def METHOD_NAME(self, app_name=None, external_only=False, jsonify=False):
    """
    Gets a list of the valid handoff handlers.
    Args:
        app_name (str, optional): The name of another app whose capabilities should be listed. Defaults to None in which case the capabilities of the current app will be listed.
        external_only (bool, optional): If True only return handlers where the internal attribute is False. Default is False.
        jsonify (bool, optional): If True return the JSON representation of the handlers is used. Default is False.
    Returns:
        A list of valid HandoffHandler objects (or a JSON string if jsonify=True) representing the capabilities of app_name, or None if no app with app_name is found.
    """  # noqa: E501
    manager = self._get_handoff_manager_for_app(app_name)
    if manager:
        handlers = manager.valid_handlers
        if external_only:
            handlers = [handler for handler in handlers if not handler.internal]
        if jsonify:
            handlers = json.dumps([handler.__dict__ for handler in handlers])
        return handlers