Return a unique identifier for the folder data

def _identifier_data(self):
        """Return a unique identifier for the folder data"""
        # Use only file names
        data = [ff.name for ff in self.files]
        data.sort()
        # also use the folder name
        data.append(self.path.name)
        # add meta data
        data += self._identifier_meta()
        return hash_obj(data)