Return the chunkattr of obj formatted with the chunkfomat specified in the constructor
        If the attr is None then ``None`` is returned not the string ``\'None\'``.

        :param obj: the fileinfo with information.
        :type obj: :class:`FileInfo`
        :returns: the chunk or None
        :rtype: str|None
        :raises: None

def get_chunk(self, obj):
        """Return the chunkattr of obj formatted with the chunkfomat specified in the constructor
        If the attr is None then ``None`` is returned not the string ``\'None\'``.

        :param obj: the fileinfo with information.
        :type obj: :class:`FileInfo`
        :returns: the chunk or None
        :rtype: str|None
        :raises: None
        """
        if self._chunkattr is None:
            return
        a = attrgetter(self._chunkattr)(obj)
        if a is None:
            return
        s = self._chunkformat % a
        return s