Fix LoadImage to raise OptionalImportError when specified reader is not available

- Modified LoadImage.__init__ to catch ValueError from look_up_option when reader name is not recognized
- Raise OptionalImportError instead of just warning when specified reader is not installed
- Added test case to verify the new behavior

This addresses issue Project-MONAI#7437 where LoadImage would silently fall back to another reader
when the specified reader (e.g., ITKReader) was not installed. Now it properly raises
an OptionalImportError to make it clear that the requested reader is not available.

Fixes: Project-MONAI#7437

Author: its-serah

monai/transforms/io/array.py

@@ -206,13 +206,19 @@ def __init__(
                 if not has_built_in:
                     the_reader = locate(f"{_r}")  # search dotted path
                 if the_reader is None:
-                    the_reader = look_up_option(_r.lower(), SUPPORTED_READERS)
+                    try:
+                        the_reader = look_up_option(_r.lower(), SUPPORTED_READERS)
+                    except ValueError:
+                        # If the reader name is not recognized at all, raise OptionalImportError
+                        raise OptionalImportError(
+                            f"Cannot find reader '{_r}'. It may not be installed or recognized."
+                        )
                 try:
                     self.register(the_reader(*args, **kwargs))
-                except OptionalImportError:
-                    warnings.warn(
+                except OptionalImportError as e:
+                    raise OptionalImportError(
                         f"required package for reader {_r} is not installed, or the version doesn't match requirement."
-                    )
+                    ) from e
                 except TypeError:  # the reader doesn't have the corresponding args/kwargs
                     warnings.warn(f"{_r} is not supported with the given parameters {args} {kwargs}.")
                     self.register(the_reader())

tests/transforms/test_load_image.py

@@ -28,7 +28,7 @@
 from monai.data.meta_obj import set_track_meta
 from monai.data.meta_tensor import MetaTensor
 from monai.transforms import LoadImage
-from monai.utils import optional_import
+from monai.utils import OptionalImportError, optional_import
 from tests.test_utils import SkipIfNoModule, assert_allclose, skip_if_downloading_fails, testing_data_config
 
 itk, has_itk = optional_import("itk", allow_namespace_pkg=True)
@@ -436,6 +436,11 @@ def test_my_reader(self):
         self.assertEqual(out.meta["name"], "my test")
         out = LoadImage(image_only=True, reader=_MiniReader, is_compatible=False)("test")
         self.assertEqual(out.meta["name"], "my test")
+
+    def test_reader_not_installed_exception(self):
+        """test if an exception is raised when a specified reader is not installed"""
+        with self.assertRaises(OptionalImportError):
+            LoadImage(image_only=True, reader="NonExistentReader")("test")
         for item in (_MiniReader, _MiniReader(is_compatible=False)):
             out = LoadImage(image_only=True, reader=item)("test")
             self.assertEqual(out.meta["name"], "my test")