> The default encoding in the C/POSIX locale is ASCII (which is the entire source of the problem).

The reality is more complex than that :-) It depends on the OS.

Some OS uses Latin1 for the POSIX locale. Some OS announces to use
Latin1 for the POSIX locale, but use ASCII in practice :-) On these
lying OS, Python decodes bytes 0x80..0xff using mbstowcs() to check if
we get ASCII or Latin1: see the check_force_ascii() function.

/* Workaround FreeBSD and OpenIndiana locale encoding issue with the C locale.
   On these operating systems, nl_langinfo(CODESET) announces an alias of the
   ASCII encoding, whereas mbstowcs() and wcstombs() functions use the
   ISO-8859-1 encoding. The problem is that os.fsencode() and os.fsdecode() use
   locale.getpreferredencoding() codec. For example, if command line arguments
   are decoded by mbstowcs() and encoded back by os.fsencode(), we get a
   UnicodeEncodeError instead of retrieving the original byte string.

   The workaround is enabled if setlocale(LC_CTYPE, NULL) returns "C",
   nl_langinfo(CODESET) announces "ascii" (or an alias to ASCII), and at least
   one byte in range 0x80-0xff can be decoded from the locale encoding. The
   workaround is also enabled on error, for example if getting the locale
   failed.

    (...) */