class_docstr: Simple event to which event handlers can be attached. For instance::
  
      class Cls:
          def __init__(self):
              # Define event. The first parameter is the sender.
              self.event = Event(self)
  
      obj = Cls()
  
      def handler(sender):
          pass
  
      # Add event handler by using the += operator.
      obj.event += handler
  
      # Fire event.
      obj.event()
docstr: Add another handler to this callback.
  (Handler should be a callable that takes exactly one parameter: the
  sender object.)
code: def add_handler(self, handler: Callable[[_Sender], None]) -> None:
      """
      Add another handler to this callback.
      (Handler should be a callable that takes exactly one parameter: the
      sender object.)
      """
      # Add to list of event handlers.
      self._handlers.append(handler)
  
imports: from __future__ import annotations
  import os
  import signal
  import sys
  import threading
  from collections import deque
  from typing import Callable, ContextManager, Dict, Generator, Generic, TypeVar, Union
  from wcwidth import wcwidth
label_desc: Use image node_id 4 for calling the Event obj's underlying member method code with example usage: obj.add_handler(handler) without return types