Wednesday, December 13, 2006

Django Su

It's always a sign of good design when adding a new feature turns out to be easy.

In the Django authentication system, I wanted a way for an administrator to view the site as if they were a particular user; effectively an equivalent to su in UNIX-land.

The first easy step was to invent a URL to correspond to this action, which gets encoded in

  (r'^su/(?P<username>.*)/$', '', {'redirect_url': '/qlockdata/'}),   

That done, the second and final step is to write some view code.

  @user_passes_test(lambda u: u.is_staff)
  def su(request, username, redirect_url='/'):
      su_user = get_object_or_404(User, username=username)
      if su_user.is_active:
          request.session[SESSION_KEY] =
      return HttpResponseRedirect(redirect_url)

Seven lines of code and we're done (modulo a bunch of import statements).

Expanding what's going on here:

    (r'^su/(?P<username>.*)/$', '', {'redirect_url': '/qlockdata/'}), 
  • When an HTTP request arrives at the framework, Django goes through its list of URLs until it finds a match. In this case, going to http://mysite/accounts/su/fred/ ends hitting the line above; the /(?P<username>.*)/ part of the regexp pulls out "fred" and this gets passed as a parameter named username into the function su in qlockweb/accounts/ This function also gets passed a parameter called redirect_url with value '/qlockdata/'.
  • @user_passes_test(lambda u: u.is_staff)
  • Actually, we need to rewind one step before we get into the su function. The line before the function definition is a Python decorator: some extra code wrapping the function that gets executed just before the function itself is executed. This decorator needs some expansion of its own:
      @user_passes_test(lambda u: u.is_staff)
    • The @ sign is the syntactic sugar that indicates that this line is a decorator for the function that comes immediately afterwards.
    • @user_passes_test(lambda u: u.is_staff)
    • This is the decorator function (from contrib/auth/; its first argument test_func is a function that does the test. This test function is given a single parameter: the current User. If the test function returns true, the wrapped view code is called; if not, then the user gets redirected to a login page.
    • @user_passes_test(lambda u: u.is_staff)
    • More syntactic sugar. We want a function is_this_a_staff_user(u) that checks whether its argument u is an administrator. However, as this is the only place that the function is used, we don't bother to give it a name—we just use a lambda expression to give the definition right here and now.
    • @user_passes_test(lambda u: u.is_staff)
    • Finally, the body of the lambda expression just uses the method of the User class that indicates whether the user is an administrator or not.
  • def su(request, username, redirect_url='/'):
  • So now we're in the su function itself, and if we've got this far we're guaranteed that the person viewing the page is logged in as an is_staff user. The function has the username and redirect_url parameters mentioned earlier; it also has a request parameter that holds all of the information about the original web request (in a HttpRequest object).
  •   su_user = get_object_or_404(User, username=username)
  • The next line of code gets a User object for the username that was specified—fred in other words. If there isn't a user called fred, then a Http404 exception gets raised, which will percolate up the stack and display a (surprise, surprise) 404 page.
  •   if su_user.is_active:
  • This particular version of our code only allows impersonation of active users, helpfully provided by the is_active field in the standard User model.
  •     request.session[SESSION_KEY] =
  • The next line of code is the one that actually does the work. The requesting user's session is modified so that its user ID is the impersonated user's.
  •   return HttpResponseRedirect(redirect_url)
  • The final line of code redirects the web browser off to the redirect_url page.

(Statutory disclaimer: I am not a security expert, nor do I play one on TV. Adding this to a production system is probably not a good idea.)

Another Django snippet: I finally discovered that the follow argument to the standard Manipulators allows you to list fields in the model that the form should leave untouched. Very helpful: the end result is more compact and less brittle than the code I'd put together to manually override all of the hidden fields.

[A:37385 B:3278 C:346 D:9187 E:62544 Total:112740]

Saturday, December 02, 2006

Mind Hack #39

[reading: Jón Árnason, Alan Boucher (translator), "Icelandic Folk Tales"]

One of the MindHacks that doesn't include an easy demonstration is Hack 39, where you're less likely to notice a trigger event if it occurs soon (say, <0.5s) after another trigger event.

(Now that I've got into the content rather than being distracted by the typesetting, the book is turning out to be rather good.)

My first attempt to test this out is below, but I have to say that the effect didn't seem all that strong to me—so maybe there's a bug or I've implemented it wrong. Or maybe I'm just too impatient to run it for long enough to get statistically significant data.

After hitting "Go", this applet will display letters for a tenth of a second each. You should hit a key whenever the letter displayed is either "X" or is displayed in white. To stop the applet, hit "Stop" or press the Escape key.

After running the applet, the bar graph shows the time between triggers (on the X axis) against how many triggers there were with that delay; the white is the total number shown, the black is the subset of them that were missed. Hitting "Go" again accumulates more data.

(Download source code)

Technical Details: letters are picked uniformly (so there's a 1/26 chance of a letter X trigger) and white is used for the colour 1/20 of the time. A trigger is considered hit if there's a keypress within 1 second after the trigger, but each keypress only counts once (so "trigger, trigger, keypress, letters…" would count as one hit then one missed trigger, with a delay of 1).

[A:37385 B:3278 C:346 D:9187 E:46445 Total:96641]