Fix a bug when setting complete chunks

changes/2851.bugfix.rst

@@ -0,0 +1 @@
+Fix a bug when setting values of a smaller last chunk.

src/zarr/core/codec_pipeline.py

@@ -296,17 +296,6 @@ def _merge_chunk_array(
         is_complete_chunk: bool,
         drop_axes: tuple[int, ...],
     ) -> NDBuffer:
-        if is_complete_chunk and value.shape == chunk_spec.shape:
-            return value
-        if existing_chunk_array is None:
-            chunk_array = chunk_spec.prototype.nd_buffer.create(
-                shape=chunk_spec.shape,
-                dtype=chunk_spec.dtype,
-                order=chunk_spec.order,
-                fill_value=fill_value_or_default(chunk_spec),
-            )
-        else:
-            chunk_array = existing_chunk_array.copy()  # make a writable copy
         if chunk_selection == () or is_scalar(value.as_ndarray_like(), chunk_spec.dtype):
             chunk_value = value
         else:
@@ -320,6 +309,20 @@ def _merge_chunk_array(
                     for idx in range(chunk_spec.ndim)
                 )
                 chunk_value = chunk_value[item]
+        if is_complete_chunk and chunk_value.shape == chunk_spec.shape:
+            # TODO: For the last chunk, we could have is_complete_chunk=True
+            #       that is smaller than the chunk_spec.shape but this throws
+            #       an error in the _decode_single
+            return chunk_value
+        if existing_chunk_array is None:
+            chunk_array = chunk_spec.prototype.nd_buffer.create(
+                shape=chunk_spec.shape,
+                dtype=chunk_spec.dtype,
+                order=chunk_spec.order,
+                fill_value=fill_value_or_default(chunk_spec),
+            )
+        else:
+            chunk_array = existing_chunk_array.copy()  # make a writable copy
         chunk_array[chunk_selection] = chunk_value
         return chunk_array
 

tests/test_indexing.py

@@ -815,6 +815,15 @@ def test_set_orthogonal_selection_1d(store: StorePath) -> None:
         _test_set_orthogonal_selection(v, a, z, selection)
 
 
+def test_set_orthogonal_selection_1d_last_two_chunks():
+    # regression test for GH2849
+    g = zarr.open_group("foo.zarr", zarr_format=3, mode="w")
+    a = g.create_array("bar", shape=(10,), chunks=(3,), dtype=int)
+    data = np.array([7, 8, 9])
+    a[slice(7, 10)] = data
+    np.testing.assert_array_equal(a[slice(7, 10)], data)
+
+
 def _test_set_orthogonal_selection_2d(
     v: npt.NDArray[np.int_],
     a: npt.NDArray[np.int_],