Restricting Access by Group in Django

Published on April 24, 2009, 1:36 p.m.

django python web

Django's authentication system provides built-in support for Groups. When developing an app, you may want to prevent users in a particular group from accessing part of your app.

For example, if you were building a tool to be used by Faculty and Students, it's quite possible that there would be parts of the app you wouldn't want Students to access (like the part that allows a User to change grades!). Luckily, there's a decorator called user_passes_test that allows you to easily perform this sort of thing. Let's see an example:

from django.contrib.auth.decorators import login_required, user_passes_test

@user_passes_test(lambda u: u.groups.filter(name='Student').count() == 0, login_url='/myapp/denied/')
def some_view(request):

# Do whatever this view should do

The view above (which lacks any content) actually uses two decorators. The first login_required simply requires that a user be logged in. The second, user_passes_test, requires a function as the first argument. This function must accept a User object and return True or False. If True, the User can view the page. If False, the user cannot view the page.

We define this function using a python Lambda Expression. It simply uses the ORM to check if a User is in the Student group. In this example, u would be an instance of django.contrib.auth.models.User.

lambda u: u.groups.filter(name='Student').count() == 0

Additionally, you can specify the keyword argument login_url to the user_passes_test decorator. Normally, if the user failed the test this would redirect them to a login page. However, in our case, they're already logged in (because of the login_required decorator), so this just acts as a redirect page. In this example, it would redirect to a url that we've specified which might contain an Access Denied message (with giant red blinking letters!)

Just kidding.

UPDATE: There is one slight caveat to this solution so far... it breaks if your user is not already logged in. In fact, this will result in

AttributeError:'NoneType' object has no attribute '_meta'

So, to account for that, we need to put the logic that tests for Student-group membership into its own function. The result would look something like the following:

def not_in_student_group(user):
if user:
return user.groups.filter(name='Student').count() == 0
return False

This makes sure the user object exists before trying the test, and if not, we assume the test fails (by returning False). Now, our decorator will look something like this:

@user_passes_test(not_in_student_group, login_url='/advising/denied/')
def some_view(request):
# ...
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