In http://docs.python.org/dev/library/io.html:

  "if newline is None, any '\n' characters written are translated to the system default line separator, os.linesep. "

But os.linesep is not referred at all. On Windows default newline is always '\r\n' on Windows, '\n' otherwise.

And that is the value of os.linesep at Python startup.  I'm don't think that we really support the mutability of os.linesep, we just don't bother to make it immutable.  I'm not sure how this would be documented, since code that does use "os.linesep" is indeed affected by changing it.

What is the change to the documentation being suggested here?  The code does reference os.linesep, so it seems like the documentation is correct, or am I missing something?

        self._writetranslate = newline != ''
        self._writenl = newline or os.linesep
        self._encoder = None

http://hg.python.org/cpython/file/20a46c73855f/Lib/_pyio.py#l1501

I don't think the C version does, though.

It doesn't.
_io can be fixed to directly support os.linesep, but I doubt if anybody really need it.

I think it can be useful for testing reasons (e.g. testing that os.linesep is respected by certain code).