On Mac OS X and Linux, Houdini uses the system’s Python. On Windows, it uses the version of Python installed with Houdini. On Linux, Houdini does not look at $PATH when trying to search and load the Python library. Instead it searches all the paths in $LD_LIBRARY_PATH, then /lib and then /usr/lib. If it can’t find a suitable Python library in the system, then it defaults to the one in $HFS. If your Python library is not in a standard location, you should add its directory path to $LD_LIBRARY_PATH.