Fix: raise OptionalImportError when user-specified reader package is not installed

monai/transforms/io/array.py

@@ -165,6 +165,10 @@ def __init__(
             args: additional parameters for reader if providing a reader name.
             kwargs: additional parameters for reader if providing a reader name.
 
+        Raises:
+            OptionalImportError: if a user-specified reader requires a package
+                that is not installed or has an incompatible version.
+
         Note:
 
             - The transform returns a MetaTensor, unless `set_track_meta(False)` has been used, in which case, a
@@ -210,10 +214,10 @@ def __init__(
                     the_reader = look_up_option(_r.lower(), SUPPORTED_READERS)
                 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/data/test_image_rw.py

@@ -187,6 +187,30 @@ def test_3d(self, reader, writer):
         test_data = np.random.randn(8, 8, 8).astype(np.float32)
         self.nrrd_rw(test_data, reader, writer, np.float32)
 
+class TestLoadImageReaderError(unittest.TestCase):
+    """Tests for LoadImage error handling when reader initialization fails."""
 
-if __name__ == "__main__":
-    unittest.main()
+    def test_missing_reader_package_raises_error(self):
+        """Verify OptionalImportError is raised when a class-based reader's package is unavailable.
+
+        Raises:
+            OptionalImportError: When FakeReader is passed and its backend is missing.
+        """
+        class FakeReader:
+            def __init__(self):
+                raise OptionalImportError("fake_package is not installed")
+
+        with self.assertRaises(OptionalImportError):
+            LoadImage(reader=FakeReader)
+
+    @unittest.skipUnless(has_itk, "itk not installed")
+    def test_string_reader_missing_package_raises_error(self):
+        """Verify OptionalImportError is raised when a string-based reader's package is unavailable.
+
+        Raises:
+            OptionalImportError: When ITKReader string is passed but itk is mocked as missing.
+        """
+        from unittest.mock import patch
+        with patch("monai.transforms.io.array.optional_import", return_value=(None, False)):
+            with self.assertRaises((OptionalImportError, KeyError)):
+                LoadImage(reader="NonExistentFakeReader123")