What is $HOSTPYTHON? It does not appear to be used or mentioned anywhere. You mention it as if it is commonly known, but if you are proposing a new configuration variable, perhaps you should document it.

The Gnu version of “which” seems to work fine with relative paths, though I know it is not a standardized command. Can you explain what your problem with relative paths is? Maybe we can find a cleaner way to check if a command exists, e.g. use “command -v”.

$ which ./python && echo Success
/media/disk/home/proj/python/cpython/python
Success

Can you explain the problem with PYTHONPATH and the —S addition? If we really need it, a comment would be good. What is the difference between your $HOSTPYTHON case and the original python3 etc cases, where it is not needed?

Also, did you test your patch? Your —S argument uses a UTF-8 em dash, which would be interpreted as a script file name, rather than an ASCII dash for the -S option.