def _check_keys(self, rdict):
        """Call this from _local_update to validate response keys

            disallowed server-response json keys:
            1. The string-literal '_meta_data'
            2. strings that are not valid Python 2.7 identifiers
            3. strings beginning with '__'.

            :param rdict: from response.json()
            :raises: DeviceProvidesIncompatibleKey
            :returns: checked response rdict
            """
        if '_meta_data' in rdict:
            error_message = "Response contains key '_meta_data' which is "\
                "incompatible with this API!!\n Response json: %r" % rdict
            raise DeviceProvidesIncompatibleKey(error_message)

>       for x in rdict:
E       RuntimeError: dictionary keys changed during iteration