Fix error when writing scalar varaibles to Zarr with region={}

doc/whats-new.rst

@@ -74,6 +74,9 @@ Bug fixes
 - Allow ``combine_attrs="drop_conflicts"`` to handle objects with ``__eq__`` methods that return
   non-bool values (e.g., numpy arrays) without raising ``ValueError`` (:pull:`10726`).
   By `Maximilian Roos <https://github.com/max-sixty>`_.
+- Fix error raised when writing scalar variables to Zarr with ``region={}``
+  (:pull:`10796`).
+  By `Stephan Hoyer <https://github.com/shoyer>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/backends/zarr.py

@@ -1359,7 +1359,7 @@ def _validate_and_autodetect_region(self, ds: Dataset) -> Dataset:
         non_matching_vars = [
             k for k, v in ds.variables.items() if not set(region).intersection(v.dims)
         ]
-        if non_matching_vars:
+        if region and non_matching_vars:
             raise ValueError(
                 f"when setting `region` explicitly in to_zarr(), all "
                 f"variables in the dataset to write must have at least "

xarray/tests/test_backends.py

@@ -3443,6 +3443,14 @@ def test_write_region(self, consolidated, compute, use_dask, write_empty) -> Non
             ) as actual:
                 assert_identical(actual, nonzeros)
 
+    def test_region_scalar(self) -> None:
+        ds = Dataset({"x": 0})
+        with self.create_zarr_target() as store:
+            ds.to_zarr(store)
+            ds.to_zarr(store, region={}, mode="r+")
+            with xr.open_zarr(store) as actual:
+                assert_identical(actual, ds)
+
     @pytest.mark.parametrize("mode", [None, "r+", "a"])
     def test_write_region_mode(self, mode) -> None:
         zeros = Dataset({"u": (("x",), np.zeros(10))})