Fix LoadImage to raise OptionalImportError when specified reader is not available

[its-serah]

Description

This PR fixes issue #7437 by making LoadImage raise an OptionalImportError when a specified reader is not installed, instead of silently falling back to another reader.

Changes

• Modified LoadImage.__init__ to catch ValueError from look_up_option when reader name is not recognized
• Raise OptionalImportError instead of just warning when specified reader is not installed
• Added test case to verify the new behavior

Why this is needed

Previously, when a user specified LoadImage(reader='ITKReader') without ITK installed, it would just warn and use PILReader instead. This could lead to confusion and unexpected behavior. Now it properly raises an OptionalImportError to make it clear that the requested reader is not available.

Fixes #7437

[coderabbitai]

Walkthrough

The changes update error handling in the LoadImage and LoadImaged classes so that specifying a non-existent or uninstalled reader raises an OptionalImportError instead of issuing a warning or falling back to another reader. This is done by adding a raise_on_missing_reader parameter controlling this behavior and wrapping reader lookup in a try-except block. Corresponding tests verify that the exception is raised when expected and that the flag behaves correctly.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~7 minutes

Assessment against linked issues

Objective	Addressed	Explanation
Raise exception if specified reader in LoadImage is not installed (#7437)	✅

Assessment against linked issues: Out-of-scope changes

No out-of-scope changes detected.

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📥 Commits

Reviewing files that changed from the base of the PR and between a741d0b and 2af0501.

📒 Files selected for processing (3)

• monai/transforms/io/array.py (4 hunks)
• monai/transforms/io/dictionary.py (3 hunks)
• tests/transforms/test_load_image.py (2 hunks)

🚧 Files skipped from review as they are similar to previous changes (2)

• monai/transforms/io/array.py
• tests/transforms/test_load_image.py

🧰 Additional context used

📓 Path-based instructions (1)

**/*.py

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Review the Python code for quality and correctness. Ensure variable names adhere to PEP8 style guides, are sensible and informative in regards to their function, though permitting simple names for loop and comprehension variables. Ensure routine names are meaningful in regards to their function and use verbs, adjectives, and nouns in a semantically appropriate way. Docstrings should be present for all definition which describe each variable, return value, and raised exception in the appropriate section of the Google-style of docstrings. Examine code for logical error or inconsistencies, and suggest what may be changed to addressed these. Suggest any enhancements for code improving efficiency, maintainability, comprehensibility, and correctness. Ensure new or modified definitions will be covered by existing or new unit tests.

Files:

• monai/transforms/io/dictionary.py

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🔇 Additional comments (4)

monai/transforms/io/dictionary.py (4)

90-90: Parameter placement and naming are correct.

The raise_on_missing_reader parameter is appropriately positioned after existing parameters and follows Python naming conventions.

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127-128: Documentation accurately describes the new parameter.

The docstring clearly explains the parameter's purpose and default behavior for backward compatibility.

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142-142: Parameter correctly passed to underlying LoadImage.

The flag is properly forwarded to the internal _loader instance, maintaining consistency between the dictionary wrapper and array transform.

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133-145: Parameter order is correct; no action needed.
The LoadImage constructor in array.py defines its parameters in the exact same sequence (reader, image_only, dtype, ensure_channel_first, simple_keys, prune_meta_pattern, prune_meta_sep, expanduser, raise_on_missing_reader, *args, **kwargs), matching the call in dictionary.py.
All good here.

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[ericspod]

Hi @its-serah thanks for the contribution. As you see from the failed tests the expectation of the reader is to not raise an exception if an optional package isn't found, and this is correct in the cases when a fallback reader does exist for some formats. If we raise an exception whenever a reader can't be loaded then this fallback behaviour can't happen. I would suggest that we add a flag as a member to the class to enable the exception behaviour, but whose default state retains the existing behaviour. We would also need tests to check that turning this on correctly raises exceptions. What do you think? There's interest in the associated issue so I'm keen to find a solution that works for everyone. Thanks!

[its-serah]

Hi @ericspod thanks for the excellent feedback! I've implemented exactly what you suggested. I added a raise_on_missing_reader flag to both LoadImage and LoadImaged classes that defaults to False to maintain existing behavior and backward compatibility. When set to True, it raises OptionalImportError for missing readers. When False (default), it preserves the current fallback behavior with warnings. I also added comprehensive tests to verify the flag correctly raises exceptions when enabled. This gives users control while maintaining the important fallback functionality for existing codebases. The failed tests should now pass since the default behavior is unchanged. What do you think of this approach?