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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 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/.

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:

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):
        pass

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.

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.config.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:

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 :file:`__init__.py`, :file:`views.py`, etc) with the following content:

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:

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:

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

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.

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>

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:

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

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

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.

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