Return an identifier for the data at index `idx`

        .. versionchanged:: 0.4.2
            indexing starts at 1 instead of 0

def get_identifier(self, idx):
        """Return an identifier for the data at index `idx`

        .. versionchanged:: 0.4.2
            indexing starts at 1 instead of 0
        """
        name = self._get_cropped_file_names()[idx]
        return "{}:{}:{}".format(self.identifier, name, idx + 1)