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fix: print install hint when no suitable ImageWriter found #8761

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fix: print install hint when no suitable ImageWriter found #8761

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b809c77
fix: print install hint when no suitable ImageWriter found
yashnaiduu Mar 1, 2026
6a5238f
clean up unnecessary comments
yashnaiduu Mar 1, 2026
e35f6fd
Refactor filetype hints for ImageWriter backendsrefactor: move instal…
yashnaiduu Mar 7, 2026
c2274ff
Add unit tests for image writer functionalitytest: add unit tests for…
yashnaiduu Mar 7, 2026
5fc5be5
style: fix test_image_rw.py imports and ordering
Mar 7, 2026
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18 changes: 17 additions & 1 deletion monai/data/image_writer.py
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Original file line number Diff line number Diff line change
Expand Up @@ -116,7 +116,23 @@ def resolve_writer(ext_name, error_if_not_found=True) -> Sequence:
except Exception: # other writer init errors indicating it exists
avail_writers.append(_writer)
if not avail_writers and error_if_not_found:
raise OptionalImportError(f"No ImageWriter backend found for {fmt}.")
# map common extensions to their required package
install_hints: dict = {
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"nii": "nibabel",
"nii.gz": "nibabel",
"mha": "itk",
"mhd": "itk",
"nrrd": "itk",
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@dzenanz dzenanz Mar 9, 2026

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This list is not exhaustive. A longer list is available here. Also, NRRD format can have .nhdr extension.

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@ericspod ericspod Mar 29, 2026

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Hi @yashnaiduu please have a look into adding more to the dictionary.

"png": "pillow",
"jpg": "pillow",
"jpeg": "pillow",
"tif": "pillow",
"tiff": "pillow",
"bmp": "pillow",
}
hint = install_hints.get(fmt)
extra = f" Try installing the required package: pip install {hint}" if hint else ""
raise OptionalImportError(f"No ImageWriter backend found for '{fmt}'.{extra}")
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@coderabbitai coderabbitai Bot Mar 1, 2026

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⚠️ Potential issue | 🟠 Major

Add regression tests for the new install-hint behavior.

Line 119 through Line 134 changes user-facing error output and format mapping, but no tests are included here. Please add cases for nii.gz -> nibabel, png -> pillow, nrrd -> itk, and unknown extensions (no hint).

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

🧰 Tools
🪛 Ruff (0.15.2)

[warning] 134-134: Avoid specifying long messages outside the exception class

(TRY003)

🤖 Prompt for AI Agents 
Verify each finding against the current code and only fix it if needed.

In `@monai/data/image_writer.py` around lines 119 - 134, Add unit tests for the
new install-hint behavior in monai.data.image_writer: exercise the mapping
stored in install_hints by calling the ImageWriter error path (the code that
raises OptionalImportError) with formats "nii.gz", "png", "nrrd" and an unknown
extension (e.g., "abc"); for each of the first three assert the raised
OptionalImportError message contains the expected package string ("nibabel",
"pillow", "itk" respectively) and for the unknown extension assert the message
does not contain a "pip install" hint; ensure tests reference the image_writer
module and OptionalImportError so they fail if the mapping or message changes.

writer_tuple = ensure_tuple(avail_writers)
SUPPORTED_WRITERS[fmt] = writer_tuple
return writer_tuple
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