colcon not adding Python module built with cmake to PYTHONPATH

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It turns out we had already solved this problem in a different project. See https://github.com/tesseract-robotics... for an example.

The solution is to use colcon hooks to update PYTHONPATH.

First, add the following to CMakeLists.txt:

file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/share/ament_index/resource_index/packages/${PROJECT_NAME} "")
  install(FILES ${CMAKE_CURRENT_BINARY_DIR}/share/ament_index/resource_index/packages/${PROJECT_NAME} DESTINATION share/ament_index/resource_index/packages)
  file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/share/${PROJECT_NAME}/hook/ament_prefix_path.dsv "prepend-non-duplicate;AMENT_PREFIX_PATH;")
  install(FILES ${CMAKE_CURRENT_BINARY_DIR}/share/${PROJECT_NAME}/hook/ament_prefix_path.dsv DESTINATION share/${PROJECT_NAME}/hook)

if (${PYTHON_VERSION_MAJOR} LESS 3)
  file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/share/${PROJECT_NAME}/hook/python_path.dsv "prepend-non-duplicate;PYTHONPATH;lib/python2.7/dist-packages")
  install(FILES ${CMAKE_CURRENT_BINARY_DIR}/share/${PROJECT_NAME}/hook/python_path.dsv DESTINATION share/${PROJECT_NAME}/hook)
else()
  file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/share/${PROJECT_NAME}/hook/python_path.dsv "prepend-non-duplicate;PYTHONPATH;lib/python3/dist-packages")
  install(FILES ${CMAKE_CURRENT_BINARY_DIR}/share/${PROJECT_NAME}/hook/python_path.dsv DESTINATION share/${PROJECT_NAME}/hook)
endif()

This assumes that PROJECT_NAMEexactly matches the ROS package name. See https://github.com/tesseract-robotics... for an example.

Then create a colcon.pkg file with the following contents, replacing <PROJECT_NAME> with the name of the package. It must match the CMakeLists PROJECT_NAME exactly.

{
    "hooks": ["share/<PROJECT_NAME>/hook/ament_prefix_path.dsv", "share/<PROJECT_NAME>/hook/python_path.dsv"]
}

Once the hook is created, the PYTHONPATH should be updated when the colcon setup scripts are sourced.