better backcompat

xarray/core/dataarray.py

@@ -6749,6 +6749,11 @@ def groupby(
         restore_coord_dims : bool, default: False
             If True, also restore the dimension order of multi-dimensional
             coordinates.
+        eagerly_compute_group: bool
+            Whether to eagerly compute ``group`` when it is a chunked array.
+            This option is to maintain backwards compatibility. Set to False
+            to opt-in to future behaviour, where ``group`` is not automatically loaded
+            into memory.
         **groupers : Mapping of str to Grouper or Resampler
             Mapping of variable name to group by to :py:class:`Grouper` or :py:class:`Resampler` object.
             One of ``group`` or ``groupers`` must be provided.
@@ -6917,6 +6922,11 @@ def groupby_bins(
             coordinates.
         duplicates : {"raise", "drop"}, default: "raise"
             If bin edges are not unique, raise ValueError or drop non-uniques.
+        eagerly_compute_group: bool
+            Whether to eagerly compute ``group`` when it is a chunked array.
+            This option is to maintain backwards compatibility. Set to False
+            to opt-in to future behaviour, where ``group`` is not automatically loaded
+            into memory.
 
         Returns
         -------

xarray/core/dataset.py

@@ -10382,6 +10382,11 @@ def groupby(
         restore_coord_dims : bool, default: False
             If True, also restore the dimension order of multi-dimensional
             coordinates.
+        eagerly_compute_group: bool
+            Whether to eagerly compute ``group`` when it is a chunked array.
+            This option is to maintain backwards compatibility. Set to False
+            to opt-in to future behaviour, where ``group`` is not automatically loaded
+            into memory.
         **groupers : Mapping of str to Grouper or Resampler
             Mapping of variable name to group by to :py:class:`Grouper` or :py:class:`Resampler` object.
             One of ``group`` or ``groupers`` must be provided.
@@ -10519,6 +10524,11 @@ def groupby_bins(
             coordinates.
         duplicates : {"raise", "drop"}, default: "raise"
             If bin edges are not unique, raise ValueError or drop non-uniques.
+        eagerly_compute_group: bool
+            Whether to eagerly compute ``group`` when it is a chunked array.
+            This option is to maintain backwards compatibility. Set to False
+            to opt-in to future behaviour, where ``group`` is not automatically loaded
+            into memory.
 
         Returns
         -------

xarray/core/groupby.py

@@ -308,21 +308,45 @@ def __post_init__(self) -> None:
         # of pd.cut
         # We do not want to modify the original object, since the same grouper
         # might be used multiple times.
+        from xarray.groupers import BinGrouper, UniqueGrouper
+
         self.grouper = copy.deepcopy(self.grouper)
 
         self.group = _resolve_group(self.obj, self.group)
 
-        if (
-            self.eagerly_compute_group
-            and not isinstance(self.group, _DummyGroup)
-            and is_chunked_array(self.group.variable._data)
+        if not isinstance(self.group, _DummyGroup) and is_chunked_array(
+            self.group.variable._data
         ):
-            emit_user_level_warning(
-                f"Eagerly computing the DataArray you're grouping by ({self.group.name!r}) "
-                "is deprecated and will be removed in v2025.05.0. "
-                "Please load this array's data manually using `.compute` or `.load`.",
-                DeprecationWarning,
-            )
+            if self.eagerly_compute_group is False:
+                # This requires a pass to discover the groups present
+                if (
+                    isinstance(self.grouper, UniqueGrouper)
+                    and self.grouper.labels is None
+                ):
+                    raise ValueError(
+                        "Please pass `labels` to UniqueGrouper when grouping by a chunked array."
+                    )
+                # this requires a pass to compute the bin edges
+                if isinstance(self.grouper, BinGrouper) and isinstance(
+                    self.grouper.bins, int
+                ):
+                    raise ValueError(
+                        "Please pass explicit bin edges to BinGrouper using the ``bins`` kwarg"
+                        "when grouping by a chunked array."
+                    )
+
+            if self.eagerly_compute_group:
+                emit_user_level_warning(
+                    f""""Eagerly computing the DataArray you're grouping by ({self.group.name!r}) "
+                    is deprecated and will raise an error in v2025.05.0.
+                    Please load this array's data manually using `.compute` or `.load`.
+                    To intentionally avoid eager loading, either (1) specify
+                    `.groupby({self.group.name}=UniqueGrouper(labels=...), eagerly_load_group=False)`
+                    or (2) pass explicit bin edges using or `.groupby({self.group.name}=BinGrouper(bins=...),
+                    eagerly_load_group=False)`; as appropriate.""",
+                    DeprecationWarning,
+                )
+                self.group = self.group.compute()
 
         self.encoded = self.grouper.factorize(self.group)
 
@@ -678,8 +702,9 @@ def _raise_if_by_is_chunked(self):
         if self._by_chunked:
             raise ValueError(
                 "This method is not supported when lazily grouping by a chunked array. "
-                "Either load the array in to memory prior to grouping, or explore another "
-                "way of applying your function, potentially using the `flox` package."
+                "Either load the array in to memory prior to grouping using .load or .compute, "
+                " or explore another way of applying your function, "
+                "potentially using the `flox` package."
             )
 
     def _raise_if_not_single_group(self):

xarray/tests/test_groupby.py

@@ -3091,6 +3091,41 @@ def test_groupby_multiple_bin_grouper_missing_groups():
     assert_identical(actual, expected)
 
 
+@requires_dask
+def test_groupby_dask_eager_load_warnings():
+    ds = xr.Dataset(
+        {"foo": (("z"), np.arange(12))},
+        coords={"x": ("z", np.arange(12)), "y": ("z", np.arange(12))},
+    ).chunk(z=6)
+
+    with pytest.warns(DeprecationWarning):
+        ds.groupby(x=UniqueGrouper())
+
+    with pytest.warns(DeprecationWarning):
+        ds.groupby("x")
+
+    with pytest.warns(DeprecationWarning):
+        ds.groupby(ds.x)
+
+    with pytest.raises(ValueError, match="Please pass"):
+        ds.groupby("x", eagerly_compute_group=False)
+
+    # This is technically fine but anyone iterating over the groupby object
+    # will see an error, so let's warn and have them opt-in.
+    with pytest.warns(DeprecationWarning):
+        ds.groupby(x=UniqueGrouper(labels=[1, 2, 3]))
+
+    ds.groupby(x=UniqueGrouper(labels=[1, 2, 3]), eagerly_compute_group=False)
+
+    with pytest.warns(DeprecationWarning):
+        ds.groupby_bins("x", bins=3)
+    with pytest.raises(ValueError, match="Please pass"):
+        ds.groupby_bins("x", bins=3, eagerly_compute_group=False)
+    with pytest.warns(DeprecationWarning):
+        ds.groupby_bins("x", bins=[1, 2, 3])
+    ds.groupby_bins("x", bins=[1, 2, 3], eagerly_compute_group=False)
+
+
 # Possible property tests
 # 1. lambda x: x
 # 2. grouped-reduce on unique coords is identical to array