model Meta attribute`_.

This example model creates three custom permissions::

    class USCitizen(models.Model):
        # ...
        class Meta:
            permissions = (
                ("can_drive", "Can drive"),
                ("can_vote", "Can vote in elections"),
                ("can_drink", "Can drink alcohol"),
            )

The only thing this does is create those extra permissions when you run
``syncdb``.

.. _model Meta attribute: http://www.djangoproject.com/documentation/model_api/#meta-options

API reference
-------------

Just like users, permissions are implemented in a Django model that lives in
`django/contrib/auth/models.py`_.

.. _django/contrib/auth/models.py: http://code.djangoproject.com/browser/django/trunk/django/contrib/auth/models.py

Fields
~~~~~~

``Permission`` objects have the following fields:

    * ``name`` -- Required. 50 characters or fewer. Example: ``'Can vote'``.
    * ``content_type`` -- Required. A reference to the ``django_content_type``
      database table, which contains a record for each installed Django model.
    * ``codename`` -- Required. 100 characters or fewer. Example: ``'can_vote'``.

Methods
~~~~~~~

``Permission`` objects have the standard data-access methods like any other
`Django model`_.

Authentication data in templates
================================

The currently logged-in user and his/her permissions are made available in the
`template context`_ when you use ``RequestContext``.

.. admonition:: Technicality

   Technically, these variables are only made available in the template context
   if you use ``RequestContext`` *and* your ``TEMPLATE_CONTEXT_PROCESSORS``
   setting contains ``"django.core.context_processors.auth"``, which is default.
   For more, see the `RequestContext docs`_.

   .. _RequestContext docs: http://www.djangoproject.com/documentation/templates_python/#subclassing-context-requestcontext

Users
-----

The currently logged-in user, either a ``User`` instance or an``AnonymousUser``
instance, is stored in the template variable ``{{ user }}``::

    {% if user.is_authenticated %}
        <p>Welcome, {{ user.username }}. Thanks for logging in.</p>    
    {% else %}
        <p>Welcome, new user. Please log in.</p>
    {% endif %}

Permissions
-----------

The currently logged-in user's permissions are stored in the template variable
``{{ perms }}``. This is an instance of ``django.core.context_processors.PermWrapper``,
which is a template-friendly proxy of permissions.

In the ``{{ perms }}`` object, single-attribute lookup is a proxy to
``User.has_module_perms``. This example would display ``True`` if the logged-in
user had any permissions in the ``foo`` app::

    {{ perms.foo }}

Two-level-attribute lookup is a proxy to ``User.has_perm``. This example would
display ``True`` if the logged-in user had the permission ``foo.can_vote``::

    {{ perms.foo.can_vote }}

Thus, you can check permissions in template ``{% if %}`` statements::

    {% if perms.foo %}
        <p>You have permission to do something in the foo app.</p>
        {% if perms.foo.can_vote %}
            <p>You can vote!</p>
        {% endif %}
        {% if perms.foo.can_drive %}
            <p>You can drive!</p>
        {% endif %}
    {% else %}
        <p>You don't have permission to do anything in the foo app.</p>
    {% endif %}

.. _template context: http://www.djangoproject.com/documentation/templates_python/

Groups
======

Groups are a generic way of categorizing users so you can apply permissions, or
some other label, to those users. A user can belong to any number of groups.

A user in a group automatically has the permissions granted to that group. For
example, if the group ``Site editors`` has the permission
``can_edit_home_page``, any user in that group will have that permission.

Beyond permissions, groups are a convenient way to categorize users to give
them some label, or extended functionality. For example, you could create a
group ``'Special users'``, and you could write code that could, say, give them
access to a members-only portion of your site, or send them members-only e-mail
messages.

Messages
========

The message system is a lightweight way to queue messages for given users.

A message is associated with a ``User``. There's no concept of expiration or
timestamps.

Messages are used by the Django admin after successful actions. For example,
``"The poll Foo was created successfully."`` is a message.

The API is simple::

    * To create a new message, use
      ``user_obj.message_set.create(message='message_text')``.
    * To retrieve/delete messages, use ``user_obj.get_and_delete_messages()``,
      which returns a list of ``Message`` objects in the user's queue (if any)
      and deletes the messages from the queue.

