def METHOD_NAME(self, mock_read):
    headers = {_OPID_HEADER: "0", "_cid": "someid"}
    mock_read.return_value = headers
    context = FContext("someid")
    self.protocol.read_response_headers(context)
    # Ensure the opid is not set when the response headers are read in
    self.assertIsNone(context.get_response_header(_OPID_HEADER))
    self.assertEqual("someid", context.get_response_header("_cid"))