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Adding Authorization

:app:`Pyramid` provides facilities for :term:`authentication` and :term:`authorization`. We'll make use of both features to provide security to our application. Our application currently allows anyone with access to the server to view, edit, and add pages to our wiki. 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.

We will do the following steps:

  • Add a :term:`root factory` with an :term:`ACL` (models.py).
  • Add an :term:`authentication policy` and an :term:`authorization policy` (__init__.py).
  • Add an authentication policy callback (new security.py module).
  • Add login and logout views (views.py).
  • Add :term:`permission` declarations to the edit_page and add_page views (views.py).
  • Make the existing views return a logged_in flag to the renderer (views.py).
  • Add a login template (new login.pt).
  • Add a "Logout" link to be shown when logged in and viewing or editing a page (view.pt, edit.pt).

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

Adding A Root Factory

Open models.py and add the following statements:

.. literalinclude:: src/authorization/tutorial/models.py
   :lines: 1-4,35-39
   :linenos:
   :language: python

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. We do this to allow :app:`Pyramid` declarative security to work properly. The context object generated by the root factory during a request will 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 the new class we created inside our models.py file.

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.

The context object generated by our root factory 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.

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

Add an Authorization Policy and an Authentication Policy

We're going to be making several changes to our __init__.py file which will help us configure 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 enable an AuthTktAuthenticationPolicy and an ACLAuthorizationPolicy to implement declarative security checking. Open tutorial/__init__.py and add these import statements:

.. literalinclude:: src/authorization/tutorial/__init__.py
   :lines: 2-3,7
   :linenos:
   :language: python

Now add those policies to the configuration:

.. literalinclude:: src/authorization/tutorial/__init__.py
   :lines: 16-22
   :linenos:
   :language: python

Note 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 groupfinder function in the current directory's security.py file. We haven't added that module yet, but we're about to.

Viewing Your Changes

When we're done configuring a root factory, adding a authentication and authorization policies, and adding routes for /login and /logout, your application's __init__.py will look like this:

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

Adding an authentication policy callback

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

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

The groupfinder function 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

To our views.py 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.

The login view callable will look something like this:

.. literalinclude:: src/authorization/tutorial/views.py
   :lines: 89-115
   :linenos:
   :language: python

The logout view callable will look something like this:

.. literalinclude:: src/authorization/tutorial/views.py
   :lines: 117-121
   :linenos:
   :language: python

The login view callable is decorated with two decorators, a @view_config decorators, which associates it with the login route, the other a @forbidden_view_config decorator which turns it in to an :term:`exception view` when Pyramid raises a :class:`pyramid.httpexceptions.HTTPForbidden` exception. The one which associates it with the login route makes it visible when we visit /login. The other one makes it a :term:`forbidden view`. The forbidden view is displayed whenever Pyramid or your application raises an HTTPForbidden exception. In this case, we'll be relying on the forbidden view to show the login form whenver someone attempts to execute an action which they're not yet authorized to perform.

The logout view callable is decorated with a @view_config decorator which associates it with the logout route. This makes it visible when we visit /login.

We'll need to import some stuff to service the needs of these two functions: the pyramid.view.forbidden_view_config class, a number of values from the pyramid.security module, and a value from our newly added tutorial.security package. Add the following import statements to the head of the views.py file:

.. literalinclude:: src/authorization/tutorial/views.py
   :lines: 9-18,24-25
   :linenos:
   :language: python

Changing Existing Views

Add permision declarations

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

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

Return a logged_in flag to the renderer

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

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

We'll also need to add a permission value to the @view_config decorator for each of the add_page and edit_page view callables. For each, we'll add permission='edit', for example:

@view_config(route_name='edit_page', renderer='templates/edit.pt',
             permission='edit')

See the permission='edit' we added there? This indicates that the view callables which these views reference cannot be invoked without the authenticated user possessing the edit permission with respect to the current :term:`context`.

Adding these permission arguments causes Pyramid to make 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.

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 views.py.

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

Add a "Logout" link when logged in

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 id="right" class="app-welcome align-right"> div:

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

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
   :language: xml

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

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

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.
  • After logging 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.