# Τεκμηρίωση του Django - 1.10.7

# Πηγαίος κώδικας για το django.contrib.auth

    import inspect
    import re

    from django.apps import apps as django_apps
    from django.conf import settings
    from django.core.exceptions import ImproperlyConfigured, PermissionDenied
    from django.middleware.csrf import rotate_token
    from django.utils.crypto import constant_time_compare
    from django.utils.module_loading import import_string
    from django.utils.translation import LANGUAGE_SESSION_KEY

    from .signals import user_logged_in, user_logged_out, user_login_failed

    SESSION_KEY = '_auth_user_id'
    BACKEND_SESSION_KEY = '_auth_user_backend'
    HASH_SESSION_KEY = '_auth_user_hash'
    REDIRECT_FIELD_NAME = 'next'

    def load_backend(path):
        return import_string(path)()

    def _get_backends(return_tuples=False):
        backends = []
        for backend_path in settings.AUTHENTICATION_BACKENDS:
            backend = load_backend(backend_path)
            backends.append((backend, backend_path) if return_tuples else backend)
        if not backends:
            raise ImproperlyConfigured(
                'No authentication backends have been defined. Does '
                'AUTHENTICATION_BACKENDS contain anything?'
            )
        return backends

    def get_backends():
        return _get_backends(return_tuples=False)

    def _clean_credentials(credentials):
        """
        Cleans a dictionary of credentials of potentially sensitive info before
        sending to less secure functions.

        Not comprehensive - intended for user_login_failed signal
        """
        SENSITIVE_CREDENTIALS = re.compile('api|token|key|secret|password|signature', re.I)
        CLEANSED_SUBSTITUTE = '********************'
        for key in credentials:
            if SENSITIVE_CREDENTIALS.search(key):
                credentials[key] = CLEANSED_SUBSTITUTE
        return credentials

    def _get_user_session_key(request):
        # This value in the session is always serialized to a string, so we need
        # to convert it back to Python whenever we access it.
        return get_user_model()._meta.pk.to_python(request.session[SESSION_KEY])

    def authenticate(**credentials):
        """
        If the given credentials are valid, return a User object.
        """
        for backend, backend_path in _get_backends(return_tuples=True):
            try:
                inspect.getcallargs(backend.authenticate, **credentials)
            except TypeError:
                # This backend doesn't accept these credentials as arguments. Try the next one.
                continue

            try:
                user = backend.authenticate(**credentials)
            except PermissionDenied:
                # This backend says to stop in our tracks - this user should not be allowed in at all.
                break
            if user is None:
                continue
            # Annotate the user object with the path of the backend.
            user.backend = backend_path
            return user

        # The credentials supplied are invalid to all backends, fire signal
        user_login_failed.send(sender=__name__, credentials=_clean_credentials(credentials))

    def login(request, user, backend=None):
        """
        Persist a user id and a backend in the request. This way a user doesn't
        have to reauthenticate on every request. Note that data set during
        the anonymous session is retained when the user logs in.
        """
        session_auth_hash = ''
        if user is None:
            user = request.user
        if hasattr(user, 'get_session_auth_hash'):
            session_auth_hash = user.get_session_auth_hash()

        if SESSION_KEY in request.session:
            if _get_user_session_key(request) != user.pk or (
                    session_auth_hash and
                    not constant_time_compare(request.session.get(HASH_SESSION_KEY, ''), session_auth_hash)):
                # 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()

        try:
            backend = backend or user.backend
        except AttributeError:
            backends = _get_backends(return_tuples=True)
            if len(backends) == 1:
                _, backend = backends[0]
            else:
                raise ValueError(
                    'You have multiple authentication backends configured and '
                    'therefore must provide the `backend` argument or set the '
                    '`backend` attribute on the user.'
                )

        request.session[SESSION_KEY] = user._meta.pk.value_to_string(user)
        request.session[BACKEND_SESSION_KEY] = backend
        request.session[HASH_SESSION_KEY] = session_auth_hash
        if hasattr(request, 'user'):
            request.user = user
        rotate_token(request)
        user_logged_in.send(sender=user.__class__, request=request, user=user)

    def logout(request):
        """
        Removes the authenticated user's ID from the request and flushes their
        session data.
        """
        # Dispatch the signal before the user is logged out so the receivers have a
        # chance to find out *who* logged out.
        user = getattr(request, 'user', None)
        if hasattr(user, 'is_authenticated') and not user.is_authenticated:
            user = None
        user_logged_out.send(sender=user.__class__, request=request, user=user)

        # remember language choice saved to session
        language = request.session.get(LANGUAGE_SESSION_KEY)

        request.session.flush()

        if language is not None:
            request.session[LANGUAGE_SESSION_KEY] = language

        if hasattr(request, 'user'):
            from django.contrib.auth.models import AnonymousUser
            request.user = AnonymousUser()

    def get_user_model():
        """
        Returns the User model that is active in this project.
        """
        try:
            return django_apps.get_model(settings.AUTH_USER_MODEL)
        except ValueError:
            raise ImproperlyConfigured("AUTH_USER_MODEL must be of the form 'app_label.model_name'")
        except LookupError:
            raise ImproperlyConfigured(
                "AUTH_USER_MODEL refers to model '%s' that has not been installed" % settings.AUTH_USER_MODEL
            )

    def get_user(request):
        """
        Returns the user model instance associated with the given request session.
        If no user is retrieved an instance of `AnonymousUser` is returned.
        """
        from .models import AnonymousUser
        user = None
        try:
            user_id = _get_user_session_key(request)
            backend_path = request.session[BACKEND_SESSION_KEY]
        except KeyError:
            pass
        else:
            if backend_path in settings.AUTHENTICATION_BACKENDS:
                backend = load_backend(backend_path)
                user = backend.get_user(user_id)
                # Verify the session
                if hasattr(user, 'get_session_auth_hash'):
                    session_hash = request.session.get(HASH_SESSION_KEY)
                    session_hash_verified = session_hash and constant_time_compare(
                        session_hash,
                        user.get_session_auth_hash()
                    )
                    if not session_hash_verified:
                        request.session.flush()
                        user = None

        return user or AnonymousUser()

    def get_permission_codename(action, opts):
        """
        Returns the codename of the permission for the specified action.
        """
        return '%s_%s' % (action, opts.model_name)

    def update_session_auth_hash(request, user):
        """
        Updating a user's password logs out all sessions for the user.

        This function takes the current request and the updated user object from
        which the new session hash will be derived and updates the session hash
        appropriately to prevent a password change from logging out the session
        from which the password was changed.
        """
        if hasattr(user, 'get_session_auth_hash') and request.user == user:
            request.session[HASH_SESSION_KEY] = user.get_session_auth_hash()

    default_app_config = 'django.contrib.auth.apps.AuthConfig'