In this example view, the system saves a message for the user after creating
a playlist::

    def create_playlist(request, songs):
        # Create the playlist with the given songs.
        # ...
        request.user.message_set.create(message="Your playlist was added successfully.")
        return render_to_response("playlists/create.html",
            context_instance=RequestContext(request))

When you use ``RequestContext``, the currently logged-in user and his/her
messages are made available in the `template context`_ as the template variable
``{{ messages }}``. Here's an example of template code that displays messages::

    {% if messages %}
    <ul>
        {% for message in messages %}
        <li>{{ message.message }}</li>
        {% endfor %}
    </ul>
    {% endif %}

Note that ``RequestContext`` calls ``get_and_delete_messages`` behind the
scenes, so any messages will be deleted even if you don't display them.

Finally, note that this messages framework only works with users in the user
database. To send messages to anonymous users, use the `session framework`_.

.. _session framework: http://www.djangoproject.com/documentation/sessions/

Other authentication sources
============================

The authentication that comes with Django is good enough for most common cases,
but you may have the need to hook into another authentication source -- that
is, another source of usernames and passwords or authentication methods.

For example, your company may already have an LDAP setup that stores a username
and password for every employee. It'd be a hassle for both the network
administrator and the users themselves if users had separate accounts in LDAP
and the Django-based applications.

So, to handle situations like this, the Django authentication system lets you
plug in another authentication sources. You can override Django's default
database-based scheme, or you can use the default system in tandem with other
systems.

Specifying authentication backends
----------------------------------

Behind the scenes, Django maintains a list of "authentication backends" that it
checks for authentication. When somebody calls
``django.contrib.auth.authenticate()`` -- as described in "How to log a user in"
above -- Django tries authenticating across all of its authentication backends.
If the first authentication method fails, Django tries the second one, and so
on, until all backends have been attempted.

The list of authentication backends to use is specified in the
``AUTHENTICATION_BACKENDS`` setting. This should be a tuple of Python path
names that point to Python classes that know how to authenticate. These classes
can be anywhere on your Python path.

By default, ``AUTHENTICATION_BACKENDS`` is set to::

    ('django.contrib.auth.backends.ModelBackend',)

That's the basic authentication scheme that checks the Django users database.

The order of ``AUTHENTICATION_BACKENDS`` matters, so if the same username and
password is valid in multiple backends, Django will stop processing at the
first positive match.

Writing an authentication backend
---------------------------------

An authentication backend is a class that implements two methods:
``get_user(id)`` and ``authenticate(**credentials)``.

The ``get_user`` method takes an ``id`` -- which could be a username, database
ID or whatever -- and returns a ``User`` object.

The  ``authenticate`` method takes credentials as keyword arguments. Most of
the time, it'll just look like this::

    class MyBackend:
        def authenticate(username=None, password=None):
            # Check the username/password and return a User.

But it could also authenticate a token, like so::

    class MyBackend:
        def authenticate(token=None):
            # Check the token and return a User.

Either way, ``authenticate`` should check the credentials it gets, and it
should return a ``User`` object that matches those credentials, if the
credentials are valid. If they're not valid, it should return ``None``.

The Django admin system is tightly coupled to the Django ``User`` object
described at the beginning of this document. For now, the best way to deal with
this is to create a Django ``User`` object for each user that exists for your
backend (e.g., in your LDAP directory, your external SQL database, etc.) You
can either write a script to do this in advance, or your ``authenticate``
method can do it the first time a user logs in.

Here's an example backend that authenticates against a username and password
variable defined in your ``settings.py`` file and creates a Django ``User``
object the first time a user authenticates::

    from django.conf import settings
    from django.contrib.auth.models import User, check_password

    class SettingsBackend:
        """
        Authenticate against the settings ADMIN_LOGIN and ADMIN_PASSWORD.

        Use the login name, and a hash of the password. For example:

        ADMIN_LOGIN = 'admin'
        ADMIN_PASSWORD = 'sha1$4e987$afbcf42e21bd417fb71db8c66b321e9fc33051de'
        """
        def authenticate(self, username=None, password=None):
            login_valid = (settings.ADMIN_LOGIN == username)
            pwd_valid = check_password(password, settings.ADMIN_PASSWORD)
            if login_valid and pwd_valid:
                try:
                    user = User.objects.get(username=username)
                except User.DoesNotExist:
                    # Create a new user. Note that we can set password
                    # to anything, because it won't be checked; the password
                    # from settings.py will.
                    user = User(username=username, password='get from settings.py')
                    user.is_staff = True
                    user.is_superuser = True
                    user.save()
                return user
            return None

        def get_user(self, user_id):
            try:
                return User.objects.get(pk=user_id)
            except User.DoesNotExist:
                return None