import rclpy
from rclpy.waitable import Waitable
from rclpy.callback_groups import MutuallyExclusiveCallbackGroup


class MutuallyExclusiveWaitable(Waitable):

    def __init__(self):
        super().__init__(MutuallyExclusiveCallbackGroup())

    def is_ready(self, wait_set):
        return False

    def take_data(self):
        return None

    async def execute(self, taken_data):
        pass

    def get_num_entities(self):
        return NumberOfEntities(0, 0, 0, 0, 0)

    def add_to_wait_set(self, wait_set):
        pass

rclpy.init()
waitable = MutuallyExclusiveWaitable()
node = rclpy.create_node('mynode')
node.add_waitable(waitable)
rclpy.spin(node)
rclpy.shutdown()

Traceback (most recent call last):
  File "waitable_mux.py", line 30, in <module>
    rclpy.spin(node)
  File "/home/jacob/ws/ros2_actions_ws/install/rclpy/lib/python3.5/site-packages/rclpy/__init__.py", line 119, in spin
    executor.spin_once()
  File "/home/jacob/ws/ros2_actions_ws/install/rclpy/lib/python3.5/site-packages/rclpy/executors.py", line 557, in spin_once
    handler, entity, node = self.wait_for_ready_callbacks(timeout_sec=timeout_sec)
  File "/home/jacob/ws/ros2_actions_ws/install/rclpy/lib/python3.5/site-packages/rclpy/executors.py", line 540, in wait_for_ready_callbacks
    return next(self._cb_iter)
  File "/home/jacob/ws/ros2_actions_ws/install/rclpy/lib/python3.5/site-packages/rclpy/executors.py", line 388, in _wait_for_ready_callbacks
    waitables.extend(filter(self.can_execute, node.waitables))
  File "/home/jacob/ws/ros2_actions_ws/install/rclpy/lib/python3.5/site-packages/rclpy/executors.py", line 339, in can_execute
    return not entity._executor_event and entity.callback_group.can_execute(entity)
  File "/home/jacob/ws/ros2_actions_ws/install/rclpy/lib/python3.5/site-packages/rclpy/callback_groups.py", line 98, in can_execute
    assert weakref.ref(entity) in self.entities
AssertionError