fix: avoid mutable default arguments in exp_manager timer callbacks

nemo/utils/exp_manager.py

@@ -296,13 +296,15 @@ class TimingCallback(Callback):
     Logs execution time of train/val/test steps
     """
 
-    def __init__(self, log_tokens_per_sec: bool = False, timer_kwargs={}):
+    def __init__(self, log_tokens_per_sec: bool = False, timer_kwargs: Optional[dict] = None):
         """init for TimitCallback
 
         Args:
             log_tokens_per_sec (bool, optional): _description_. Defaults to False.
-            timer_kwargs (dict, optional): _description_. Defaults to {}.
+            timer_kwargs (dict, optional): _description_. Defaults to None.
         """
+        if timer_kwargs is None:
+            timer_kwargs = {}
         self.log_tokens_per_sec = log_tokens_per_sec
         self.timer = timers.NamedTimer(**timer_kwargs)
 
@@ -422,12 +424,14 @@ class DeltaTimingCallback(Callback):
     lightning logger used in the latter.
     """
 
-    def __init__(self, timer_kwargs={}):
+    def __init__(self, timer_kwargs: Optional[dict] = None):
         """init
 
         Args:
-            timer_kwargs (dict, optional): _description_. Defaults to {}.
+            timer_kwargs (dict, optional): _description_. Defaults to None.
         """
+        if timer_kwargs is None:
+            timer_kwargs = {}
         self._sync_cuda = timer_kwargs.get("sync_cuda", False)
         self.timers = defaultdict(defaultdict)
 

tests/core/test_exp_manager_timer_callbacks.py

@@ -0,0 +1,43 @@
+# Copyright (c) 2025, NVIDIA CORPORATION.  All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""
+Tests for exp_manager timer callback defaults.
+"""
+
+import inspect
+from unittest.mock import patch
+
+from nemo.utils.exp_manager import DeltaTimingCallback, TimingCallback
+
+
+class TestTimerCallbackDefaults:
+    """Tests that timer callbacks avoid mutable default arguments."""
+
+    def test_timing_callback_passes_fresh_empty_kwargs_by_default(self):
+        """Each TimingCallback should receive its own empty timer_kwargs dict."""
+        with patch('nemo.utils.exp_manager.timers.NamedTimer') as mock_timer:
+            TimingCallback()
+            TimingCallback()
+
+        kwargs1 = mock_timer.call_args_list[0].kwargs
+        kwargs2 = mock_timer.call_args_list[1].kwargs
+        assert kwargs1 == {}
+        assert kwargs2 == {}
+        assert kwargs1 is not kwargs2
+
+    def test_delta_timing_callback_default_timer_kwargs_is_none(self):
+        """DeltaTimingCallback should use None as the default timer_kwargs to avoid shared mutable state."""
+        sig = inspect.signature(DeltaTimingCallback.__init__)
+        assert sig.parameters['timer_kwargs'].default is None