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Wiki Flow of Authentication

This tutorial describes the "flow of authentication" of the result of the completing the :ref:`wiki2_adding_authorization` tutorial chapter from the main Pyramid documentation.

This text was contributed by John Shipman.

Overall flow of an authentication

Now that you have seen all the pieces of the authentication mechanism, here are some examples that show how they all work together.

  1. Failed login: The user requests /FrontPage/edit_page. The site presents the login form. The user enters editor as the login, but enters an invalid password bad. The site redisplays the login form with the message "Failed login". See :ref:`failed_login`.
  2. The user again requests /FrontPage/edit_page. The site presents the login form, and this time the user enters login editor and password editor. The site presents the edit form with the content of /FrontPage. The user makes some changes and saves them. See :ref:`good_login`.
  3. The user again revisits /FrontPage/edit_page. The site goes immediately to the edit form without requesting credentials. See :ref:`revisit`.
  4. The user clicks the Logout link. See :ref:`logging_out`.

Failed login

The process starts when the user enters URL http://localhost:6543/FrontPage/edit_page. Let's assume that this is the first request ever made to the application and the page database is empty except for the Page instance created for the front page by the initialize_sql function in :file:`models.py`.

This process involves two complete request/response cycles.

  1. From the front page, the user clicks :guilabel:`Edit page`. The request is to /FrontPage/edit_page. The view callable is login.login. The response is the login.pt template with blank fields.
  2. The user enters invalid credentials and clicks :guilabel:`Log in`. A POST request is sent to /FrontPage/edit_page. The view callable is again login.login. The response is the login.pt template showing the message "Failed login", with the entry fields displaying their former values.

Cycle 1:

  1. During URL dispatch, the route '/{pagename}/edit_page' is considered for matching. The associated view has a view_permission='edit' permission attached, so the dispatch logic has to verify that the user has that permission or the route is not considered to match.

    The context for all route matching comes from the configured root factory, :meth:`RootFactory` in :file:`models.py`. This class has an __acl__ attribute that defines the access control list for all routes:

    __acl__ = [ (Allow, Everyone, 'view'),
                (Allow, 'group:editors', 'edit') ]
    

    In practice, this means that for any route that requires the edit permission, the user must be authenticated and have the group:editors principal or the route is not considered to match.

  2. To find the list of the user's principals, the authorization first policy checks to see if the user has a paste.auth.auth_tkt cookie. Since the user has never been to the site, there is no such cookie, and the user is considered to be unauthenticated.

  3. Since the user is unauthenticated, the groupfinder function in :file:`security.py` is called with None as its userid argument. The function returns an empty list of principals.

  4. Because that list does not contain the group:editors principal, the '/{pagename}/edit_page' route's edit permission fails, and the route does not match.

  5. Because no routes match, the forbidden view callable is invoked: the login function in module login.py.

  6. Inside the login function, the value of login_url is http://localhost:6543/login, and the value of referrer is http://localhost:6543/FrontPage/edit_page.

    Because request.params has no key for 'came_from', the variable came_from is also set to http://localhost:6543/FrontPage/edit_page. Variables message, login, and password are set to the empty string.

    Because request.params has no key for 'form.submitted', the login function returns this dictionary:

    {'message': '', 'url':'http://localhost:6543/login',
     'came_from':'http://localhost:6543/FrontPage/edit_page',
     'login':'', 'password':''}
    
  7. This dictionary is used to render the login.pt template. In the form, the action attribute is http://localhost:6543/login, and the value of came_from is included in that form as a hidden field by this line in the template:

    <input type="hidden" name="came_from" value="${came_from}"/>
    

Cycle 2:

  1. The user enters incorrect credentials and clicks the :guilabel:`Log in` button, which does a POST request to URL http://localhost:6543/login. The name of the :guilabel:`Log in` button in this form is form.submitted.

  2. The route with pattern '/login' matches this URL, so control is passed again to the login view callable.

  3. The login_url and referrer have the same value this time (http://localhost:6543/login), so variable referrer is set to '/'.

    Since request.params does have a key 'form.submitted', the values of login and password are retrieved from request.params.

    Because the login and password do not match any of the entries in the USERS dictionary in security.py, variable message is set to 'Failed login'.

    The view callable returns this dictionary:

    {'message':'Failed login',
     'url':'http://localhost:6543/login', 'came_from':'/',
     'login':'editor', 'password':'bad'}
    
  4. The login.pt template is rendered using those values.

Successful login

In this scenario, the user again requests URL /FrontPage/edit_page.

This process involves four complete request/response cycles.

  1. The user clicks :guilabel:`Edit page`. The view callable is login.login. The response is template login.pt, with all the fields blank.
  2. The user enters valid credentials and clicks :guilabel:`Log in`. The view callable is login.login. The response is a redirect to /FrontPage/edit_page.
  3. The view callable is views.edit_page. The response renders template edit.pt, displaying the current page content.
  4. The user edits the content and clicks :guilabel:`Save`. The view callable is views.edit_page. The response is a redirect to /FrontPage.

