Address review: shallow-copy lazy backend and assert laziness

- Use copy.copy instead of copy.deepcopy for the preserved lazy backend
  in Image.__deepcopy__: the backend is a read-only access adapter, so a
  shallow copy preserves the derived crop/pad view without deep-copying
  nibabel/zarr internals or duplicating open I/O resources.
- Assert the padded lazy image stays unloaded before and after deepcopy
  so the test exercises the lazy-backend code path.

Author: fepegar

src/torchio/data/image.py

@@ -1165,7 +1165,7 @@ def __deepcopy__(self, memo: dict) -> Self:
                 new._data = self._data.clone()
                 new._affine = affine_copy
             elif self._backend is not None:
-                new._backend = copy.deepcopy(self._backend, memo)
+                new._backend = copy.copy(self._backend)
         elif self._path is not None:
             new = type(self)(
                 self._path,
@@ -1183,8 +1183,10 @@ def __deepcopy__(self, memo: dict) -> Self:
                 # Preserve a derived lazy backend (e.g. the cropped/padded
                 # view installed by CropOrPad). Rebuilding from the source
                 # path alone would discard the crop/pad and revert to the
-                # full-resolution image.
-                new._backend = copy.deepcopy(self._backend, memo)
+                # full-resolution image. A shallow copy is enough: the backend
+                # is a read-only access adapter, so sharing the underlying
+                # storage object avoids deep-copying nibabel/zarr internals.
+                new._backend = copy.copy(self._backend)
         elif self._zarr_store is not None:
             new = type(self)(
                 self._zarr_store,
@@ -1198,7 +1200,7 @@ def __deepcopy__(self, memo: dict) -> Self:
                 new._data = self._data.clone()
                 new._affine = affine_copy
             elif self._backend is not None:
-                new._backend = copy.deepcopy(self._backend, memo)
+                new._backend = copy.copy(self._backend)
         elif self._data is not None:
             new = type(self)(
                 self._data.clone(),

tests/test_crop_or_pad.py

@@ -580,9 +580,11 @@ def test_deepcopy_padded_lazy_preserves_shape(self, tmp_path) -> None:
         self._make_nii(path, shape=(8, 8, 8))
         result = tio.CropOrPad(target_shape=12)(tio.Subject(t1=tio.ScalarImage(path)))
         image = result.t1
+        assert not image.is_loaded
 
         copied = copy.deepcopy(image)
         assert copied.shape == (1, 12, 12, 12)
+        assert not copied.is_loaded
         torch.testing.assert_close(copied.data, image.data)
 
     def test_transform_after_lazy_crop_uses_cropped_shape(self, tmp_path) -> None: