fix: raise exception when LoadImage reader is specified but not installed

monai/transforms/io/array.py

@@ -209,10 +209,12 @@ def __init__(
                     the_reader = look_up_option(_r.lower(), SUPPORTED_READERS)
                 try:
                     self.register(the_reader(*args, **kwargs))
-                except OptionalImportError:
-                    warnings.warn(
-                        f"required package for reader {_r} is not installed, or the version doesn't match requirement."
-                    )
+                except OptionalImportError as e:
+                    raise RuntimeError(
+                        f"The required package for reader '{_r}' is not installed, or the version doesn't match "
+                        f"the requirement. If you want to use '{_r}', please install the required package. "
+                        f"If you want to use an alternative reader, do not specify the `reader` argument."
+                    ) 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

@@ -498,5 +498,36 @@ def test_correct(self, input_param, expected_shape, track_meta):
             self.assertFalse(hasattr(r, "affine"))
 
 
+
+
+class TestLoadImageMissingReader(unittest.TestCase):
+    """Test that LoadImage raises RuntimeError when a user-specified reader is not installed."""
+
+    def test_explicit_reader_not_installed_raises_runtime_error(self):
+        """When the user explicitly names a reader whose package is missing, a RuntimeError must be raised."""
+        from unittest.mock import patch
+        from monai.utils import OptionalImportError
+
+        # Patch the reader class so that instantiation raises OptionalImportError,
+        # simulating a missing optional dependency (e.g. itk not installed).
+        with patch("monai.data.ITKReader.__init__", side_effect=OptionalImportError("itk")):
+            with self.assertRaises(RuntimeError) as ctx:
+                LoadImage(reader="ITKReader")
+        self.assertIn("ITKReader", str(ctx.exception))
+        self.assertIn("not installed", str(ctx.exception))
+
+    def test_unspecified_reader_falls_back_silently(self):
+        """When no reader is specified, missing optional readers should be silently skipped (no exception)."""
+        # This should not raise even if some optional readers are unavailable.
+        loader = LoadImage()
+        self.assertIsInstance(loader, LoadImage)
+
+    def test_explicit_reader_available_succeeds(self):
+        """When the user explicitly names a reader whose package IS installed, no exception is raised."""
+        # NibabelReader is always available (nibabel is a core dep)
+        loader = LoadImage(reader="NibabelReader")
+        self.assertIsInstance(loader, LoadImage)
+
+
 if __name__ == "__main__":
     unittest.main()