From bdce0a8207f5ff410e21635ca31d963eb7d87d30 Mon Sep 17 00:00:00 2001
From: dcherian <deepak@cherian.net>
Date: Fri, 15 Jul 2022 09:09:28 -0600
Subject: [PATCH 1/2] Warn when grouping by dask array.

---
 xarray/core/groupby.py       | 12 +++++++++++-
 xarray/tests/test_groupby.py |  7 +++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/xarray/core/groupby.py b/xarray/core/groupby.py
index 5fa78ae76de..c4540f46be5 100644
--- a/xarray/core/groupby.py
+++ b/xarray/core/groupby.py
@@ -29,7 +29,7 @@
 from .indexes import create_default_index_implicit, filter_indexes_from_coords
 from .npcompat import QUANTILE_METHODS, ArrayLike
 from .options import _get_keep_attrs
-from .pycompat import integer_types
+from .pycompat import integer_types, is_duck_dask_array
 from .types import T_Xarray
 from .utils import (
     either_dict_or_kwargs,
@@ -377,6 +377,16 @@ def __init__(
         self._unstacked_group = group
         self._bins = bins
 
+        if is_duck_dask_array(group.data):
+            warnings.warn(
+                "Grouping by a dask array computes that array. "
+                "This will raise an error in the future. "
+                "Use `.groupby(group.compute())` to avoid an error in the future.",
+                UserWarning,
+                stacklevel=3,
+            )
+            group = group.compute()
+
         group, obj, stacked_dim, inserted_dims = _ensure_1d(group, obj)
         (group_dim,) = group.dims
 
diff --git a/xarray/tests/test_groupby.py b/xarray/tests/test_groupby.py
index a006e54468a..aadebd7e5c3 100644
--- a/xarray/tests/test_groupby.py
+++ b/xarray/tests/test_groupby.py
@@ -504,6 +504,13 @@ def test_groupby_repr_datetime(obj) -> None:
     assert actual == expected
 
 
+@requires_dask
+def test_groupby_dask_array_raises_warning():
+    ds = Dataset({"foo": ("x", np.arange(10)), "baz": ("x", np.arange(10))}).chunk()
+    with pytest.warns(UserWarning):
+        ds.groupby("baz")
+
+
 def test_groupby_drops_nans() -> None:
     # GH2383
     # nan in 2D data variable (requires stacking)

From d752672f83abad17ba37a4b13a101761b0b9fa41 Mon Sep 17 00:00:00 2001
From: dcherian <deepak@cherian.net>
Date: Fri, 15 Jul 2022 11:11:29 -0600
Subject: [PATCH 2/2] FIx mypy?

---
 xarray/core/groupby.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xarray/core/groupby.py b/xarray/core/groupby.py
index c4540f46be5..768246c2529 100644
--- a/xarray/core/groupby.py
+++ b/xarray/core/groupby.py
@@ -377,7 +377,7 @@ def __init__(
         self._unstacked_group = group
         self._bins = bins
 
-        if is_duck_dask_array(group.data):
+        if not isinstance(group, _DummyGroup) and is_duck_dask_array(group.data):
             warnings.warn(
                 "Grouping by a dask array computes that array. "
                 "This will raise an error in the future. "