From f06db796abc8fe00eeb11fec3251a6b544f5412f Mon Sep 17 00:00:00 2001
From: nightcityblade <nightcityblade@gmail.com>
Date: Mon, 22 Jun 2026 11:32:57 +0800
Subject: [PATCH] fix: prevent once handlers from refiring on epoch termination

Fixes pytorch/ignite#3625
---
 ignite/engine/engine.py            | 3 +++
 ignite/engine/events.py            | 5 ++++-
 tests/ignite/engine/test_engine.py | 1 +
 3 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/ignite/engine/engine.py b/ignite/engine/engine.py
index 98bea1563c1b..ba8800a8bc16 100644
--- a/ignite/engine/engine.py
+++ b/ignite/engine/engine.py
@@ -325,6 +325,9 @@ def execute_something():
             return RemovableEventHandle(event_name, handler, self)
         if isinstance(event_name, CallableEventWithFilter) and event_name.filter is not None:
             event_filter = event_name.filter
+            once = getattr(event_filter, "_once", None)
+            if once is not None:
+                event_filter = Events.once_event_filter(list(once))
             handler = self._handler_wrapper(handler, event_name, event_filter)
 
         self._assert_allowed_event(event_name)
diff --git a/ignite/engine/events.py b/ignite/engine/events.py
index 0a370e9d8815..ffd2354420af 100644
--- a/ignite/engine/events.py
+++ b/ignite/engine/events.py
@@ -140,12 +140,15 @@ def wrapper(engine: "Engine", event: int) -> bool:
     @staticmethod
     def once_event_filter(once: list) -> Callable:
         """A wrapper for once event filter."""
+        remaining = set(once)
 
         def wrapper(engine: "Engine", event: int) -> bool:
-            if event in once:
+            if event in remaining:
+                remaining.remove(event)
                 return True
             return False
 
+        wrapper._once = tuple(once)  # type: ignore[attr-defined]
         return wrapper
 
     @staticmethod
diff --git a/tests/ignite/engine/test_engine.py b/tests/ignite/engine/test_engine.py
index 508d29570939..bcb0cdc0a92c 100644
--- a/tests/ignite/engine/test_engine.py
+++ b/tests/ignite/engine/test_engine.py
@@ -395,6 +395,7 @@ def check_previous_events2():
 
             epoch_completed_events = [e for e in engine.called_events if e[2] == Events.EPOCH_COMPLETED.name]
             assert len(epoch_completed_events) == max_epochs - skip_epoch_completed
+            assert call_count == 1
 
     @pytest.mark.parametrize("data", [None, "mock_data_loader"])
     def test_iteration_events_are_fired(self, data):