No, you need to be able to pass bytes to Popen, just like you do to the os.exec[xx] functions.  When the OS doesn't fully support unicode, that is sometimes the only option.  As for filenames; again, as long as the underlying systems use bytes filenames we need to support it.  Currently we encode them when received using surrogateescape and decode them back to bytes when used. 

I am not sure what os.exec[xx] does with strings containing non-ascii.  Presumably it uses some default encoding or other, which seems to be utf-8 on my system.  (It doesn't seem to be explicitly documented where those functions are discussed in the os module.)