Remove special mapping of auto to {} in open_zarr

doc/whats-new.rst

@@ -18,6 +18,9 @@ New Features
 Breaking Changes
 ~~~~~~~~~~~~~~~~
 
+- Remove special mapping of ``"auto"`` to ``{}`` in ``open_zarr``. This matches the behavior of ``open_dataset(..., engine="zarr")``
+  and means that by default the chunks might be a multiple of the the on-disk chunks. (:issue:`11002` :pull:`11010`).
+  By `Julia Signell <https://github.com/jsignell>`_.
 
 Deprecations
 ~~~~~~~~~~~~

xarray/backends/zarr.py

@@ -1564,7 +1564,6 @@ def open_zarr(
                 chunked_array_type
             )  # attempt to import that parallel backend
 
-            chunks = {}
         except (ValueError, ImportError):
             chunks = None
 

xarray/tests/test_backends.py

@@ -3637,6 +3637,18 @@ def test_chunk_encoding_with_larger_dask_chunks(self) -> None:
         ) as ds1:
             assert_equal(ds1, original)
 
+    @requires_dask
+    def test_chunk_auto_with_small_dask_chunks(self) -> None:
+        original = Dataset({"u": (("x",), np.zeros(10))}).chunk({"x": 2})
+        with self.create_zarr_target() as store:
+            original.to_zarr(store, **self.version_kwargs)
+            with xr.open_zarr(store, chunks={}, **self.version_kwargs) as native:
+                assert native.chunks == {"x": (2, 2, 2, 2, 2)}
+                with xr.open_zarr(store, **self.version_kwargs) as actual:
+                    assert_identical(actual, original)
+                    assert actual.chunks == {"x": (10,)}
+                    assert actual.chunks != native.chunks
+
     @requires_cftime
     def test_open_zarr_use_cftime(self) -> None:
         ds = create_test_data()