'Reads and returns a buffer reversely from current file-pointer position.
:rtype : str'

def _read(self):

    filepos = self._fp.tell()
    if (filepos < 1):
        return ''
    destpos = max((filepos - self._chunk_size), 0)
    self._fp.seek(destpos)
    buf = self._fp.read((filepos - destpos))
    self._fp.seek(destpos)
    return buf
