django - Persisting session variables across login

Question

I want to hold information about a users preferences in a session variable. If the user chooses a preference while logged out and then later logs in, I want the preference to be maintained without needing to reselect it.

Django sessions maintain a session key in a cookie to track a users session. The way I understand it, this key is changed when a user logs in.

a) Does this mean all session variables are deleted on login or is there any sort of passover

b) In the case of not being able to save preferences across a login, is manually setting cookies the best way to proceed? I imagine a scenario like:

• while logged out, maintain preferences in cookie
• on login, copy preferences to session variable and write to db (via signal?)
• on logout, update cookies with preferences (via signal?)

Update

I managed to get this functionality by saving preferences in the User's profile object, as well as in a cookie (these preferences are not sensitive in any way). When the user is logged in, their profile setting takes preference. When not logged in, the cookie preference is chosen

Answer 1

Upon login, Django calls session.flush() or session.cycle_key(), which makes sure nothing from the old session is kept. This is a security measure that protects you against session fixation vulnerabilities. So, when applying this solution, be aware what find of variables you want to persist.

If you want to keep some state, you'll have to restore that after the login was issued.

The solution by Chase Seibert was a great start, it was very insecure due to threadsafety issues in that code. You can find an improved version here, which is safe to use:

from functools import wraps

class persist_session_vars(object):
    """
    Some views, such as login and logout, will reset all session state.
    (via a call to ``request.session.cycle_key()`` or ``session.flush()``).
    That is a security measure to mitigate session fixation vulnerabilities.

    By applying this decorator, some values are retained.
    Be very aware what find of variables you want to persist.
    """

    def __init__(self, vars):
        self.vars = vars

    def __call__(self, view_func):

        @wraps(view_func)
        def inner(request, *args, **kwargs):
            # Backup first
            session_backup = {}
            for var in self.vars:
                try:
                    session_backup[var] = request.session[var]
                except KeyError:
                    pass

            # Call the original view
            response = view_func(request, *args, **kwargs)

            # Restore variables in the new session
            for var, value in session_backup.items():
                request.session[var] = value

            return response

        return inner

and now you can write:

from django.contrib.auth import views

@persist_session_vars(['some_field'])
def login(request, *args, **kwargs):
    return views.login(request, *args, **kwargs)

And for class based views (django-allauth):

import allauth.account.views as auth_views
from django.utils.decorators import method_decorator

@method_decorator(persist_session_vars(['some_field']), name='dispatch')
class LoginView(auth_views.LoginView):
    pass

and use that view in the url patterns:

import allauth.urls
from django.conf.urls import include, url

from . import views

urlpatterns = [
    # Views that overlap the default:
    url(r'^login/$', views.LoginView.as_view(), name='account_login'),

    # default allauth urls
    url(r'', include(allauth.urls)),
]