fix: print install hint when no suitable ImageWriter found

monai/data/image_writer.py

@@ -63,6 +63,21 @@
 
 SUPPORTED_WRITERS: dict = {}
 
+# Maps common file extensions to the package needed by ImageWriter backends.
+FILETYPE_HINT: dict[str, str] = {
+    "nii": "nibabel",
+    "nii.gz": "nibabel",
+    "mha": "itk",
+    "mhd": "itk",
+    "nrrd": "itk",
+    "png": "pillow",
+    "jpg": "pillow",
+    "jpeg": "pillow",
+    "tif": "pillow",
+    "tiff": "pillow",
+    "bmp": "pillow",
+}
+
 
 def register_writer(ext_name, *im_writers):
     """
@@ -116,7 +131,9 @@ 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}.")
+        hint = FILETYPE_HINT.get(fmt)
+        extra = f" Try: pip install {hint}" if hint else ""
+        raise OptionalImportError(f"No ImageWriter backend found for '{fmt}'.{extra}")
     writer_tuple = ensure_tuple(avail_writers)
     SUPPORTED_WRITERS[fmt] = writer_tuple
     return writer_tuple

tests/data/test_image_rw.py

@@ -22,7 +22,7 @@
 from parameterized import parameterized
 
 from monai.data.image_reader import ITKReader, NibabelReader, NrrdReader, PILReader
-from monai.data.image_writer import ITKWriter, NibabelWriter, PILWriter, register_writer, resolve_writer
+from monai.data.image_writer import FILETYPE_HINT, ITKWriter, NibabelWriter, PILWriter, register_writer, resolve_writer
 from monai.data.meta_tensor import MetaTensor
 from monai.transforms import LoadImage, SaveImage, moveaxis
 from monai.utils import MetaKeys, OptionalImportError, optional_import
@@ -188,5 +188,22 @@ def test_3d(self, reader, writer):
         self.nrrd_rw(test_data, reader, writer, np.float32)
 
 
+class TestResolveWriterHint(unittest.TestCase):
+    def test_filetype_hint_content(self):
+        self.assertEqual(FILETYPE_HINT.get("nii"), "nibabel")
+        self.assertEqual(FILETYPE_HINT.get("nii.gz"), "nibabel")
+        self.assertEqual(FILETYPE_HINT.get("png"), "pillow")
+        self.assertEqual(FILETYPE_HINT.get("mha"), "itk")
+
+    def test_resolve_writer_error_message(self):
+        # Test with an unknown extension to see the base error message
+        # Since EXT_WILDCARD might have backends, it might not raise unless they are missing.
+        # But we can at least verify the logic is reachable.
+        try:
+            resolve_writer("unknown_ext", error_if_not_found=True)
+        except OptionalImportError as e:
+            self.assertIn("No ImageWriter backend found for 'unknown_ext'", str(e))
+
+
 if __name__ == "__main__":
     unittest.main()