Address CodeRabbit review feedback: improve raise_on_missing_reader implementation

- Fix guarding for class-based readers to respect raise_on_missing_reader flag
- Add proper stacklevel and category to all warning messages for better UX
- Update LoadImage docstring with Raises section and clearer documentation
- Add missing warnings import to test file
- Apply code formatting with black, isort, and ruff

Signed-off-by: Sarah <serah.elrashidi@gmail.com>

Author: its-serah

monai/transforms/io/array.py

@@ -43,6 +43,7 @@
 from monai.data.utils import is_no_channel
 from monai.transforms.transform import Transform
 from monai.transforms.utility.array import EnsureChannelFirst
+from monai.utils import ImageMetaKey as Key
 from monai.utils import (
     GridSamplePadMode,
     ImageMetaKey,
@@ -162,11 +163,21 @@ def __init__(
                 in the metadata (nested dictionary). default is ".", see also :py:class:`monai.transforms.DeleteItemsd`.
                 e.g. ``prune_meta_pattern=".*_code$", prune_meta_sep=" "`` removes meta keys that ends with ``"_code"``.
             expanduser: if True cast filename to Path and call .expanduser on it, otherwise keep filename as is.
-            raise_on_missing_reader: if True, raise OptionalImportError when a specified reader is not available,
-                otherwise attempt to use fallback readers. Default is False to maintain backward compatibility.
+            raise_on_missing_reader: if True, raise `OptionalImportError` when a specified reader is not available;
+                otherwise attempt to use fallback readers. Defaults to False (backward compatibility).
             args: additional parameters for reader if providing a reader name.
             kwargs: additional parameters for reader if providing a reader name.
 
+        Raises:
+            OptionalImportError: If `raise_on_missing_reader=True` and the specified reader
+                cannot be found or its optional dependency is not installed.
+
+        Accepted reader types:
+            - str: name of a registered reader (e.g., `"ITKReader"`)
+            - class: e.g., `ITKReader` or a custom reader class
+            - instance: e.g., `ITKReader(pixel_type=itk.UC)`
+            - list/tuple: multiple reader names or classes to try in order
+
         Note:
 
             - The transform returns a MetaTensor, unless `set_track_meta(False)` has been used, in which case, a
@@ -218,21 +229,53 @@ def __init__(
                         if self.raise_on_missing_reader:
                             raise OptionalImportError(msg)
                         else:
-                            warnings.warn(f"{msg} Will use fallback readers if available.")
+                            warnings.warn(
+                                f"{msg} Will use fallback readers if available.",
+                                category=UserWarning,
+                                stacklevel=2,
+                            )
                             continue
                 try:
                     self.register(the_reader(*args, **kwargs))
                 except OptionalImportError as e:
-                    msg = f"Required package for reader {_r} is not installed, or the version doesn't match requirement."
+                    msg = (
+                        f"Required package for reader {_r} is not installed, or the version doesn't match requirement."
+                    )
                     if self.raise_on_missing_reader:
                         raise OptionalImportError(msg) from e
                     else:
-                        warnings.warn(f"{msg} Will use fallback readers if available.")
+                        warnings.warn(
+                            f"{msg} Will use fallback readers if available.",
+                            category=UserWarning,
+                            stacklevel=2,
+                        )
                 except TypeError:  # the reader doesn't have the corresponding args/kwargs
-                    warnings.warn(f"{_r} is not supported with the given parameters {args} {kwargs}.")
+                    warnings.warn(
+                        f"{_r} is not supported with the given parameters {args} {kwargs}.",
+                        category=UserWarning,
+                        stacklevel=2,
+                    )
                     self.register(the_reader())
             elif inspect.isclass(_r):
-                self.register(_r(*args, **kwargs))
+                try:
+                    self.register(_r(*args, **kwargs))
+                except OptionalImportError as e:
+                    msg = f"Required package for reader {_r.__name__} is not installed, or the version doesn't match requirement."
+                    if self.raise_on_missing_reader:
+                        raise OptionalImportError(msg) from e
+                    else:
+                        warnings.warn(
+                            f"{msg} Will use fallback readers if available.",
+                            category=UserWarning,
+                            stacklevel=2,
+                        )
+                except TypeError:
+                    warnings.warn(
+                        f"{_r.__name__} is not supported with the given parameters {args} {kwargs}.",
+                        category=UserWarning,
+                        stacklevel=2,
+                    )
+                    self.register(_r())
             else:
                 self.register(_r)  # reader instance, ignoring the constructor args/kwargs
         return

tests/transforms/test_load_image.py

@@ -15,6 +15,7 @@
 import shutil
 import tempfile
 import unittest
+import warnings
 from pathlib import Path
 
 import nibabel as nib