Adding Authorization

Pyramid provides facilities for authentication and 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 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 root factory with an ACL (models.py).
  • Add an authentication policy and an authorization policy (__init__.py).
  • Add an authentication policy callback (new security.py module).
  • Add login and logout views (views.py).
  • Add 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/1.3-branch/docs/tutorials/wiki2/src/authorization/.

Adding A Root Factory

Open models.py and add the following statements:

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from pyramid.security import (
    Allow,
    Everyone,
    )
class RootFactory(object):
    __acl__ = [ (Allow, Everyone, 'view'),
                (Allow, 'group:editors', 'edit') ]
    def __init__(self, request):
        pass

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.

Note

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.

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:

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from pyramid.authentication import AuthTktAuthenticationPolicy
from pyramid.authorization import ACLAuthorizationPolicy
from tutorial.security import groupfinder

Now add those policies to the configuration:

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    authn_policy = AuthTktAuthenticationPolicy(
        'sosecret', callback=groupfinder)
    authz_policy = ACLAuthorizationPolicy()
    config = Configurator(settings=settings,
                          root_factory='tutorial.models.RootFactory')
    config.set_authentication_policy(authn_policy)
    config.set_authorization_policy(authz_policy)

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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from pyramid.config import Configurator
from pyramid.authentication import AuthTktAuthenticationPolicy
from pyramid.authorization import ACLAuthorizationPolicy

from sqlalchemy import engine_from_config

from tutorial.security import groupfinder

from .models import DBSession

def main(global_config, **settings):
    """ This function returns a Pyramid WSGI application.
    """
    engine = engine_from_config(settings, 'sqlalchemy.')
    DBSession.configure(bind=engine)
    authn_policy = AuthTktAuthenticationPolicy(
        'sosecret', callback=groupfinder)
    authz_policy = ACLAuthorizationPolicy()
    config = Configurator(settings=settings,
                          root_factory='tutorial.models.RootFactory')
    config.set_authentication_policy(authn_policy)
    config.set_authorization_policy(authz_policy)
    config.add_static_view('static', 'static', cache_max_age=3600)
    config.add_route('view_wiki', '/')
    config.add_route('login', '/login')
    config.add_route('logout', '/logout')
    config.add_route('view_page', '/{pagename}')
    config.add_route('add_page', '/add_page/{pagename}')
    config.add_route('edit_page', '/{pagename}/edit_page')
    config.scan()
    return config.make_wsgi_app()

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:

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USERS = {'editor':'editor',
          'viewer':'viewer'}
GROUPS = {'editor':['group:editors']}

def groupfinder(userid, request):
    if userid in USERS:
        return GROUPS.get(userid, [])

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.

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:

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@view_config(route_name='login', renderer='templates/login.pt')
@forbidden_view_config(renderer='templates/login.pt')
def login(request):
    login_url = request.route_url('login')
    referrer = request.url
    if referrer == login_url:
        referrer = '/' # never use the login form itself as came_from
    came_from = request.params.get('came_from', referrer)
    message = ''
    login = ''
    password = ''
    if 'form.submitted' in request.params:
        login = request.params['login']
        password = request.params['password']
        if USERS.get(login) == password:
            headers = remember(request, login)
            return HTTPFound(location = came_from,
                             headers = headers)
        message = 'Failed login'

    return dict(
        message = message,
        url = request.application_url + '/login',
        came_from = came_from,
        login = login,
        password = password,
        )

The logout view callable will look something like this:

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@view_config(route_name='logout')
def logout(request):
    headers = forget(request)
    return HTTPFound(location = request.route_url('view_wiki'),
                     headers = headers)

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:

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from pyramid.view import (
    view_config,
    forbidden_view_config,
    )

from pyramid.security import (
    remember,
    forget,
    authenticated_userid,
    )

from .security import USERS

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:

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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.:

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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:

