Raise exception when user-specified reader package is not installed

monai/transforms/io/array.py

@@ -70,6 +70,16 @@
     "nibabelreader": NibabelReader,
 }
 
+# Maps reader names (lower-cased) to pip install commands so error messages
+# can tell users exactly how to fix a missing-package error.
+_READER_INSTALL_HINTS: dict[str, str] = {
+    "nibabelreader": "pip install nibabel",
+    "itkreader": "pip install itk",
+    "pilreader": "pip install Pillow",
+    "pydicomreader": "pip install pydicom",
+    "nrrdreader": "pip install pynrrd",
+}
+
 
 def switch_endianness(data, new="<"):
     """
@@ -209,10 +219,10 @@ 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:
+                    hint = _READER_INSTALL_HINTS.get(_r.lower(), "")
+                    install_msg = f" Install with: {hint}" if hint else ""
+                    raise OptionalImportError(f"{e}{install_msg}") 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/data/test_init_reader.py

@@ -15,6 +15,7 @@
 
 from monai.data import ITKReader, NibabelReader, NrrdReader, NumpyReader, PILReader, PydicomReader
 from monai.transforms import LoadImage, LoadImaged
+from monai.utils import OptionalImportError
 from tests.test_utils import SkipIfNoModule
 
 
@@ -26,8 +27,13 @@ def test_load_image(self):
         self.assertIsInstance(instance2, LoadImage)
 
         for r in ["NibabelReader", "PILReader", "ITKReader", "NumpyReader", "NrrdReader", "PydicomReader", None]:
-            inst = LoadImaged("image", reader=r)
-            self.assertIsInstance(inst, LoadImaged)
+            try:
+                inst = LoadImaged("image", reader=r)
+                self.assertIsInstance(inst, LoadImaged)
+            except OptionalImportError:
+                # Reader's backend package is not installed — expected in
+                # minimal-dependency environments after the fix for #7437.
+                pass
 
     @SkipIfNoModule("nibabel")
     @SkipIfNoModule("cupy")

tests/transforms/test_load_image.py

@@ -498,5 +498,47 @@ 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 with an install hint instead of silently falling back."""
+        from monai.utils import OptionalImportError
+
+        with self.assertRaises(OptionalImportError) as ctx:
+            LoadImage(reader="ITKReader")
+        self.assertIn("pip install itk", str(ctx.exception))
+
+    def test_specified_reader_not_installed_raises_mocked(self):
+        """Mock test to verify OptionalImportError is raised with original message preserved
+        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":
+
+                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) as ctx:
+                LoadImage(reader="MockMissingReader")
+            # Original error message should be preserved in the re-raised exception
+            self.assertIn("mock package is not installed", str(ctx.exception))
+
+
 if __name__ == "__main__":
     unittest.main()