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

Previously, when a user explicitly specified a reader (e.g. LoadImage(reader='ITKReader'))
but the required optional package was not installed, MONAI would silently warn and fall
back to the next available reader. This silent fallback is surprising and hard to debug.

This change raises a RuntimeError instead, giving the user a clear actionable message
that explains what happened and how to fix it (install the package or omit the reader
argument to use automatic fallback).

Backward compatibility is preserved: if no reader is specified, the existing warn-and-
skip behavior for missing optional packages is unchanged.

Fixes #7437

Signed-off-by: Alan Bishop <alanbishop@users.noreply.github.com>

Author: alanbishop

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()