diff --git a/tools/libinput-analyze-per-slot-delta.py b/tools/libinput-analyze-per-slot-delta.py
index b60c1317..3363ea74 100755
--- a/tools/libinput-analyze-per-slot-delta.py
+++ b/tools/libinput-analyze-per-slot-delta.py
@@ -81,7 +81,6 @@ class SlotState:
 
 
 class Slot:
-    index = 0
     state = SlotState.NONE
     x = 0
     y = 0
@@ -90,6 +89,9 @@ class Slot:
     used = False
     dirty = False
 
+    def __init__(self, index):
+        self.index = index
+
 
 class InputEvent:
     def __init__(self, data):
@@ -130,7 +132,7 @@ def main(argv):
     if args.use_st:
         nslots = 1
 
-    slots = [Slot() for _ in range(0, nslots)]
+    slots = [Slot(i) for i in range(0, nslots)]
 
     marker_begin_slot = "   ++++++    | "  # noqa
     marker_end_slot   = "   ------    | "  # noqa