Automatically chunk other in GroupBy binary ops. (#7684)

* Automatically chunk `other` in GroupBy binary ops.

Closes #7683

* Update xarray/core/groupby.py

* Add test

* Update xarray/core/groupby.py

xarray/core/groupby.py

@@ -888,6 +888,21 @@ def _binary_op(self, other, f, reflexive=False):
             group = group.where(~mask, drop=True)
             codes = codes.where(~mask, drop=True).astype(int)
 
+        # if other is dask-backed, that's a hint that the
+        # "expanded" dataset is too big to hold in memory.
+        # this can be the case when `other` was read from disk
+        # and contains our lazy indexing classes
+        # We need to check for dask-backed Datasets
+        # so utils.is_duck_dask_array does not work for this check
+        if obj.chunks and not other.chunks:
+            # TODO: What about datasets with some dask vars, and others not?
+            # This handles dims other than `name``
+            chunks = {k: v for k, v in obj.chunksizes.items() if k in other.dims}
+            # a chunk size of 1 seems reasonable since we expect individual elements of
+            # other to be repeated multiple times across the reduced dimension(s)
+            chunks[name] = 1
+            other = other.chunk(chunks)
+
         # codes are defined for coord, so we align `other` with `coord`
         # before indexing
         other, _ = align(other, coord, join="right", copy=False)

xarray/tests/test_groupby.py

@@ -12,6 +12,7 @@
 from xarray import DataArray, Dataset, Variable
 from xarray.core.groupby import _consolidate_slices
 from xarray.tests import (
+    InaccessibleArray,
     assert_allclose,
     assert_array_equal,
     assert_equal,
@@ -2392,3 +2393,17 @@ def test_min_count_error(use_flox: bool) -> None:
     with xr.set_options(use_flox=use_flox):
         with pytest.raises(TypeError):
             da.groupby("labels").mean(min_count=1)
+
+
+@requires_dask
+def test_groupby_math_auto_chunk():
+    da = xr.DataArray(
+        [[1, 2, 3], [1, 2, 3], [1, 2, 3]],
+        dims=("y", "x"),
+        coords={"label": ("x", [2, 2, 1])},
+    )
+    sub = xr.DataArray(
+        InaccessibleArray(np.array([1, 2])), dims="label", coords={"label": [1, 2]}
+    )
+    actual = da.chunk(x=1, y=2).groupby("label") - sub
+    assert actual.chunksizes == {"x": (1, 1, 1), "y": (2, 1)}