This package is used to populate the PYTHONPATH with the true path of all python packages in catkin workspaces.
Many completion tools rely on PYTHONPATH to find the modules for auto-completion.
With the catkin workspace, the PYTHONPATH is set to <workspace>/devel/python<xx>/dist-packages.
This folder contains all your python packages as sub-folders.
Each folder does not have the actually source code, but only has an __init__.py inside,
which is used to locate the true path with the source code (under the src folder).
Unfortunately, most completion tools do not handle this well (example):
since no source code can be found under dist-packages, no auto-completion can be done.
This package reads .catkin under the devel folder of all the sourced workspace (extracted from CMAKE_PREFIX_PATH).
This file contains the paths of all packages inside the workspace (<workspace>/src/<package>).
Then each package path is used to construct the python source code path (<workspace>/src/<package>/src) as if it is a python package.
Finally, the constructed paths are prepended to the existing PYTHONPATH.
This is done by the environment hook provided by catkin.
Simply copy it into your workspace. After your workspace is built, the PYTHONPATH will get populated every time you source your workspace.