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> "The fact that write() -> open() relies on sys.getfilesystemencoding()
> (respectively locale.getpreferredencoding()) at default as encoding is
> either a defect or a bad design (I leave the decision to you)."
>
> Or am I overlooking something?
First, you should probably just drop mentioning write() or print() or any of the functions that actually perform I/O. The crucial decisions about decoding are made inside open().
Second, open() is implemented in C. It cannot "rely on sys.getfilesystemencoding()" as it never calls it. Internally, sys.getfilesystemencoding() simply returns a C global called Py_FileSystemDefaultEncoding. But open() doesn't examine that, either.
Instead, open() determines the default encoding by calling the same function that's used to initialize Py_FileSystemDefaultEncoding: get_locale_encoding() in Python/pythonrun.c. Which on POSIX systems calls the POSIX function nl_langinfo().
If you want to see the actual mechanisms involved, you should read the C source code in Modules/_io in the Python trunk. open() is implemented as the C function io_open() in _iomodule.c. When it opens a file in text mode without an explicit encoding, it wraps it in a TextIOWrapper object; the __init__ function for this class is the C function textiowrapper_init() in textio.c.
As for your assertion that this is "either a defect or a bad design": I leave the critique of that to others.