[DiapDealer]

I'm still not following at all (but then I don't have to, either). Subprocess or no, calibredb won't know a thing about any generic Python temporary environment variables. Nor will calibre or calibre-debug. Calibre's Python (including plugins) is launched with specific instructions to ignore any outside Python environment variables. For good reason.

Never mind: My confusion lies with the fact that you keep implying that the environment variables need to be set for the PLUGIN to work. When in fact, the environment variables need to be set so your PROGRAM (and its Python subprocesses) interacts with a calibre plugin correctly.