diff --git a/rhodecode/apps/debug_style/__init__.py b/rhodecode/apps/debug_style/__init__.py
--- a/rhodecode/apps/debug_style/__init__.py
+++ b/rhodecode/apps/debug_style/__init__.py
@@ -26,7 +26,7 @@ class DebugStylePredicate(object):
         self.val = val
 
     def text(self):
-        return 'debug style route = %s' % self.val
+        return f'debug style route = {self.val}'
 
     phash = text