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.

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📝 Walkthrough

Walkthrough

OptionalImportError now records the missing package name when raised by require_pkg. Image writer resolution collects unavailable backend dependencies, maps module names to PyPI packages, and includes single or multiple pip install commands in error messages. Tests cover package-name propagation, single and multiple dependency hints, and fallback behavior 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: improved missing image writer dependency hints.
Description check	✅ Passed	The description matches the template and includes the issue, summary, change list, and test status.
Linked Issues check	✅ Passed	The PR satisfies #7980 by surfacing installable package hints, including Pillow for PNG writer failures.
Out of Scope Changes check	✅ Passed	The code and tests stay focused on error-message improvements and dependency hinting, with no clear unrelated changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[NiketGirdhar22]

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🧹 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.

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📥 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