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@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 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 in the root factory via its ACL, so only a user who is a member of the group named group:editors will be 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.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"
      xmlns:tal="http://xml.zope.org/namespaces/tal">
<head>
  <title>Login - Pyramid tutorial wiki (based on TurboGears
    20-Minute Wiki)</title>
  <meta http-equiv="Content-Type" content="text/html;charset=UTF-8"/>
  <meta name="keywords" content="python web application" />
  <meta name="description" content="pyramid web application" />
  <link rel="shortcut icon"
        href="${request.static_url('tutorial:static/favicon.ico')}" />
  <link rel="stylesheet"
        href="${request.static_url('tutorial:static/pylons.css')}"
        type="text/css" media="screen" charset="utf-8" />
  <!--[if lte IE 6]>
  <link rel="stylesheet"
        href="${request.static_url('tutorial:static/ie6.css')}"
        type="text/css" media="screen" charset="utf-8" />
  <![endif]-->
</head>
<body>
  <div id="wrap">
    <div id="top-small">
      <div class="top-small align-center">
        <div>
          <img width="220" height="50" alt="pyramid"
        src="${request.static_url('tutorial:static/pyramid-small.png')}" />
        </div>
      </div>
    </div>
    <div id="middle">
      <div class="middle align-right">
        <div id="left" class="app-welcome align-left">
          <b>Login</b><br/>
          <span tal:replace="message"/>
        </div>
        <div id="right" class="app-welcome align-right"></div>
      </div>
    </div>
    <div id="bottom">
      <div class="bottom">
        <form action="${url}" method="post">
          <input type="hidden" name="came_from" value="${came_from}"/>
          <input type="text" name="login" value="${login}"/><br/>
          <input type="password" name="password"
                 value="${password}"/><br/>
          <input type="submit" name="form.submitted" value="Log In"/>
        </form>
      </div>
    </div>
  </div>
  <div id="footer">
    <div class="footer"
         >&copy; Copyright 2008-2011, Agendaless Consulting.</div>
  </div>
</body>
</html>

Seeing Our Changes To views.py and our Templates

Our views.py module will look something like this when we’re done:

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import re
from docutils.core import publish_parts

from pyramid.httpexceptions import (
    HTTPFound,
    HTTPNotFound,
    )

from pyramid.view import (
    view_config,
    forbidden_view_config,
    )

from pyramid.security import (
    remember,
    forget,
    authenticated_userid,
    )

from .models import (
    DBSession,
    Page,
    )

from .security import USERS

# regular expression used to find WikiWords
wikiwords = re.compile(r"\b([A-Z]\w+[A-Z]+\w+)")

@view_config(route_name='view_wiki')
def view_wiki(request):
    return HTTPFound(location = request.route_url('view_page',
                                                  pagename='FrontPage'))

@view_config(route_name='view_page', renderer='templates/view.pt')
def view_page(request):
    pagename = request.matchdict['pagename']
    page = DBSession.query(Page).filter_by(name=pagename).first()
    if page is None:
        return HTTPNotFound('No such page')

    def check(match):
        word = match.group(1)
        exists = DBSession.query(Page).filter_by(name=word).all()
        if exists:
            view_url = request.route_url('view_page', pagename=word)
            return '<a href="%s">%s</a>' % (view_url, word)
        else:
            add_url = request.route_url('add_page', pagename=word)
            return '<a href="%s">%s</a>' % (add_url, word)

    content = publish_parts(page.data, writer_name='html')['html_body']
    content = wikiwords.sub(check, content)
    edit_url = request.route_url('edit_page', pagename=pagename)
    return dict(page=page, content=content, edit_url=edit_url,
                logged_in=authenticated_userid(request))

@view_config(route_name='add_page', renderer='templates/edit.pt',
             permission='edit')
def add_page(request):
    name = request.matchdict['pagename']
    if 'form.submitted' in request.params:
        body = request.params['body']
        page = Page(name, body)
        DBSession.add(page)
        return HTTPFound(location = request.route_url('view_page',
                                                      pagename=name))
    save_url = request.route_url('add_page', pagename=name)
    page = Page('', '')
    return dict(page=page, save_url=save_url,
                logged_in=authenticated_userid(request))

@view_config(route_name='edit_page', renderer='templates/edit.pt',
             permission='edit')
def edit_page(request):
    name = request.matchdict['pagename']
    page = DBSession.query(Page).filter_by(name=name).one()
    if 'form.submitted' in request.params:
        page.data = request.params['body']
        DBSession.add(page)
        return HTTPFound(location = request.route_url('view_page',
                                                      pagename=name))
    return dict(
        page=page,
        save_url = request.route_url('edit_page', pagename=name),
        logged_in=authenticated_userid(request),
        )

@view_config(route_name='login', renderer='templates/login.pt')
@forbidden_view_config(renderer='templates/login.pt')
def login(request):
    login_url = request.route_url('login')
    referrer = request.url
    if referrer == login_url:
        referrer = '/' # never use the login form itself as came_from
    came_from = request.params.get('came_from', referrer)
    message = ''
    login = ''
    password = ''
    if 'form.submitted' in request.params:
        login = request.params['login']
        password = request.params['password']
        if USERS.get(login) == password:
            headers = remember(request, login)
            return HTTPFound(location = came_from,
                             headers = headers)
        message = 'Failed login'

    return dict(
        message = message,
        url = request.application_url + '/login',
        came_from = came_from,
        login = login,
        password = password,
        )

@view_config(route_name='logout')
def logout(request):
    headers = forget(request)
    return HTTPFound(location = request.route_url('view_wiki'),
                     headers = headers)

(Only the highlighted lines need to be added.)

