build_depend vs build_export_depend vs exec_depend in ROS with Python?

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You are correct, that there is no build process per se with python-only packages. Note though, that you still need the CMakeLists.txt for catkin to function properly.

In the end, the build_depend and build_export_depend tags are practically irrelevant for python-only packages. You only need to specify your python-based dependencies via exec_depend.

The catkin docs state, however, to import any message packages using the depend tag, even though exec_depend would be enough. Though I'm not sure why that's the case...