Improve missing image writer dependency hints

[NiketGirdhar22]

Fixes #7980.

Description

Improve the error raised when resolve_writer cannot find an available image writer by including actionable installation commands for the actual candidate backends.

Rather than maintaining a separate extension-to-package table, this change records the missing dependency name on OptionalImportError in require_pkg. resolve_writer collects that information from its registered candidate writers and formats one or more installation hints. This keeps the guidance aligned with SUPPORTED_WRITERS, including wildcard formats and custom registrations that use require_pkg.

For example, saving PNG without Pillow now reports:

No ImageWriter backend found for png. Install `pillow` with `pip install pillow`.

This is a non-breaking, error-message-only enhancement.

I am aware of the existing proposals for #7980, particularly #8761, #8625, and #8240. This draft uses the package metadata approach previously suggested in the review of #8240 and is opened for maintainer guidance on whether to proceed independently or coordinate with an existing PR.

Changes

• Preserve the missing package name through OptionalImportError.name.
• Generate installation guidance from unavailable registered writers.
• Translate the PIL import name to the installable pillow distribution.
• Preserve the generic error when dependency metadata is unavailable.
• Add tests for single, multiple, and unknown dependency cases.
• Document the enhanced OptionalImportError behavior.

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.
• Focused unit tests passed locally.
• Black, isort, Ruff, compilation, and whitespace checks passed.
• In-line docstrings updated.
• Documentation pages updated.

Test results

• tests.utils.test_require_pkg: 7 passed
• tests.utils.test_optional_import: 12 passed
• New writer dependency-hint tests: 3 passed

The complete tests.data.test_image_rw module requires optional image backends not installed in the minimal local environment. Its pre-existing NRRD assertion requires itk or nibabel; the new tests do not require those dependencies and pass independently.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📥 Commits

Reviewing files that changed from the base of the PR and between 94f317a and efb6588.

📒 Files selected for processing (1)

• monai/data/image_writer.py

🚧 Files skipped from review as they are similar to previous changes (1)

• monai/data/image_writer.py

---

📝 Walkthrough

Walkthrough

OptionalImportError now records missing package names from require_pkg. Image writer resolution aggregates unavailable backend dependencies, maps module names to PyPI packages, and includes single or multiple pip install commands in error messages. Tests cover these changes and the fallback when no package name is available.

Estimated code review effort: 3 (Moderate) | ~20 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: improving missing image-writer dependency hints.
Description check	✅ Passed	The description matches the template and includes scope, change list, test status, and type checkboxes.
Linked Issues check	✅ Passed	The changes satisfy #7980 by reporting installable package hints when no suitable image writer is available.
Out of Scope Changes check	✅ Passed	The added mapping, error handling, and tests all support the stated writer-hint improvement.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

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[NiketGirdhar22]

@coderabbitai review

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[coderabbitai]

🧹 Nitpick comments (1)

monai/data/image_writer.py (1)

95-140: 📐 Maintainability & Code Quality | 🔵 Trivial | 💤 Low value

Add Returns section to resolve_writer docstring.

The docstring now has Args and Raises but is missing a Returns section documenting the return type and value. As per coding guidelines, Google-style docstrings should describe each return value.

📝 Proposed docstring addition

         error_if_not_found: whether to raise an error if no suitable image writer is found.
             if True , raise an ``OptionalImportError``, otherwise return an empty tuple. Default is ``True``.

+    Returns:
+        A tuple of available ``ImageWriter`` classes matching the given extension.
+
     Raises:
         OptionalImportError: When no registered writer is available. If candidate writers require missing optional
             dependencies, the error includes commands for installing them.

🤖 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` around lines 95 - 140, The resolve_writer
docstring is missing documentation for its return value. Add a Google-style
Returns section to resolve_writer describing that it returns a tuple of
available ImageWriter classes, or an empty tuple when error_if_not_found is
false and no writer is available.

Source: Path instructions

🤖 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.

Nitpick comments:
In `@monai/data/image_writer.py`:
- Around line 95-140: The resolve_writer docstring is missing documentation for
its return value. Add a Google-style Returns section to resolve_writer
describing that it returns a tuple of available ImageWriter classes, or an empty
tuple when error_if_not_found is false and no writer is available.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 28492799-bf75-4726-b58a-c3e68c762102

📥 Commits

Reviewing files that changed from the base of the PR and between fd12fdc and 94f317a.

📒 Files selected for processing (4)

• monai/data/image_writer.py
• monai/utils/module.py
• tests/data/test_image_rw.py
• tests/utils/test_require_pkg.py

[NiketGirdhar22]

Addressed the CodeRabbit Returns-section nitpick in commit efb6588.

[ericspod]

Thanks @NiketGirdhar22 for this version of the solution, it aligns with my original idea and looks good so far. If there's further work to be done please do so and we can review again.