=============================
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/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
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, 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``.

    * ``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 field directly unless you know what you're doing. This
is explained in the next section.

Passwords
---------

Previous versions, such as Django 0.90, used simple MD5 hashes without password
salts.

The ``password`` field 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``. 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.

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

``django.contrib.auth.models.AnonymousUser`` is a class that implements
the ``django.contirb.auth.models.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()``, ``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.

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`` and ``AuthenticationMiddleware`` 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/

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

To log a user in, do the following within a view::

    from django.contrib.auth.models import SESSION_KEY
    request.session[SESSION_KEY] = some_user.id

Because this uses sessions, you'll need to make sure you have
``SessionMiddleware`` enabled. See the `session documentation`_ for more
information.

This assumes ``some_user`` is your ``User`` instance. Depending on your task,
you'll probably want to make sure to validate the user's username and password.

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

    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.

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.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 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_permission`` database table when you run
``django-admin.py syncdb``.

Note that if your model doesn't have ``admin`` set when you run
``django-admin.py syncdb``, the permissions won't be created. If you
initialize your database and add ``admin`` to models after the fact, you'll
need to run ``django-admin.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"),
            )

.. _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_types`` 
      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-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.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 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 create a new message, use ``user.message_set.create(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.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/
