Colcon build fail, rclpy

Question

Trying to Colcon build ROS2 Rolling (https://docs.ros.org/en/rolling/Installation/Ubuntu-Development-Setup.html), I've faced this error:

colcon build --symlink-install

Starting >>> rclpy   
--- stderr: rclpy                         
CMake Error at /usr/share/cmake-3.16/Modules/FindPackageHandleStandardArgs.cmake:146 (message):
  Could NOT find Python3 (missing: Python3_LIBRARIES Python3_INCLUDE_DIRS
  Development) (found version "3.9.10")
Call Stack (most recent call first):
  /usr/share/cmake-3.16/Modules/FindPackageHandleStandardArgs.cmake:393 (_FPHSA_FAILURE_MESSAGE)
  /usr/share/cmake-3.16/Modules/FindPython/Support.cmake:2214 (find_package_handle_standard_args)
  /usr/share/cmake-3.16/Modules/FindPython3.cmake:300 (include)
  CMakeLists.txt:40 (find_package)


---
Failed   <<< rclpy [5.33s, exited with code 1]

My system Python is 3.8.10, my system Ubuntu 20.04. From the logs, it looks like 3.8 is not being found, while 3.9 is picked instead (3.9 is installed too on my system). The other 300s or so packages of ROS2 Rolling installed without issues.

Answer 1

I believe the error was due to colcon picking up the anaconda python, instead of the default OS python (3.8.10 in my case).

Sometimes rclpy fails to be imported because the expected C extension libraries are not found. If so, compare the libraries present in the directory with the one mentioned in the error message. Assuming a file with a similar name exists (same prefix like _rclpy. and same suffix like .so but a different Python version / architecture) you are using a different Python interpreter than which was used to build the C extension. Be sure to use the same Python interpreter as the one used to build the binary.

https://docs.ros.org/en/rolling/How-To-Guides/Installation-Troubleshooting.html#import-failing-without-library-present-on-the-system