code: def singleton_init_by(init_fn=None):
      
  
      if not init_fn:
          def wrap_init(origin_init):
              return origin_init
      else:
          def wrap_init(origin_init):
              def __init__(self):
                  origin_init(self)
                  init_fn(self)
  
              return __init__
  
      def inner(cls_def: type):
          if not hasattr(cls_def, '__instancecheck__') or isinstance(cls_def.__instancecheck__,
                                                                     (types.BuiltinMethodType, _slot_wrapper)):
              def __instancecheck__(self, instance):
                  return instance is self
  
              cls_def.__instancecheck__ = __instancecheck__
  
          _origin_init = cls_def.__init__
          cls_def.__init__ = wrap_init(_origin_init)
  
          return cls_def()
  
      return inner
docs: >>> from Redy.Magic.Classic import singleton
      >>> @singleton
      >>> class S:
      >>>     pass
      >>> assert isinstance(S, S)