fix: capture optional_import availability flag for SummaryWriter

optional_import() returns a lazy-raising stub on failure, never None, so
the previous 'SummaryWriter is None' guard could never trigger and the
RuntimeError was unreachable. Capture and check the availability flag
instead, document the exception in the docstring, and add a regression
test per CodeRabbit review.

Author: Sumit Kumar Das

monai/handlers/tensorboard_handlers.py

@@ -31,7 +31,7 @@
     Engine, _ = optional_import(
         "ignite.engine", IgniteInfo.OPT_IMPORT_VERSION, min_version, "Engine", as_type="decorator"
     )
-    SummaryWriter, _ = optional_import("torch.utils.tensorboard", name="SummaryWriter")
+    SummaryWriter, _tb_available = optional_import("torch.utils.tensorboard", name="SummaryWriter")
     SummaryWriterX, _ = optional_import("tensorboardX", name="SummaryWriter")
 
 DEFAULT_TAG = "Loss"
@@ -46,11 +46,14 @@ class TensorBoardHandler:
             default to create a new TensorBoard writer.
         log_dir: if using default SummaryWriter, write logs to this directory, default is `./runs`.
 
+    Raises:
+        RuntimeError: When ``summary_writer`` is ``None`` and the ``tensorboard`` package is not installed.
+
     """
 
     def __init__(self, summary_writer: SummaryWriter | SummaryWriterX | None = None, log_dir: str = "./runs"):
         if summary_writer is None:
-            if SummaryWriter is None:
+            if not _tb_available:
                 raise RuntimeError(
                     "TensorBoardHandler requires tensorboard to be installed. "
                     "Please install it with: pip install tensorboard"

tests/handlers/test_handler_tb_stats.py

@@ -14,12 +14,13 @@
 import glob
 import tempfile
 import unittest
-from unittest.mock import MagicMock
+from unittest.mock import MagicMock, patch
 
 from ignite.engine import Engine, Events
 from parameterized import parameterized
 
 from monai.handlers import TensorBoardStatsHandler
+from monai.handlers.tensorboard_handlers import TensorBoardHandler
 from monai.utils import optional_import
 
 SummaryWriter, has_tb = optional_import("torch.utils.tensorboard", name="SummaryWriter")
@@ -162,5 +163,13 @@ def _update_metric(engine):
                 )  # 2 = len([1, 3]) from event_filter
 
 
+class TestTensorBoardHandlerMissingDependency(unittest.TestCase):
+
+    def test_raises_when_tensorboard_unavailable(self):
+        with patch("monai.handlers.tensorboard_handlers._tb_available", False):
+            with self.assertRaises(RuntimeError):
+                TensorBoardHandler()
+
+
 if __name__ == "__main__":
     unittest.main()