def head_object(self, bucket, object_name):
        """
        Retrieve object metadata only.
        """
        details = self._details(
            method=b"HEAD",
            url_context=self._url_context(bucket=bucket, object_name=object_name),
        )
        d = self._submit(self._query_factory(details))
        d.addCallback(lambda (response, body): _to_dict(response.responseHeaders))
        return d