def __read(self, size):
        """Return size bytes from stream. If internal buffer is empty,
           read another block from the stream.
        """
        c = len(self.buf)
        while c < size:
            buf = self.fileobj.read(self.bufsize)
            if not buf:
                break
            self.buf += buf
            c += len(buf)
        buf = self.buf[:size]
        self.buf = self.buf[size:]
        return buf
Return size bytes from stream. If internal buffer is empty,
           read another block from the stream.