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37.7. ADDING AUTHORIZATION

37.7.1 Adding A Root Factory

Open models.py and add the following statements:

We’re going to start to use a custom root factory within our __init__.py file. The objects generated by the root factory will be used as the context of each request to our application. Those context objects will be decorated with security declarations. When we use a custom root factory to generate our contexts, we can begin to make use of the declarative security features of Pyramid.

We’ll modify our __init__.py, passing in a root factory to our 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 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 pyramid.security.Everyone (a special principal) to view all pages, while allowing only a principal named group:editors to edit and add pages. The __acl__ attribute attached to a context is interpreted specially by Pyramid as an access control list during view callable execution. See Assigning ACLs to your Resource Objects for more information about what an ACL represents.

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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 pyramid.config.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

Configurator.

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37.7.2 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 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 authentication policy and an 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:

Now add those policies to the configuration:

Note that the 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:

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37.7. ADDING AUTHORIZATION

1 from pyramid.config import Configurator

2 from pyramid.authentication import AuthTktAuthenticationPolicy

3 from pyramid.authorization import ACLAuthorizationPolicy

4

5 from sqlalchemy import engine_from_config

6

7 from tutorial.security import groupfinder

8

9 from .models import DBSession

10

11def main(global_config, **settings):

12""" This function returns a Pyramid WSGI application.

13"""

14engine = engine_from_config(settings, ’sqlalchemy.’)

15DBSession.configure(bind=engine)

16authn_policy = AuthTktAuthenticationPolicy(

17’sosecret’, callback=groupfinder)

18authz_policy = ACLAuthorizationPolicy()

19config = Configurator(settings=settings,

21config.set_authentication_policy(authn_policy)

22config.set_authorization_policy(authz_policy)

23config.add_static_view(’static’, ’static’, cache_max_age=3600)

24config.add_route(’view_wiki’, ’/’)

25config.add_route(’login’, ’/login’)

26config.add_route(’logout’, ’/logout’)

27config.add_route(’view_page’, ’/{pagename}’)

28config.add_route(’add_page’, ’/add_page/{pagename}’)

29config.add_route(’edit_page’, ’/{pagename}/edit_page’)

30config.scan()

31return config.make_wsgi_app()

37.7.3 Adding an authentication policy callback

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

1 USERS = {’editor’:’editor’,

2’viewer’:’viewer’}

3 GROUPS = {’editor’:[’group:editors’]}

4

5 def groupfinder(userid, request):

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The groupfinder function defined here is an 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 context object 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.

37.7.4 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:

5referrer = request.url

6if referrer == login_url:

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The logout view callable will look something like this:

1 @view_config(route_name=’logout’) 2 def logout(request):

3headers = forget(request)

4return HTTPFound(location = request.route_url(’view_wiki’),

The login view callable is decorated with two decorators, a @view_config decorator, which associates it with the login route, and a @forbidden_view_config decorator which turns it in to an exception view. The one which associates it with the login route makes it visible when we visit /login. The other one makes it a forbidden view. The forbidden view is displayed whenever Pyramid or your application raises an pyramid.httpexceptions.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 /logout.

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:

5

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