from django.core.context_processors import PermWrapper

from django.utils.functional import lazy, memoize, SimpleLazyObject

from django.contrib import messages

def auth(request):

    """

    Returns context variables required by apps that use Django's authentication

    system.

    If there is no 'user' attribute in the request, uses AnonymousUser (from

    django.contrib.auth).

    """

    # If we access request.user, request.session is accessed, which results in

    # 'Vary: Cookie' being sent in every request that uses this context

    # processor, which can easily be every request on a site if

    # TEMPLATE_CONTEXT_PROCESSORS has this context processor added.  This kills

    # the ability to cache.  So, we carefully ensure these attributes are lazy.

    # We don't use django.utils.functional.lazy() for User, because that

    # requires knowing the class of the object we want to proxy, which could

    # break with custom auth backends.  LazyObject is a less complete but more

    # flexible solution that is a good enough wrapper for 'User'.

    def get_user():

        if hasattr(request, 'user'):

            return request.user

        else:

            from django.contrib.auth.models import AnonymousUser

            return AnonymousUser()

    return {

        'user': SimpleLazyObject(get_user),

        'messages': messages.get_messages(request),

        'perms':  lazy(lambda: PermWrapper(get_user()), PermWrapper)(),

    }