ROS on Ubuntu 20.04, No module named 'rclpy._rclpy_pybind11'

After installing ROS2 Galactic to Ubuntu 20.04 following the instruction here, when run the examples, I stumped at this error:

Traceback (most recent call last):
  File "/opt/ros/galactic/lib/demo_nodes_py/listener", line 11, in <module>
    load_entry_point('demo-nodes-py==0.14.3', 'console_scripts', 'listener')()
  File "/usr/lib/python3/dist-packages/pkg_resources/", line 490, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/usr/lib/python3/dist-packages/pkg_resources/", line 2854, in load_entry_point
    return ep.load()
  File "/usr/lib/python3/dist-packages/pkg_resources/", line 2445, in load
    return self.resolve()
  File "/usr/lib/python3/dist-packages/pkg_resources/", line 2451, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/opt/ros/galactic/lib/python3.8/site-packages/demo_nodes_py/topics/", line 16, in <module>
    from rclpy.node import Node
  File "/opt/ros/galactic/lib/python3.8/site-packages/rclpy/", line 43, in <module>
    from rclpy.client import Client
  File "/opt/ros/galactic/lib/python3.8/site-packages/rclpy/", line 22, in <module>
    from rclpy.impl.implementation_singleton import rclpy_implementation as _rclpy
  File "/opt/ros/galactic/lib/python3.8/site-packages/rclpy/impl/", line 32, in <module>
    rclpy_implementation = import_c_library('._rclpy_pybind11', package)
  File "/opt/ros/galactic/lib/python3.8/site-packages/rpyutils/", line 39, in import_c_library
    return importlib.import_module(name, package=package)
  File "/usr/lib/python3.9/importlib/", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
ModuleNotFoundError: No module named 'rclpy._rclpy_pybind11'
The C extension '/opt/ros/galactic/lib/python3.8/site-packages/' isn't present on the system. Please refer to '' for possible solutions

This is due to the mismatch between my default Python version (Python 3.9) and the version used to build the binary .deb (Python 3.8).

To solve this, run update the default Python version to 3.8 by:

sudo update-alternatives --config python3


There are 2 choices for the alternative python3 (providing /usr/bin/python3).

  Selection    Path                Priority   Status
* 0            /usr/bin/python3.9   2         auto mode
  1            /usr/bin/python3.8   1         manual mode
  2            /usr/bin/python3.9   2         manual mode

Press <enter> to keep the current choice[*], or type selection number: 1

Python with * is the default version. Choose Python 3.8.

After that, you can run the example in the ROS2 installation guide as follows:

If you installed ros-galactic-desktop above you can try some examples.

In one terminal, source the setup file and then run a C++ talker:

source /opt/ros/galactic/setup.bash
ros2 run demo_nodes_cpp talker

In another terminal source the setup file and then run a Python listener:

source /opt/ros/galactic/setup.bash
ros2 run demo_nodes_py listener

You should see the talker saying that it’s Publishing messages and the listener saying I heard those messages. This verifies both the C++ and Python APIs are working properly. Hooray!

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