diff --git a/data/custom_extra_functionality.py b/data/custom_extra_functionality.py
index 98b0c026..0ac24cea 100644
--- a/data/custom_extra_functionality.py
+++ b/data/custom_extra_functionality.py
@@ -1,7 +1,9 @@
+# -*- coding: utf-8 -*-
 # SPDX-FileCopyrightText: : 2023- The PyPSA-Eur Authors
 #
 # SPDX-License-Identifier: MIT
 
+
 def custom_extra_functionality(n, snapshots):
     """
     Add custom extra functionality constraints.