Second question:

The following looks as if a BOM might be written for writeable, non-seekable streams:

        # don't write a BOM in the middle of a file
        if self._seekable and self.writable():
            position = self.buffer.tell()
            if position != 0:
                try:
                    self._get_encoder().setstate(0)
                except LookupError:
                    # Sometimes the encoder doesn't exist
                    pass

Is that really desirable? It seems to me the safe choice is to *not* write the BOM, except when we know it's safe. Eg:

        # don't write a BOM unless we know we're at the beginning of a file
        if (self.writeable() and not
           (self._seekable and self.buffer.tell() == 0)):
            try:
                self._get_encoder().setstate(0)
            except LookupError:
                # Sometimes the encoder doesn't exist
                pass