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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 that to :allow only people who 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.

We will also add a login page and a logout link on all the pages. The login page will be shown when a user is denied access to any of the views that require permission, instead of a default "403 Forbidden" page.

We will implement the access control with the following steps:

Then we will add the login and logout feature:

  • Add login and logout views (views.py).
  • Add a login template (login.pt).
  • Make the existing views return a logged_in flag to the renderer (views.py).
  • Add a "Logout" link to be shown when logged in and viewing or editing a page (view.pt, edit.pt).

Access control

Add users and groups

Create a new tutorial/tutorial/security.py module with the following content:

The groupfinder function accepts a userid and a request and returns one of these values:

  • If the userid exists in the system, it 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, it will return None.

For example, groupfinder('editor', request ) returns ['group:editor'], groupfinder('viewer', request) returns [], and groupfinder('admin', request) returns None. We will use groupfinder() as an :term:`authentication policy` "callback" that will provide the :term:`principal` or principals for a user.

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.

Add an ACL

Open tutorial/tutorial/models.py and add the following import statement at the head:

Add the following lines to the Wiki class:

We import :data:`~pyramid.security.Allow`, an action that means that permission is allowed, and :data:`~pyramid.security.Everyone`, a special :term:`principal` that is associated to all requests. Both are used in the :term:`ACE` entries that make up the ACL.

The ACL is a list that needs to be named __acl__ and be an attribute of a class. We define an :term:`ACL` with two :term:`ACE` entries: the first entry allows any user the view permission. The second entry allows the group:editors principal the edit permission.

The Wiki class that contains the ACL is the :term:`resource` constructor for the :term:`root` resource, which is a Wiki instance. The ACL is provided to each view in the :term:`context` of the request as the context attribute.

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 need only one ACL for the entire system, however, because our security requirements are simple, so this feature is not demonstrated. See :ref:`assigning_acls` for more information about what an :term:`ACL` represents.

Add authentication and authorization policies

Open tutorial/tutorial/__init__.py and add the highlighted import statements:

Now add those policies to the configuration:

Only the highlighted lines need to be added.

We are enabling an AuthTktAuthenticationPolicy, which is based in an auth ticket that may be included in the request. We are also enabling an ACLAuthorizationPolicy, which uses an ACL to determine the allow or deny outcome for a view.

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 the groupfinder() function that we created before.

Add permission declarations

Open tutorial/tutorial/views.py and add a permission='edit' parameter to the @view_config decorators for add_page() and edit_page():

Only the highlighted lines, along with their preceding commas, need to be edited and added.

The result is that only users who possess the edit permission at the time of the request may invoke those two views.

Add a permission='view' parameter to the @view_config decorator for view_wiki() and view_page() as follows:

Only the highlighted lines, along with their preceding commas, need to be edited and added.

This allows anyone to invoke these two views.

We are done with the changes needed to control access. The changes that follow will add the login and logout feature.

Login, logout

Add 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 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.

Add the following import statements to the head of tutorial/tutorial/views.py:

All the highlighted lines need to be added or edited.

:meth:`~pyramid.view.forbidden_view_config` will be used to customize the default 403 Forbidden page. :meth:`~pyramid.security.remember` and :meth:`~pyramid.security.forget` help to create and expire an auth ticket cookie.

Now add the login and logout views at the end of the file:

login() has two decorators:

  • a @view_config decorator which associates it with the login route and makes it visible when we visit /login,
  • a @forbidden_view_config decorator which turns it into a :term:`forbidden view`. login() will be invoked when a user tries to execute a view callable for which they lack authorization. For example, if a user has not logged in and tries to add or edit a Wiki page, they will be shown the login form before being allowed to continue.

The order of these two :term:`view configuration` decorators is unimportant.

logout() is decorated with a @view_config decorator which associates it with the logout route. It will be invoked when we visit /logout.

Add the login.pt Template

Create tutorial/tutorial/templates/login.pt with the following content:

The above template is referenced in the login view that we just added in views.py.

Return a logged_in flag to the renderer

Open tutorial/tutorial/views.py again. Add a logged_in parameter to the return value of view_page(), edit_page(), and add_page() as follows:

Only the highlighted lines need to be added or edited.

The :meth:`pyramid.request.Request.authenticated_userid` will be None if the user is not authenticated, or a userid if the user is authenticated.

Add a "Logout" link when logged in

Open tutorial/tutorial/templates/edit.pt and tutorial/tutorial/templates/view.pt and add the following code as indicated by the highlighted lines.

The attribute tal:condition="logged_in" will make the element be included when logged_in is any user id. The link will invoke the logout view. The above element will not be included if logged_in is None, such as when a user is not authenticated.

Reviewing our changes

Our tutorial/tutorial/__init__.py will look like this when we're done:

Only the highlighted lines need to be added or edited.

Our tutorial/tutorial/models.py will look like this when we're done:

Only the highlighted lines need to be added or edited.

Our tutorial/tutorial/views.py will look like this when we're done:

Only the highlighted lines need to be added or edited.

Our tutorial/tutorial/templates/edit.pt template will look like this when we're done:

Only the highlighted lines need to be added or edited.

Our tutorial/tutorial/templates/view.pt template will look like this when we're done:

Only the highlighted lines need to be added or edited.

Viewing the application in a browser

We can finally examine our application in a browser (See :ref:`wiki-start-the-application`). Launch a browser and visit each of the following URLs, checking that the result is as expected:

  • http://localhost:6543/ invokes the view_wiki view. This always redirects to the view_page view of the FrontPage Page resource. It is executable by any user.
  • http://localhost:6543/FrontPage 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.
  • http://localhost:6543/FrontPage/edit_page 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.
  • http://localhost:6543/add_page/SomePageName 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.
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