.. _wiki2_adding_authorization:

====================
Adding Authorization
====================

Our application currently allows anyone with access to the server to
view, edit, and add pages to our wiki.  For purposes of demonstration
we'll change our application to allow only people whom possess a
specific username (`editor`) to add and edit wiki pages but we'll
continue allowing anyone with access to the server to view pages.
:app:`Pyramid` provides facilities for *authorization* and
*authentication*.  We'll make use of both features to provide security
to our application.

The source code for this tutorial stage can be browsed at
`http://github.com/Pylons/pyramid/tree/master/docs/tutorials/wiki2/src/authorization/
<http://github.com/Pylons/pyramid/tree/master/docs/tutorials/wiki2/src/authorization/>`_.

Changing ``__init__.py`` For Authorization
-------------------------------------------

We're going to be making several changes to our ``__init__.py`` file which
will help us configure an authorization policy.

Adding A Root Factory
~~~~~~~~~~~~~~~~~~~~~

We're going to start to use a custom :term:`root factory` within our
``__init__.py`` file.  The objects generated by the root factory will be
used as the :term:`context` of each request to our application.  In
order for :app:`Pyramid` declarative security to work properly, the
context object generated during a request must be decorated with
security declarations; when we begin to use a custom root factory to
generate our contexts, we can begin to make use of the declarative
security features of :app:`Pyramid`.

We'll modify our ``__init__.py``, passing in a :term:`root factory` to our
:term:`Configurator` constructor.  We'll point it at a new class we create
inside our ``models.py`` file.  Add the following statements to your
``models.py`` file:

.. code-block:: python

   from pyramid.security import Allow
   from pyramid.security import Everyone

   class RootFactory(object):
       __acl__ = [ (Allow, Everyone, 'view'), 
                   (Allow, 'group:editors', 'edit') ]
       def __init__(self, request):
           self.__dict__.update(request.matchdict)

The ``RootFactory`` class we've just added will be used by
:app:`Pyramid` to construct a ``context`` object.  The context is
attached to the request object passed to our view callables as the
``context`` attribute.

All of our context objects will possess an ``__acl__`` attribute that
allows :data:`pyramid.security.Everyone` (a special principal) to
view all pages, while allowing only a :term:`principal` named
``group:editors`` to edit and add pages.  The ``__acl__`` attribute
attached to a context is interpreted specially by :app:`Pyramid` as
an access control list during view callable execution.  See
:ref:`assigning_acls` for more information about what an :term:`ACL`
represents.

.. note: Although we don't use the functionality here, the ``factory`` used
   to create route contexts may differ per-route as opposed to globally.  See
   the ``factory`` argument to
   :meth:`pyramid.configuration.Configurator.add_route` for more info.

We'll pass the ``RootFactory`` we created in the step above in as the
``root_factory`` argument to a :term:`Configurator`.  

Configuring an Authorization Policy
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

For any :app:`Pyramid` application to perform authorization, we need to add a
``security.py`` module (we'll do that shortly) and we'll need to change our
``__init__.py`` file to add an :term:`authentication policy` and an
:term:`authorization policy` which uses the ``security.py`` file for a
*callback*.

We'll change our ``__init__.py`` file to enable an
``AuthTktAuthenticationPolicy`` and an ``ACLAuthorizationPolicy`` to enable
declarative security checking.  We'll also change ``__init__.py`` to add a
:meth:`pyramid.configuration.Configurator.add_view` call to points at our
``login`` :term:`view callable`, also known as a :term:`forbidden view`.
This configures our newly created login view to show up when :app:`Pyramid`
detects that a view invocation can not be authorized.  Also, we'll add
``view_permission`` arguments with the value ``edit`` to the ``edit_page``
and ``add_page`` routes.  This indicates that the view callables which these
routes reference cannot be invoked without the authenticated user possessing
the ``edit`` permission with respect to the current context.

This makes the assertion that only users who possess the effective ``edit``
permission at the time of the request may invoke those two views.  We've
granted the ``group:editors`` principal the ``edit`` permission at the root
model via its ACL, so only the a user whom is a member of the group named
``group:editors`` will able to invoke the views associated with the
``add_page`` or ``edit_page`` routes.

Viewing Your Changes
~~~~~~~~~~~~~~~~~~~~

When we're done configuring a root factory, adding an authorization policy,
and adding views, your application's ``__init__.py`` will look like this:

.. literalinclude:: src/authorization/tutorial/__init__.py
   :linenos:
   :language: python

Note that that the
:class:`pyramid.authentication.AuthTktAuthenticationPolicy` constructor
accepts two arguments: ``secret`` and ``callback``.  ``secret`` is a string
representing an encryption key used by the "authentication ticket" machinery
represented by this policy: it is required.  The ``callback`` is a string,
representing a :term:`dotted Python name`, which points at the
``groupfinder`` function in the current directory's ``security.py`` file.  We
haven't added that module yet, but we're about to.


