Ros2 AsyncIO libray

Introduction

Asynchronous programming is a method of concurrent programming that enables efficient execution of tasks, particularly in I/O-bound and network-driven applications.

It enables a program to handle multiple tasks at the same time, while not using OS level threads.

When using the ROS2 Python API (rclpy) together with ros2_asyncio, we can use some features of the Python language to achieve more readable code and efficient execution.

Why not Python's asyncio?

We need to be clear on one thing. ROS 2 has no relation with Python’s asyncio library https://docs.python.org/3/library/asyncio.html

Yes, the asyncio library has many great features, and we normally use Python asynchronous programming always in tandem with it.

But ROS 2 is not making use of it and has its own Event Loop, Futures and Task implementations. See details in ros2/rclpy#279

Hence, when trying it use asyncio with ROS 2 you typically run into problems like this:

  File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/executors.py", line 294, in spin
    self.spin_once()
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/executors.py", line 739, in spin_once
    self._spin_once_impl(timeout_sec)
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/executors.py", line 736, in _spin_once_impl
    raise handler.exception()
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/task.py", line 239, in __call__
    self._handler.send(None)
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/executors.py", line 437, in handler
    await call_coroutine(entity, arg)
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/executors.py", line 391, in _execute_service
    response = await await_or_execute(srv.callback, request, srv.srv_type.Response())
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/executors.py", line 104, in await_or_execute
    return await callback(*args)
  File "/workspace/aescape_perception/example_event_loop_ros2.py", line 43, in sleep10_callback
    await asyncio.sleep(5)
  File "/usr/lib/python3.10/asyncio/tasks.py", line 599, in sleep
    loop = events.get_running_loop()
RuntimeError: no running event loop

Installation

pip3 install git+https://github.com/tlangmo/ros2_asyncio.git

How to use ros2_asyncio?

ros2_asyncio has very similar API to asyncio but it is ROS 2 aware.

import ros2_asyncio
context = rclpy.context.Context()
rclpy.init(context=context)
node = rclpy.create_node(
    'TestExecutor', namespace='/rclpy', context=context)
try:
    executor = SingleThreadedExecutor(context=context)
    executor.add_node(node)
    async def _do_task(sleep_sec: int):
        for i in range(5):
            await ros2_asyncio.sleep(simple_node, sleep_sec)
            print(f'Hello from task')
    f = executor.create_task(_do_task(1))
    executor.spin_until_future_complete(f)
finally:
    # Teardown code: destroy the node and shutdown rclpy
    node.destroy_node()
    rclpy.shutdown(context=context)