> There are very few places where the bytes/string difference matters for posix paths.
It's nothing to do with "places", points in your program, or entry points into the stdlib. It's entire about what path names you need to process, and for large classes of software you have zero control over that. If you have a path that doesn't encode properly with your LC_CTYPE, you're in for a bad time with Python 3. (Of course you won't if you control all your own path names, but then you also don't have a problem assuming and enforcing ASCII.)
People were still migrating home systems to Unicode-compatible encodings long after Py3 came out. I still find files in archives with paths in weird (and undeclared/undeclarable) encodings. Lots of people had such files; non-native English speakers were the most likely to have them.
> Python is far from the only popular tool to assume paths must be valid unicode.
It and Java are the only ones I use regularly. Java doesn't have a good reputation for playing well with the outside world, vs. Python which had been sold for years as "better shell scripts."