Adding ``security.py``
~~~~~~~~~~~~~~~~~~~~~~

Add a ``security.py`` module within your package (in the same directory as
"__init__.py", "views.py", etc) with the following content:

.. literalinclude:: src/authorization/tutorial/security.py
   :linenos:
   :language: python

The groupfinder defined here is an :term:`authentication policy`
"callback"; it is a callable that accepts a userid and a request.  If
the userid exists in the system, the callback will return a sequence
of group identifiers (or an empty sequence if the user isn't a member
of any groups).  If the userid *does not* exist in the system, the
callback will return ``None``.  In a production system, user and group
data will most often come from a database, but here we use "dummy"
data to represent user and groups sources. Note that the ``editor``
user is a member of the ``group:editors`` group in our dummy group
data (the ``GROUPS`` data structure).

We've given the ``editor`` user membership to the ``group:editors`` by
mapping him to this group in the ``GROUPS`` data structure (``GROUPS =
{'editor':['group:editors']}``).  Since the ``groupfinder`` function
consults the ``GROUPS`` data structure, this will mean that, as a
result of the ACL attached to the root returned by the root factory,
and the permission associated with the ``add_page`` and ``edit_page``
views, the ``editor`` user should be able to add and edit pages.

Adding Login and Logout Views
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We'll add a ``login`` view callable which renders a login form and
processes the post from the login form, checking credentials.

We'll also add a ``logout`` view callable to our application and
provide a link to it.  This view will clear the credentials of the
logged in user and redirect back to the front page.

We'll add a different file (for presentation convenience) to add login
and logout view callables.  Add a file named ``login.py`` to your
application (in the same directory as ``views.py``) with the following
content:

.. literalinclude:: src/authorization/tutorial/login.py
   :linenos:
   :language: python

Changing Existing Views
~~~~~~~~~~~~~~~~~~~~~~~

Then we need to change each of our ``view_page``, ``edit_page`` and
``add_page`` views in ``views.py`` to pass a "logged in" parameter to
its template.  We'll add something like this to each view body:

.. ignore-next-block
.. code-block:: python
   :linenos:

   from pyramid.security import authenticated_userid
   logged_in = authenticated_userid(request)

We'll then change the return value of these views to pass the
`resulting `logged_in`` value to the template, e.g.:

.. ignore-next-block
.. code-block:: python
   :linenos:

   return dict(page = context,
               content = content,
               logged_in = logged_in,
               edit_url = edit_url)

Adding the ``login.pt`` Template
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Add a ``login.pt`` template to your templates directory.  It's
referred to within the login view we just added to ``login.py``.

.. literalinclude:: src/authorization/tutorial/templates/login.pt
   :linenos:
   :language: xml

Change ``view.pt`` and ``edit.pt``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We'll also need to change our ``edit.pt`` and ``view.pt`` templates to
display a "Logout" link if someone is logged in.  This link will
invoke the logout view.

To do so we'll add this to both templates within the ``<div
class="main_content">`` div:

.. code-block:: xml
   :linenos:

   <span tal:condition="logged_in">
      <a href="${request.application_url}/logout">Logout</a>
   </span>

Viewing the Application in a Browser
------------------------------------

We can finally examine our application in a browser.  The views we'll
try are as follows:

- Visiting ``http://localhost:6543/`` in a browser invokes the
  ``view_wiki`` view.  This always redirects to the ``view_page`` view
  of the FrontPage page object.  It is executable by any user.

- Visiting ``http://localhost:6543/FrontPage`` in a browser invokes
  the ``view_page`` view of the FrontPage page object.

- Visiting ``http://localhost:6543/FrontPage/edit_page`` in a browser
  invokes the edit view for the FrontPage object.  It is executable by
  only the ``editor`` user.  If a different user (or the anonymous
  user) invokes it, a login form will be displayed.  Supplying the
  credentials with the username ``editor``, password ``editor`` will
  display the edit page form.

- Visiting ``http://localhost:6543/add_page/SomePageName`` in a
  browser invokes the add view for a page.  It is executable by only
  the ``editor`` user.  If a different user (or the anonymous user)
  invokes it, a login form will be displayed.  Supplying the
  credentials with the username ``editor``, password ``editor`` will
  display the edit page form.

Seeing Our Changes To ``views.py`` and our Templates
----------------------------------------------------

Our ``views.py`` module will look something like this when we're done:

.. literalinclude:: src/authorization/tutorial/views.py
   :linenos:
   :language: python

Our ``edit.pt`` template will look something like this when we're done:

.. literalinclude:: src/authorization/tutorial/templates/edit.pt
   :linenos:
   :language: xml

Our ``view.pt`` template will look something like this when we're done:

.. literalinclude:: src/authorization/tutorial/templates/view.pt
   :linenos:
   :language: xml

Revisiting the Application
---------------------------

When we revisit the application in a browser, and log in (as a result
of hitting an edit or add page and submitting the login form with the
``editor`` credentials), we'll see a Logout link in the upper right
hand corner.  When we click it, we're logged out, and redirected back
to the front page.