Remove special mapping of auto to {} in open_zarr

doc/whats-new.rst

@@ -18,6 +18,11 @@ New Features
 Breaking Changes
 ~~~~~~~~~~~~~~~~
 
+- Change the default value for ``chunk`` in ``open_zarr`` to ``_default`` and remove special mapping of ``"auto"``
+  to ``{}`` or ``None`` in ``open_zarr``. If ``chunks`` is not set, the default behavior is the same as before.
+  Explicitly setting ``chunks="auto"`` will match the behavior of ``chunks="auto"`` in
+  ``open_dataset(..., engine="zarr")`` (:issue:`11002` :pull:`11010`).
+  By `Julia Signell <https://github.com/jsignell>`_.
 
 Deprecations
 ~~~~~~~~~~~~

xarray/backends/zarr.py

@@ -30,6 +30,7 @@
 from xarray.core.utils import (
     FrozenDict,
     HiddenKeyDict,
+    _default,
     attempt_import,
     close_on_error,
     emit_user_level_warning,
@@ -1400,7 +1401,7 @@ def open_zarr(
     store,
     group=None,
     synchronizer=None,
-    chunks="auto",
+    chunks=_default,
     decode_cf=True,
     mask_and_scale=True,
     decode_times=True,
@@ -1436,8 +1437,9 @@ def open_zarr(
         Array synchronizer provided to zarr
     group : str, optional
         Group path. (a.k.a. `path` in zarr terminology.)
-    chunks : int, dict, 'auto' or None, default: 'auto'
-        If provided, used to load the data into dask arrays.
+    chunks : int, dict, "auto" or None, optional
+        Used to load the data into dask arrays. Default behavior is to use
+        ``chunks={}`` if dask is available, otherwise ``chunks=None``.
 
         - ``chunks='auto'`` will use dask ``auto`` chunking taking into account the
           engine preferred chunks.
@@ -1558,7 +1560,7 @@ def open_zarr(
     if from_array_kwargs is None:
         from_array_kwargs = {}
 
-    if chunks == "auto":
+    if chunks is _default:
         try:
             guess_chunkmanager(
                 chunked_array_type

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, **self.version_kwargs) as default:
+                assert default.chunks == {"x": (2, 2, 2, 2, 2)}
+                with xr.open_zarr(store, chunks="auto", **self.version_kwargs) as auto:
+                    assert_identical(auto, original)
+                    assert auto.chunks == {"x": (10,)}
+                    assert auto.chunks != default.chunks
+
     @requires_cftime
     def test_open_zarr_use_cftime(self) -> None:
         ds = create_test_data()