import datetime

from django.core.exceptions import ImproperlyConfigured

from django.utils.importlib import import_module

SESSION_KEY = '_auth_user_id'

BACKEND_SESSION_KEY = '_auth_user_backend'

REDIRECT_FIELD_NAME = 'next'

def load_backend(path):

    i = path.rfind('.')

    module, attr = path[:i], path[i+1:]

    try:

        mod = import_module(module)

    except ImportError, e:

    except ValueError, e:

    try:

        cls = getattr(mod, attr)

    except AttributeError:

    return cls()

def get_backends():

    from django.conf import settings

    backends = []

    for backend_path in settings.AUTHENTICATION_BACKENDS:

        backends.append(load_backend(backend_path))

    return backends

def authenticate(**credentials):

    """

    If the given credentials are valid, return a User object.

    """

    for backend in get_backends():

        try:

            user = backend.authenticate(**credentials)

        except TypeError:

            # This backend doesn't accept these credentials as arguments. Try the next one.

            continue

        if user is None:

            continue

        # Annotate the user object with the path of the backend.

        user.backend = "%s.%s" % (backend.__module__, backend.__class__.__name__)

        return user

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.flush()

    else:

        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.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