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

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"
      xmlns:tal="http://xml.zope.org/namespaces/tal">
<head>
  <title>${page.name} - Pyramid tutorial wiki (based on
    TurboGears 20-Minute Wiki)</title>
  <meta http-equiv="Content-Type" content="text/html;charset=UTF-8"/>
  <meta name="keywords" content="python web application" />
  <meta name="description" content="pyramid web application" />
  <link rel="shortcut icon"
        href="${request.static_url('tutorial:static/favicon.ico')}" />
  <link rel="stylesheet"
        href="${request.static_url('tutorial:static/pylons.css')}"
        type="text/css" media="screen" charset="utf-8" />
  <!--[if lte IE 6]>
  <link rel="stylesheet"
        href="${request.static_url('tutorial:static/ie6.css')}"
        type="text/css" media="screen" charset="utf-8" />
  <![endif]-->
</head>
<body>
  <div id="wrap">
    <div id="top-small">
      <div class="top-small align-center">
        <div>
          <img width="220" height="50" alt="pyramid"
        src="${request.static_url('tutorial:static/pyramid-small.png')}" />
        </div>
      </div>
    </div>
    <div id="middle">
      <div class="middle align-right">
        <div id="left" class="app-welcome align-left">
          Editing <b><span tal:replace="page.name">Page Name
            Goes Here</span></b><br/>
          You can return to the
          <a href="${request.application_url}">FrontPage</a>.<br/>
        </div>
        <div id="right" class="app-welcome align-right">
          <span tal:condition="logged_in">
              <a href="${request.application_url}/logout">Logout</a>
          </span>
        </div>
      </div>
    </div>
    <div id="bottom">
      <div class="bottom">
        <form action="${save_url}" method="post">
          <textarea name="body" tal:content="page.data" rows="10"
                    cols="60"/><br/>
          <input type="submit" name="form.submitted" value="Save"/>
        </form>
      </div>
    </div>
  </div>
  <div id="footer">
    <div class="footer"
         >&copy; Copyright 2008-2011, Agendaless Consulting.</div>
  </div>
</body>
</html>

(Only the highlighted lines need to be added.)

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

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"
      xmlns:tal="http://xml.zope.org/namespaces/tal">
<head>
  <title>${page.name} - Pyramid tutorial wiki (based on
    TurboGears 20-Minute Wiki)</title>
  <meta http-equiv="Content-Type" content="text/html;charset=UTF-8"/>
  <meta name="keywords" content="python web application" />
  <meta name="description" content="pyramid web application" />
  <link rel="shortcut icon"
        href="${request.static_url('tutorial:static/favicon.ico')}" />
  <link rel="stylesheet"
        href="${request.static_url('tutorial:static/pylons.css')}"
        type="text/css" media="screen" charset="utf-8" />
  <!--[if lte IE 6]>
  <link rel="stylesheet"
        href="${request.static_url('tutorial:static/ie6.css')}"
        type="text/css" media="screen" charset="utf-8" />
  <![endif]-->
</head>
<body>
  <div id="wrap">
    <div id="top-small">
      <div class="top-small align-center">
        <div>
          <img width="220" height="50" alt="pyramid"
        src="${request.static_url('tutorial:static/pyramid-small.png')}" />
        </div>
      </div>
    </div>
    <div id="middle">
      <div class="middle align-right">
        <div id="left" class="app-welcome align-left">
          Viewing <b><span tal:replace="page.name">Page Name
            Goes Here</span></b><br/>
          You can return to the
          <a href="${request.application_url}">FrontPage</a>.<br/>
        </div>
        <div id="right" class="app-welcome align-right">
          <span tal:condition="logged_in">
            <a href="${request.application_url}/logout">Logout</a>
          </span>
        </div>
      </div>
    </div>
    <div id="bottom">
      <div class="bottom">
        <div tal:replace="structure content">
          Page text goes here.
        </div>
        <p>
          <a tal:attributes="href edit_url" href="">
            Edit this page
          </a>
        </p>
      </div>
    </div>
  </div>
  <div id="footer">
    <div class="footer"
         >&copy; Copyright 2008-2011, Agendaless Consulting.</div>
  </div>
</body>
</html>

(Only the highlighted lines need to be added.)

Viewing the Application in a Browser

We can finally examine our application in a browser (See Starting the Application). Launch a browser and visit each of the following URLs, check 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 object. It is executable by any user.
  • http://localhost:6543/FrontPage invokes the view_page view of the FrontPage page object.
  • 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.