class_docstr: A configurable that only allows one instance.
  
  This class is for classes that should only have one instance of itself
  or *any* subclass. To create and retrieve such a class use the
  :meth:`SingletonConfigurable.instance` method.
docstr: Has an instance been created?
code: def initialized(cls) -> bool:
      """Has an instance been created?"""
      return hasattr(cls, "_instance") and cls._instance is not None
  
imports: from __future__ import annotations
  import logging
  import typing
  from copy import deepcopy
  from textwrap import dedent
  from traitlets.traitlets import Any, Container, Dict, HasTraits, Instance, TraitType, default, observe, observe_compat, validate
  from traitlets.utils import warnings
  from traitlets.utils.bunch import Bunch
  from traitlets.utils.text import indent, wrap_paragraphs
  from .loader import Config, DeferredConfig, LazyConfigValue, _is_section_key
label_desc: Use image node_id 4 for calling the SingletonConfigurable obj's underlying member method code with example usage: obj.initialized(cls) and returns: unknown