def uuids(self):
        """ Extract uuid from each item of specified ``seq``.
        """
        for f in self._seq:
            if isinstance(f, File):
                yield f.uuid
            elif isinstance(f, six.string_types):
                yield f
            else:
                raise ValueError(
                    'Invalid type for sequence item: {0}'.format(type(f)))
Extract uuid from each item of specified ``seq``.