class_docstr: Class used to generate nicer error messages if sessions are not
  available.  Will still allow read-only access to the empty session
  but fail on setting.
code: def _fail(self, *args: t.Any, **kwargs: t.Any) -> t.NoReturn:
      raise RuntimeError(
          "The session is unavailable because no secret "
          "key was set.  Set the secret_key on the "
          "application to something unique and secret."
      )
  
imports: from __future__ import annotations
  import hashlib
  import typing
  from collections.abc import MutableMapping
  from datetime import datetime
  from datetime import timezone
  from itsdangerous import BadSignature
  from itsdangerous import URLSafeTimedSerializer
  from werkzeug.datastructures import CallbackDict
  from .json.tag import TaggedJSONSerializer
label_desc: Use image node_id 1 for calling the NullSession obj's underlying member method code with example usage: obj._fail() without return types