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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 people whom are members of a group named group:editors 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 via http://github.com/Pylons/pyramid/tree/master/docs/tutorials/wiki/src/authorization/.

Configuring a pyramid Authentication Policy

For any :app:`Pyramid` application to perform authorization, we need to add a security.py module and we'll need to change our :term:`application registry` to add an :term:`authentication policy` and a :term:`authorization policy`.

Adding Authentication and Authorization Policies

We'll change our package's __init__.py file to enable an AuthTktAuthenticationPolicy and an ACLAuthorizationPolicy to enable declarative security checking. When you're done, your __init__.py will look like so:

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

Note that the creation of an AuthTktAuthenticationPolicy requires 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 reference to a groupfinder function in the tutorial package'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 function defined here is an authorization policy "callback"; it is a callable that accepts a userid and a request. If the userid exists in the set of users known by 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 this 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).

Adding Login and Logout Views

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

We'll also add a logout view 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 views. 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

Note that the login view callable in the login.py file has two view configuration decorators. The order of these decorators is unimportant. Each just adds a different :term:`view configuration` for the login view callable.

The first view configuration decorator configures the login view callable so it will be invoked when someone visits /login (when the context is a Wiki and the view name is login). The second decorator (with context of pyramid.exceptions.Forbidden) specifies a :term:`forbidden view`. This configures our login view to be presented to the user when :app:`Pyramid` detects that a view invocation can not be authorized. Because we've configured a forbidden view, the login view callable will be invoked whenever one of our users tries to execute a view callable that they are not allowed to invoke as determined by the :term:`authorization policy` in use. In our application, for example, this means that if a user has not logged in, and he tries to add or edit a Wiki page, he will be shown the login form. Before being allowed to continue on to the add or edit form, he will have to provide credentials that give him permission to add or edit via this login form.

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 into its template. We'll add something like this to each view body:

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

We'll then change the return value of each view that has an associated renderer to pass the resulting `logged_in` value to the template. For example:

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

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

Giving Our Root Resource an ACL

We need to give our root resource object an :term:`ACL`. This ACL will be sufficient to provide enough information to the :app:`Pyramid` security machinery to challenge a user who doesn't have appropriate credentials when he attempts to invoke the add_page or edit_page views.

We need to perform some imports at module scope in our models.py file:

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

Our root resource object is a Wiki instance. We'll add the following line at class scope to our Wiki class:

__acl__ = [ (Allow, Everyone, 'view'),
            (Allow, 'group:editors', 'edit') ]

It's only happenstance that we're assigning this ACL at class scope. An ACL can be attached to an object instance too; this is how "row level security" can be achieved in :app:`Pyramid` applications. We actually only need one ACL for the entire system, however, because our security requirements are simple, so this feature is not demonstrated.

Our resulting models.py file will now look like so:

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

Adding permission Declarations to our view_config Decorators

To protect each of our views with a particular permission, we need to pass a permission argument to each of our :class:`pyramid.view.view_config` decorators. To do so, within views.py:

  • We add permission='view' to the decorator attached to the view_wiki view function. This makes the assertion that only users who possess the view permission against the context resource at the time of the request may invoke this view. We've granted :data:`pyramid.security.Everyone` the view permission at the root model via its ACL, so everyone will be able to invoke the view_wiki view.
  • We add permission='view' to the decorator attached to the view_page view function. This makes the assertion that only users who possess the effective view permission against the context resource at the time of the request may invoke this view. We've granted :data:`pyramid.security.Everyone` the view permission at the root model via its ACL, so everyone will be able to invoke the view_page view.
  • We add permission='edit' to the decorator attached to the add_page view function. This makes the assertion that only users who possess the effective edit permission against the context resource at the time of the request may invoke this view. 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 add_page view. We've likewise given the editor user membership to this group via thes security.py file by mapping him to the group:editors group in the GROUPS data structure (GROUPS = {'editor':['group:editors']}); the groupfinder function consults the GROUPS data structure. This means that the editor user can add pages.
  • We add permission='edit' to the decorator attached to the edit_page view function. This makes the assertion that only users who possess the effective edit permission against the context resource at the time of the request may invoke this view. 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 edit_page view. We've likewise given the editor user membership to this group via thes security.py file by mapping him to the group:editors group in the GROUPS data structure (GROUPS = {'editor':['group:editors']}); the groupfinder function consults the GROUPS data structure. This means that the editor user can edit pages.

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 resource. It is executable by any user.
  • Visiting http://localhost:6543/FrontPage/ in a browser invokes the view_page view of the FrontPage Page resource. This is because it's the :term:`default view` (a view without a name) for Page resources. It is executable by any user.
  • Visiting http://localhost:6543/FrontPage/edit_page in a browser invokes the edit view for the FrontPage Page resource. 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 show the edit page form being displayed.
  • 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 show the edit page form being displayed.

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.