By portable, do you mean not using an environment variable?

Command line parsing is potentially affected by this on Windows - I'd have to look deeper - as command lines are provided as UTF-16. But we may not ever expose them as bytes.

I don't even know that this matters on the UNIX/BSD side as the file system encoding provided there is correct, no? It's just Windows where the file system encoding used for bytes doesn't match what the file system actually uses.

I was afraid a PEP would be necessary out of this, but I want to see how the python-dev discussion goes first.