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.
class_docstr_tok: ["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?
docstr_tok: ["Has","an","instance","been","created","?"]
code: def initialized(cls):
      """Has an instance been created?"""
      return hasattr(cls, "_instance") and cls._instance is not None
  
imports: from __future__ import print_function, absolute_import
  from copy import deepcopy
  import warnings
  from .loader import Config, LazyConfigValue, _is_section_key
  from traitlets.traitlets import HasTraits, Instance, observe, observe_compat, default
  from ipython_genutils.text import indent, dedent, wrap_paragraphs
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