Execution proceeds as in :ref:`failed_login`, up to the point where the password editor is successfully matched against the value from the USERS dictionary.

Cycle 2:

  1. Within the login.login view callable, the value of login_url is http://localhost:6543/login, and the value of referrer is '/', and came_from is http://localhost:6543/FrontPage/edit_page when this block is executed:

    if USERS.get(login) == password:
        headers = remember(request, login)
        return HTTPFound(location=came_from, headers=headers)
    
  2. Because the password matches this time, :mod:`pyramid.security.remember` returns a sequence of header tuples that will set a paste.auth.auth_tkt authentication cookie in the user's browser for the login 'editor'.

  3. The HTTPFound exception returns a response that redirects the browser to http://localhost:6543/FrontPage/edit_page, including the headers that set the authentication cookie.

Cycle 3:

  1. Route pattern '/{pagename}/edit_page' matches this URL, but the corresponding view is restricted by an 'edit' permission.

  2. Because the user now has an authentication cookie defining their login name as 'editor', the groupfinder function is called with that value as its userid argument.

  3. The groupfinder function returns the list ['group:editors']. This satisfies the access control entry (Allow, 'group:editors', 'edit'), which grants the edit permission. Thus, this route matches, and control passes to view callable edit_page.

  4. Within edit_page, name is set to 'FrontPage', the page name from request.matchdict['pagename'], and page is set to an instance of :class:`models.Page` that holds the current content of FrontPage.

  5. Since this request did not come from a form, request.params does not have a key for 'form.submitted'.

  6. The edit_page function calls :meth:`pyramid.security.authenticated_userid` to find out whether the user is authenticated. Because of the cookies set previously, the variable logged_in is set to the userid 'editor'.

  7. The edit_page function returns this dictionary:

    {'page':page, 'logged_in':'editor',
     'save_url':'http://localhost:6543/FrontPage/edit_page'}
    
  8. Template :file:`edit.pt` is rendered with those values. Among other features of this template, these lines cause the inclusion of a :guilabel:`Logout` link:

    <span tal:condition="logged_in">
      <a href="${request.application_url}/logout">Logout</a>
    </span>
    

    For the example case, this link will refer to http://localhost:6543/logout.

    These lines of the template display the current page's content in a form whose action attribute is http://localhost:6543/FrontPage/edit_page:

    <form action="${save_url}" method="post">
      <textarea name="body" tal:content="page.data" rows="10" cols="60"/>
      <input type="submit" name="form.submitted" value="Save"/>
    </form>
    

Cycle 4:

  1. The user edits the page content and clicks :guilabel:`Save`.

  2. URL http://localhost:6543/FrontPage/edit_page goes through the same routing as before, up until the line that checks whether request.params has a key 'form.submitted'. This time, within the edit_page view callable, these lines are executed:

    page.data = request.params['body']
    session.add(page)
    return HTTPFound(location = route_url('view_page', request,
                                          pagename=name))
    

    The first two lines replace the old page content with the contents of the body text area from the form, and then update the page stored in the database. The third line causes a response that redirects the browser to http://localhost:6543/FrontPage.

Revisiting after authentication

In this case, the user has an authentication cookie set in their browser that specifies their login as 'editor'. The requested URL is http://localhost:6543/FrontPage/edit_page.

This process requires two request/response cycles.

  1. The user clicks :guilabel:`Edit page`. The view callable is views.edit_page. The response is edit.pt, showing the current page content.
  2. The user edits the content and clicks :guilabel:`Save`. The view callable is views.edit_page. The response is a redirect to /Frontpage.

Cycle 1:

  1. The route with pattern /{pagename}/edit_page matches the URL, and because of the authentication cookie, groupfinder returns a list containing the group:editors principal, which models.RootFactory.__acl__ uses to grant the edit permission, so this route matches and dispatches to the view callable :meth:`views.edit_page`.

  2. In edit_page, because the request did not come from a form submission, request.params has no key for 'form.submitted'.

  3. The variable logged_in is set to the login name 'editor' by calling authenticated_userid, which extracts it from the authentication cookie.

  4. The function returns this dictionary:

    {'page':page,
     'save_url':'http://localhost:6543/FrontPage/edit_page',
     'logged_in':'editor'}
    
  5. Template :file:`edit.pt` is rendered with the values from that dictionary. Because of the presence of the 'logged_in' entry, a :guilabel:`Logout` link appears.

Cycle 2:

  1. The user edits the page content and clicks :guilabel:`Save`.
  2. The POST operation works as in :ref:`good_login`.

Logging out

This process starts with a request URL http://localhost:6543/logout.

  1. The route with pattern '/logout' matches and dispatches to the view callable logout in :file:`login.py`.
  2. The call to :meth:`pyramid.security.forget` returns a list of header tuples that will, when returned with the response, cause the browser to delete the user's authentication cookie.
  3. The view callable returns an HTTPFound exception that redirects the browser to named route view_wiki, which will translate to URL http://localhost:6543. It also passes along the headers that delete the authentication cookie.
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