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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. Pyramid provides facilities for authorization and authentication. We’ll make use of both features to provide security to our application.

We will add an authentication policy and an authorization policy to our application registry, add a security.py module, create a root factory with an ACL, and add permission declarations to the edit_page and add_page views.

Then we will add login and logout views, and modify the existing views to make them return a logged_in flag to the renderer.

Finally, we will add a login.pt template and change the existing view.pt and edit.pt to show a “Logout” link when not logged in.

The source code for this tutorial stage can be browsed at http://github.com/Pylons/pyramid/tree/1.1-branch/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 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. We do this to allow Pyramid declarative security to work properly. The context object generated by the root factory during a request will 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 Pyramid.

We’ll modify our __init__.py, passing in a root factory to our 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:

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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 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.

We’ll pass the RootFactory we created in the step above in as the root_factory argument to a Configurator.

Configuring 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 change our __init__.py file to enable an AuthTktAuthenticationPolicy and an ACLAuthorizationPolicy to enable declarative security checking. We need to import the new policies:

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

Then, we’ll 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',
                          authentication_policy=authn_policy,
                          authorization_policy=authz_policy)

Note that 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.

We’ll also change __init__.py, adding a call to pyramid.config.Configurator.add_view() that points at our login view callable. This is also known as a forbidden view:

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    config.add_route('login', '/login')
    config.add_view('tutorial.login.login',
                    context='pyramid.httpexceptions.HTTPForbidden',
                    renderer='tutorial:templates/login.pt')

A forbidden view configures our newly created login view to show up when Pyramid detects that a view invocation can not be authorized.

A logout view callable will allow users to log out later:

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    config.add_route('logout', '/logout')
    config.add_view('tutorial.login.logout', route_name='logout')

We’ll also add permission arguments with the value edit to the edit_page and add_page views. 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.

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    config.add_view('tutorial.views.add_page', route_name='add_page',
                    renderer='tutorial:templates/edit.pt', permission='edit')
    config.add_view('tutorial.views.edit_page', route_name='edit_page',
                    renderer='tutorial:templates/edit.pt', permission='edit')

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 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:

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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.models import initialize_sql
from tutorial.security import groupfinder

def main(global_config, **settings):
    """ This function returns a WSGI application.
    """
    engine = engine_from_config(settings, 'sqlalchemy.')
    initialize_sql(engine)
    authn_policy = AuthTktAuthenticationPolicy(
        'sosecret', callback=groupfinder)
    authz_policy = ACLAuthorizationPolicy()
    config = Configurator(settings=settings,
                          root_factory='tutorial.models.RootFactory',
                          authentication_policy=authn_policy,
                          authorization_policy=authz_policy)
    config.add_static_view('static', 'tutorial:static')

    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.add_view('tutorial.views.view_wiki', route_name='view_wiki')
    config.add_view('tutorial.login.login', route_name='login', 
                    renderer='tutorial:templates/login.pt')
    config.add_view('tutorial.login.logout', route_name='logout')
    config.add_view('tutorial.views.view_page', route_name='view_page',
                    renderer='tutorial:templates/view.pt')
    config.add_view('tutorial.views.add_page', route_name='add_page',
                    renderer='tutorial:templates/edit.pt', permission='edit')
    config.add_view('tutorial.views.edit_page', route_name='edit_page',
                    renderer='tutorial:templates/edit.pt', permission='edit')
    config.add_view('tutorial.login.login',
                    context='pyramid.httpexceptions.HTTPForbidden',
                    renderer='tutorial:templates/login.pt')
    return config.make_wsgi_app()

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:

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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 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 the logout view callables. Add a file named login.py to your application (in the same directory as views.py) with the following content:

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from pyramid.httpexceptions import HTTPFound
from pyramid.security import remember
from pyramid.security import forget
from pyramid.url import route_url

from tutorial.security import USERS

def login(request):
    login_url = route_url('login', request)
    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,
        )
    
def logout(request):
    headers = forget(request)
    return HTTPFound(location = route_url('view_wiki', request),
                     headers = headers)
    

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:

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

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)

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.

<!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>

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>

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.security import authenticated_userid
from pyramid.url import route_url

from tutorial.models import DBSession
from tutorial.models import Page

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

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

def view_page(request):
    pagename = request.matchdict['pagename']
    session = DBSession()
    page = session.query(Page).filter_by(name=pagename).first()
    if page is None:
        return HTTPNotFound('No such page')

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

def add_page(request):
    name = request.matchdict['pagename']
    if 'form.submitted' in request.params:
        session = DBSession()
        body = request.params['body']
        page = Page(name, body)
        session.add(page)
        return HTTPFound(location = route_url('view_page', request,
                                              pagename=name))
    save_url = route_url('add_page', request, pagename=name)
    page = Page('', '')
    logged_in = authenticated_userid(request)
    return dict(page=page, save_url=save_url, logged_in=logged_in)

def edit_page(request):
    name = request.matchdict['pagename']
    session = DBSession()
    page = session.query(Page).filter_by(name=name).one()
    if 'form.submitted' in request.params:
        page.data = request.params['body']
        session.add(page)
        return HTTPFound(location = route_url('view_page', request,
                                              pagename=name))

    logged_in = authenticated_userid(request)
    return dict(
        page=page,
        save_url = route_url('edit_page', request, pagename=name),
        logged_in = logged_in,
        )

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>

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>

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.
  • 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.