code: def initialize(cls) -> None:
          
          if cls._initialized:
              return
          io_loop = ioloop.IOLoop.current()
          cls._old_sigchld = signal.signal(
              signal.SIGCHLD,
              lambda sig, frame: io_loop.add_callback_from_signal(cls._cleanup),
          )
          cls._initialized = True
docs: Initializes the ``SIGCHLD`` handler.
  
          The signal handler is run on an `.IOLoop` to avoid locking issues.
          Note that the `.IOLoop` used for signal handling need not be the
          same one used by individual Subprocess objects (as long as the
          ``IOLoops`` are each running in separate threads).
  
          .. versionchanged:: 5.0
             The ``io_loop`` argument (deprecated since version 4.1) has been
             removed.
  
          Availability: Unix