Raise OptionalImportError for unavailable explicit image readers (#9006)

### Description

Fixes #7437.

When `LoadImage` is given an explicit reader string whose optional
dependency is unavailable, the original `OptionalImportError` is now
propagated instead of being converted into a warning and silently
falling back to another registered reader.

Automatic reader selection with `reader=None` remains unchanged.

### Implementation

- Propagate `OptionalImportError` for explicitly requested unavailable
string readers.
- Add an environment-independent regression test using mock readers.
- Update reader initialization tests to reflect the new explicit-reader
behavior when optional dependencies are unavailable.

### Compatibility

This is an intentional behavior change for explicitly requested
unavailable readers.

The following behavior remains unchanged:

- automatic reader selection with `reader=None`
- default registration skipping unavailable optional readers
- runtime fallback when an installed reader cannot read a file
- public APIs and signatures

This PR does not redesign explicit tuple/list reader semantics.

### Validation

Executed locally:

- `python -m tests.transforms.test_load_image`
- `python -m tests.transforms.test_load_imaged`
- `python -m tests.data.test_init_reader`
- Ruff

All executed tests passed.

Some optional-backend tests were skipped as expected in the current
environment.

---------

Signed-off-by: Minsu Kim <minsu.kim08@gmail.com>

Author: minsuking

monai/transforms/io/array.py

@@ -210,9 +210,7 @@ def __init__(
                 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."
-                    )
+                    raise
                 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

@@ -19,7 +19,7 @@
 
 from monai.data import ITKReader, NibabelReader, NrrdReader, NumpyReader, PILReader, PydicomReader
 from monai.transforms import LoadImage, LoadImaged
-from monai.utils import MetaKeys
+from monai.utils import MetaKeys, OptionalImportError, optional_import
 from tests.test_utils import SkipIfNoModule
 
 
@@ -30,9 +30,27 @@ def test_load_image(self):
         self.assertIsInstance(instance1, LoadImage)
         self.assertIsInstance(instance2, LoadImage)
 
-        for r in ["NibabelReader", "PILReader", "ITKReader", "NumpyReader", "NrrdReader", "PydicomReader", None]:
-            inst = LoadImaged("image", reader=r)
-            self.assertIsInstance(inst, LoadImaged)
+        optional_readers = {
+            "NibabelReader": "nibabel",
+            "PILReader": "PIL",
+            "ITKReader": "itk",
+            "NrrdReader": "nrrd",
+            "PydicomReader": "pydicom",
+        }
+        for r, module in optional_readers.items():
+            with self.subTest(reader=r):
+                _, has_module = optional_import(module, allow_namespace_pkg=module in ("itk", "nrrd"))
+                if has_module:
+                    inst = LoadImaged("image", reader=r)
+                    self.assertIsInstance(inst, LoadImaged)
+                else:
+                    with self.assertRaises(OptionalImportError):
+                        LoadImaged("image", reader=r)
+
+        inst = LoadImaged("image", reader="NumpyReader")
+        self.assertIsInstance(inst, LoadImaged)
+        inst = LoadImaged("image", reader=None)
+        self.assertIsInstance(inst, LoadImaged)
 
     @SkipIfNoModule("nibabel")
     @SkipIfNoModule("cupy")

tests/transforms/test_load_image.py

@@ -15,7 +15,9 @@
 import shutil
 import tempfile
 import unittest
+import warnings
 from pathlib import Path
+from unittest.mock import patch
 
 import nibabel as nib
 import numpy as np
@@ -24,11 +26,11 @@
 from PIL import Image
 
 from monai.apps import download_and_extract
-from monai.data import NibabelReader, PydicomReader
+from monai.data import ImageReader, NibabelReader, PydicomReader
 from monai.data.meta_obj import get_track_meta, set_track_meta
 from monai.data.meta_tensor import MetaTensor
 from monai.transforms import LoadImage
-from monai.utils import optional_import
+from monai.utils import OptionalImportError, optional_import
 from tests.test_utils import SkipIfNoModule, assert_allclose, skip_if_downloading_fails, testing_data_config
 
 itk, has_itk = optional_import("itk", allow_namespace_pkg=True)
@@ -52,6 +54,38 @@ def get_data(self, _obj):
         return np.zeros((1, 1, 1)), {"name": "my test"}
 
 
+class _MissingDependencyReader(ImageReader):
+    """a test reader that simulates a missing optional dependency"""
+
+    def __init__(self):
+        raise OptionalImportError("mock missing dependency")
+
+    def verify_suffix(self, _filename):
+        return True
+
+    def read(self, _data, **_kwargs):
+        return None
+
+    def get_data(self, _img):
+        return np.zeros((1, 1)), {}
+
+
+class _FallbackReader(ImageReader):
+    """a test reader that should not be used after an explicit reader import failure"""
+
+    read_called = False
+
+    def verify_suffix(self, _filename):
+        return True
+
+    def read(self, data, **_kwargs):
+        type(self).read_called = True
+        return data
+
+    def get_data(self, _img):
+        return np.zeros((1, 1)), {"name": "fallback"}
+
+
 TEST_CASE_1 = [{}, ["test_image.nii.gz"], (128, 128, 128)]
 
 TEST_CASE_2 = [{}, ["test_image.nii.gz"], (128, 128, 128)]
@@ -184,6 +218,25 @@ def get_data(self, _obj):
     TESTS_META.append([{"reader": "ITKReader", "fallback_only": False}, (128, 128, 128), track_meta])
 
 
+class TestLoadImageReaderSelection(unittest.TestCase):
+    def test_explicit_string_reader_missing_dependency_raises(self):
+        """test explicitly requested string readers don't fall back when their dependency is missing"""
+        _FallbackReader.read_called = False
+        readers = {"missingreader": _MissingDependencyReader, "fallbackreader": _FallbackReader}
+        with patch("monai.transforms.io.array.SUPPORTED_READERS", readers):
+            loader = LoadImage()
+            self.assertEqual(len(loader.readers), 1)
+            self.assertIsInstance(loader.readers[0], _FallbackReader)
+
+            with warnings.catch_warnings(record=True) as caught:
+                warnings.simplefilter("always")
+                with self.assertRaises(OptionalImportError):
+                    LoadImage(reader="missingreader")
+
+            self.assertEqual(len(caught), 0)
+            self.assertFalse(_FallbackReader.read_called)
+
+
 @unittest.skipUnless(has_itk, "itk not installed")
 class TestLoadImage(unittest.TestCase):
     @classmethod