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When building my workspace with colcon build, it fails because of missing Python packages:

ModuleNotFoundError: No module named 'em'

'em' is just an example, different project give different errors. When trying to fix this, I see the packages are already installed:

> python -m pip install empy
Requirement already satisfied: empy in c:\dev\ros2_humble\.venv\lib\site-packages (3.3.4)

After a little investigation I read the following in the logs:

--- stderr: geographic_msgs CMake Error at C:/dev/ros2_humble/share/rosidl_adapter/cmake/rosidl_adapt_interfaces.cmake:59 (message): execute_process(C:/Program Files/Python310/python.exe -m rosidl_adapter

colcon is using the wrong version of Python... But I am running all the above after sourcing ROS2, from inside the ROS2 virtual environment.

How can I get colcon to stick with the right Python version?

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2 Answers 2

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It seems CMake is at fault here, even without colcon it just refuses to use the activated Python 3.8 venv, and instead falling back on the highest available Python version.

You can work around with by adding a CMake flag:

colcon build --cmake-args -DPython3_FIND_VIRTUALENV="ONLY"

Note that the "3" in "Python3_" is important here.


A permanent solution can be modifying the CMake scripts in the ROS install.

In [ros_install]/share/ament_cmake_core/cmake/core/python.cmake, modify this line:

  find_package(Python3 REQUIRED COMPONENTS Interpreter)

To look like:

  find_package(Python3 3.8 EXACT REQUIRED COMPONENTS Interpreter)

This does not enforce virtual environment specifically, but it does lock into Python 3.8 specifically.

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The issue was only fixed after I edited the file: [ros_install]/share/ament_cmake_core/cmake/core/python.cmake Thanks to Roberto My arguments were ignored when i used the command: colcon build --cmake-args -DPython3_FIND_VIRTUALENV="ONLY"

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