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: Walk the cls.mro() for parent classes that are also singletons
  
  For use in instance()
code: def _walk_mro(
      cls,
  ) -> t.Generator[type[SingletonConfigurable], None, None]:
      """Walk the cls.mro() for parent classes that are also singletons
  
      For use in instance()
      """
  
      for subclass in cls.mro():
          if (
              issubclass(cls, subclass)
              and issubclass(subclass, SingletonConfigurable)
              and subclass != SingletonConfigurable
          ):
              yield subclass
  
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 1 for calling the SingletonConfigurable obj's underlying member method code with example usage: obj._walk_mro(cls) without return types