code: def singleton(wrapped, _, args, kwargs):
      
      if wrapped not in _INSTANCES:
          _INSTANCES[wrapped] = wrapped(*args, **kwargs)
      return _INSTANCES[wrapped]
docs: Return the single instance of wrapped.
      :param wrapped: the wrapped class.
      :param _: unused
      :param args: optional arguments for wrapped object.
      :param kwargs: optional arguments for wrapped object.