Why do you need to call getpreferredencoding()? Isn't that already the default if you call TextIOWrapper with encoding as None? For example:

    text_mode = encoding or errors or universal_newlines

    self.stdin = io.open(p2cwrite, 'wb', bufsize)
    if text_mode:
        self.stdin = io.TextIOWrapper(self.stdin, write_through=True,
                                      line_buffering=(bufsize == 1),
                                      encoding=encoding, errors=errors)