My ROS2 system is on an RPi and Ubuntu 22.04 installed with the arm64 raspy image. Ubuntu's system python version is 3.10.

I first set up update-alternatives to install Python3.9 and set that as the version displayed using python (not python3).

$ --python version
Python 3.9.18
$ python3 --version
Python 3.10.12

I installed ROS2 Humble in ~ from source per https://docs.ros.org/en/humble/Installation/Alternatives/Ubuntu-Development-Setup.html

The resulting build is in ~/ros2_humble. All packages using python were built to include Python3.10.

I also need to use opencv and cv_bridge from vision_opencv but when I build that, it also includes Python3.10.

I have an object detection node that needs to use pycoral and other libraries which require Python3.9. After struggling - Python 3.10 will not work with these packages.

I can build the node using colcon build --packages-select <my_node> and that build includes Python3.9.

After sourcing install, when I run my node it errors out including:

ModuleNotFoundError: No module named 'rclpy._rclpy_pybind11'

I tried setting up a venv using python3.9 and building the humble source in that venv - but still building humble from source uses the system python 3.10.

How can I build a node that uses a python package that depends on python3.9 in an ROS2 Humble install?

Could anyone please share a current link on the successful use of running an ROS2 node (>= Humble) in a venv with a different python version?

The troubles ...

My ROS2 system is on an RPi and Ubuntu 22.04 installed with the arm64 raspy image. Ubuntu's system python version is 3.10.

I need to create an ROS2 package for a Coral device which uses pycoral, which requires python <=3.9. Ubuntu 22.04 uses python3.10.

To test this I'm using the base Ubuntu install and only adding python3.9 through apt-get.

I installed ROS2 Humble using the Debian packages (not from source).

I installed the simple published example from https://docs.ros.org/en/humble/Tutorials/Beginner-Client-Libraries/Writing-A-Simple-Py-Publisher-And-Subscriber.html and that builds and runs fine.

I then added "import pycoral" to the publisher_member_function.py, built it, sourced the install dir and ran it and as expected the module could not be imported.

I then created a venv named venv-3.9.2

python3.9 -m venv venv-3.9.2 --system-site-packages --symlinks

Note - I also tried this without the extras:

python3.9 -m venv venv-3.9.1

Then activated that and added pycoral using python -m pip install pycoral. I ran a test.py with import pycoral and the import succeeded.

Still in the venv, I ran the talker

ros2 run py_pubsub talker

Again, the pycoral module could not be found, but the log indicates the script is looking in /usr/lib/python3.10.

I also tried adding this to setup.cfg and rebuilding:

[build_scripts]
 executable = /home/mark/venv-3.9.2/bin/python

Running this after build creates an error including:

ModuleNotFoundError: No module named 'rclpy._rclpy_pybind11'

I also tried the order of building and activating the venv and installing colcon in the venv per https://github.com/ros2/ros2/issues/1094 - still no luck.

This seems to be a common issue but I can't find a clear example on how to successfully do this.