def login(request, user):
    """
    Persist a user id and a backend in the request. This way a user doesn't
    have to reauthenticate on every request.
    """
    if user is None:
        user = request.user
    # TODO: It would be nice to support different login methods, like signed cookies.
    user.last_login = datetime.datetime.now()
    user.save()

    if SESSION_KEY in request.session:
        if request.session[SESSION_KEY] != user.id:
            # To avoid reusing another user's session, create a new, empty
            # session if the existing session corresponds to a different
            # authenticated user.
            request.session.clear() #request.session.flush()
    else:
        pass #request.session.cycle_key()
    request.session[SESSION_KEY] = user.id
    request.session[BACKEND_SESSION_KEY] = user.backend
    if hasattr(request, 'user'):
        request.user = user

def logout(request):
    """
    Removes the authenticated user's ID from the request and flushes their
    session data.
    """
    request.session.clear() #request.session.flush()
    if hasattr(request, 'user'):
        from django.contrib.auth.models import AnonymousUser
        request.user = AnonymousUser()

def get_user(request):
    from django.contrib.auth.models import AnonymousUser
    try:
        user_id = request.session[SESSION_KEY]
        backend_path = request.session[BACKEND_SESSION_KEY]
        backend = load_backend(backend_path)
        user = backend.get_user(user_id) or AnonymousUser()
    except KeyError:
        user = AnonymousUser()
    return user