Raise exception when user-specified reader package is not installed

When a user explicitly specifies a reader (e.g., reader="ITKReader") but the
required package is not installed, LoadImage now raises an OptionalImportError
instead of silently falling back to another reader with only a warning.

This makes the behavior explicit: if the user specifically requested a reader,
they should be informed immediately that it cannot be used, rather than having
the system silently use a different reader which may produce unexpected results.

Default behavior (reader=None) remains unchanged — missing optional packages
are still handled gracefully with debug logging.

Fixes #7437

Signed-off-by: haoyu-haoyu <haoyu-haoyu@users.noreply.github.com>
Signed-off-by: SexyERIC0723 <haoyuwang144@gmail.com>

Author: haoyu-haoyu

monai/transforms/io/array.py

@@ -209,10 +209,11 @@ 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 OptionalImportError(
+                        f"Required package for reader {_r} is not installed, or the version doesn't match requirement. "
+                        f"Please install the required package to use {_r}."
+                    ) 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 +498,45 @@ def test_correct(self, input_param, expected_shape, track_meta):
             self.assertFalse(hasattr(r, "affine"))
 
 
+class TestLoadImageReaderNotInstalled(unittest.TestCase):
+    """Test that specifying a reader whose required package is not installed raises an error.
+
+    Addresses https://github.com/Project-MONAI/MONAI/issues/7437
+    """
+
+    @unittest.skipIf(has_itk, "test requires itk to NOT be installed")
+    def test_specified_reader_not_installed_raises(self):
+        """When a user explicitly specifies a reader that is not installed, LoadImage should raise
+        an OptionalImportError instead of silently falling back to another reader."""
+        from monai.utils import OptionalImportError
+
+        with self.assertRaises(OptionalImportError):
+            LoadImage(reader="ITKReader")
+
+    def test_specified_reader_not_installed_raises_mocked(self):
+        """Mock test to verify OptionalImportError is raised (not just warned) when a user-specified
+        reader's required package is not installed."""
+        from unittest.mock import patch
+
+        from monai.utils import OptionalImportError
+
+        _original = __import__("monai.transforms.io.array", fromlist=["optional_import"]).optional_import
+
+        def _mock_optional_import(module, name="", *args, **kwargs):
+            if name == "MockMissingReader":
+                # Return a class that raises OptionalImportError on instantiation,
+                # simulating a reader whose backend package is not installed
+                class _Unavailable:
+                    def __init__(self, *a, **kw):
+                        raise OptionalImportError("mock package is not installed")
+
+                return _Unavailable, True
+            return _original(module, *args, name=name, **kwargs)
+
+        with patch("monai.transforms.io.array.optional_import", side_effect=_mock_optional_import):
+            with self.assertRaises(OptionalImportError):
+                LoadImage(reader="MockMissingReader")
+
+
 if __name__ == "__main__":
     unittest.main()