Fix the detection of the last chunk

xarray/backends/zarr.py

@@ -229,15 +229,14 @@ def _determine_zarr_chunks(
                     raise ValueError(base_error)
 
             if not allow_partial_chunks:
-                chunk_start = sum(dchunks[:-1]) + region_start
-                if chunk_start % zchunk:
+                region_stop = interval.stop if interval.stop else size
+
+                if region_start % zchunk:
                     # The last chunk which can also be the only one is a partial chunk
                     # if it is not aligned at the beginning
                     raise ValueError(base_error)
 
-                region_stop = interval.stop if interval.stop else size
-
-                if size - region_stop + 1 < zchunk:
+                if np.ceil(region_stop / zchunk) == np.ceil(size / zchunk):
                     # If the region is covering the last chunk then check
                     # if the reminder with the default chunk size
                     # is equal to the size of the last chunk

xarray/tests/test_backends.py

@@ -6256,3 +6256,13 @@ def test_zarr_safe_chunk_region(tmp_path):
 
     with pytest.raises(ValueError):
         arr.isel(a=slice(5, -1)).chunk(a=5).to_zarr(store, region="auto", mode="r+")
+
+    # Test if the code is detecting the last chunk correctly
+    data = np.random.RandomState(0).randn(2920, 25, 53)
+    ds = xr.Dataset({'temperature': (('time', 'lat', 'lon'), data)})
+    chunks = {'time': 1000, 'lat': 25, 'lon': 53}
+    ds.chunk(chunks).to_zarr(store, compute=False)
+    region = {'time': slice(1000, 2000, 1)}
+    chunk = ds.isel(region)
+    chunk = chunk.chunk()
+    chunk.chunk().to_zarr(store, region=region)