root/django/branches/0.90-bugfixes/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.

The basics
==========

Django supports authentication out of the box. The ``django-admin.py init``
command, used to initialize a database with Django's core database tables,
creates the infrastructure for the auth system. You don't have to do anything
else to use authentication.

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.

Users
=====

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

.. _django/models/auth.py: http://code.djangoproject.com/browser/django/trunk/django/models/auth.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_md5`` -- Required. An MD5 hash of the password. (Django
      doesn't store the raw password.) Raw passwords can be arbitrarily long
      and can contain any character.
    * ``is_staff`` -- Boolean. Designates whether this user can access the
      admin site.
    * ``is_active`` -- Boolean. Designates whether this user account is valid.
      Set this to ``False`` instead of deleting accounts.
    * ``is_superuser`` -- Boolean. Designates whether this user has permission
      to do anything (according to the permission system).
    * ``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``. Because of those relationships, ``User`` objects get
data-access methods like any other `Django model`_:

    * ``get_group_list(**kwargs)``
    * ``set_groups(id_list)``
    * ``get_permission_list(**kwargs)``
    * ``set_user_permissions(id_list)``

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

    * ``is_anonymous()`` -- Always returns ``False``. This is a way of
      comparing ``User`` objects to anonymous users.

    * ``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 MD5 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.

    * ``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.

    * ``has_perms(perm_list)`` -- Returns ``True`` if the user has each of the
      specified permissions.

    * ``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.models.auth.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

Module functions
~~~~~~~~~~~~~~~~

The ``django.models.auth.users`` module 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``.

    * ``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 standard Django
`database API`_. Just create and save a ``User`` object::

    >>> from django.models.auth import users
    >>> import md5
    >>> p = md5.new('johnpassword').hexdigest()
    >>> u = users.User(username='john', first_name='John', last_name='lennon',
    ...     email='lennon@thebeatles.com', password_md5=p, is_staff=True,
    ...     is_active=True, is_superuser=False)
    >>> u.save()

Note that ``password_md5`` requires the raw MD5 hash (as created by
``md5.new().hexdigest()``). Because that's a pain, there's a ``create_user``
helper function::

    >>> from django.models.auth import users
    >>> u = users.create_user('john', 'lennon@thebeatles.com', 'johnpassword')

.. _database API: http://www.djangoproject.com/documentation/db_api/

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

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

    >>> from django.models.auth import users
    >>> u = users.get_object(username__exact='john')
    >>> u.set_password('new password')
    >>> u.save()

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

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

    * ``id`` is always ``None``.
    * ``is_anonymous()`` returns ``True`` instead of ``False``.
    * ``has_perm()`` always returns ``False``.
    * ``set_password()``, ``check_password()``, ``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.

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 hooks this
authentication framework into its system of `request objects`_.

In any Django view, ``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_anonymous()``, like so::

    if request.user.is_anonymous():
        # Do something for anonymous users.
    else:
        # Do something for logged-in users.

If you want to use ``request.user`` in your view code, make sure you have
``SessionMiddleware`` enabled. See the `session documentation`_ for more
information.

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

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

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

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

    from django.utils.httpwrappers import HttpResponseRedirect

    def my_view(request):
        if request.user.is_anonymous():
            return HttpResponseRedirect('/login/?next=%s' % request.path)
        # ...

...or display an error message::

    def my_view(request):
        if request.user.is_anonymous():
            return render_to_response('myapp/login_error')
        # ...

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

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

    from django.views.decorators.auth import login_required

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

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

    from django.views.decorators.auth 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.

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 request.user.is_anonymous() or not 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.views.decorators.auth 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.

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 or publication date." 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 ``admin`` set. Behind the scenes, these
permissions are added to the ``auth_permissions`` database table when you run
``django-admin.py install [app]``. You can view the exact SQL ``INSERT``
statements by running ``django-admin.py sqlinitialdata [app]``.

Note that if your model doesn't have ``admin`` set when you run
``django-admin.py install``, the permissions won't be created. If you
initialize your database and add ``admin`` to models after the fact, you'll
need to add the permissions to the database manually. Do this by running
``django-admin.py installperms [app]``, which creates any missing permissions
for the given app.

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

.. _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/models/auth.py`_.

Fields
~~~~~~

``Permission`` objects have the following fields:

    * ``name`` -- Required. 50 characters or fewer. Example: ``'Can vote'``.
    * ``package`` -- Required. A reference to the ``packages`` database table,
      which contains a record for each installed Django application.
    * ``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 ``DjangoContext``.

Users
-----

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

    {% if user.is_anonymous %}
        <p>Welcome, new user. Please log in.</p>
    {% else %}
        <p>Welcome, {{ user.username }}. Thanks for logging 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.extensions.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 to 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 apply
some label, or extended functionality, to them. For example, you could create
a group ``'Special users'``, and you could write code that would do special
things to those users -- such as giving them access to a members-only portion
of your site, or sending 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 add messages, use ``user.add_message(message_text)``.
    * To retrieve/delete messages, use ``user.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.add_message("Your playlist was added successfully.")
        return render_to_response("playlists/create", context_instance=DjangoContext(request))

When you use ``DjangoContext``, 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 ``DjangoContext`` 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/