fix: improve error message when no ImageWriter backend found

[vikasreddy11]

Fixes # .

Description

Improved the error message when no ImageWriter backend is found to suggest
installing pillow
Before:
No ImageWriter backend found for png.

After:
No ImageWriter backend found for png. Please install pillow (pip install pillow)

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[coderabbitai]

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Reviewing files that changed from the base of the PR and between 1ab9df1 and a2eb5dc.

📒 Files selected for processing (1)

• monai/data/image_writer.py

✅ Files skipped from review due to trivial changes (1)

• monai/data/image_writer.py

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📝 Walkthrough

Walkthrough

Update in monai/data/image_writer.py: resolve_writer now builds an extension-specific install hint and raises OptionalImportError with that hint. The error message changed from a generic "No ImageWriter backend found for {fmt}." to "No ImageWriter backend found for {fmt}. Try: {hint}." Hints map common extensions to package installs (e.g., png/jpg/jpeg → pip install pillow, nii/nii.gz → pip install nibabel, dcm → pip install itk, otherwise a default suggestion).

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main change: improving error messages for missing ImageWriter backends.
Description check	✅ Passed	The description includes required sections with clear before/after examples, though some optional checklist items are unchecked.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[coderabbitai]

Actionable comments posted: 1

🧹 Nitpick comments (1)

monai/data/image_writer.py (1)

119-119: ⚡ Quick win

Add tests for the new error messaging path.

Please add/extend unit tests for resolve_writer(..., error_if_not_found=True) to verify message content for at least one PIL format and one non-PIL format.

As per coding guidelines, "Ensure new or modified definitions will be covered by existing or new unit tests."

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@monai/data/image_writer.py` at line 119, Add unit tests that call
resolve_writer(fmt, error_if_not_found=True) and assert it raises
OptionalImportError with the expected message content: for a PIL-backed format
(e.g., "PNG") assert the exception message includes the format name and the
pillow install hint ("Please install pillow (`pip install pillow`)"), and for a
non-PIL format (pick another unsupported fmt) assert the message includes the
format name and the generic "No ImageWriter backend found for {fmt}" text; use
the OptionalImportError class from image_writer.py and the resolve_writer symbol
to locate the behavior to test.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@monai/data/image_writer.py`:
- Line 119: The current OptionalImportError raised in image_writer.py hardcodes
a Pillow install hint for all formats; update the raise in the ImageWriter
selection path (where fmt and candidate writers are known) to produce a generic
"No ImageWriter backend found for {fmt}" message and then append
format-appropriate installation hints: e.g., if fmt corresponds to medical
volumes (nifti/nii/nii.gz or any fmt present in candidate_writers that map to
nibabel/itk) append "install nibabel or itk (`pip install nibabel itk`)", if fmt
is an image extension (jpeg/png/tiff or candidates that would use PILLOW) append
"install pillow (`pip install pillow`)", otherwise only return the generic
message; implement this conditional composition where the original raise occurs
(reference variable fmt and candidate_writers/ImageWriter selection logic) so
the error guidance is accurate per format.

---

Nitpick comments:
In `@monai/data/image_writer.py`:
- Line 119: Add unit tests that call resolve_writer(fmt,
error_if_not_found=True) and assert it raises OptionalImportError with the
expected message content: for a PIL-backed format (e.g., "PNG") assert the
exception message includes the format name and the pillow install hint ("Please
install pillow (`pip install pillow`)"), and for a non-PIL format (pick another
unsupported fmt) assert the message includes the format name and the generic "No
ImageWriter backend found for {fmt}" text; use the OptionalImportError class
from image_writer.py and the resolve_writer symbol to locate the behavior to
test.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

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📥 Commits

Reviewing files that changed from the base of the PR and between 586dea1 and 1ab9df1.

📒 Files selected for processing (1)

• monai/data/image_writer.py