Add cumsum to DatasetGroupBy

Fixes pydata#3141

xarray/core/groupby.py

@@ -13,6 +13,7 @@
 from .concat import concat
 from .formatting import format_array_flat
 from .indexes import create_default_index_implicit, filter_indexes_from_coords
+from .ops import IncludeCumMethods
 from .options import _get_keep_attrs
 from .pycompat import integer_types
 from .utils import (
@@ -970,7 +971,7 @@ def reduce_array(ar):
         return self.map(reduce_array, shortcut=shortcut)
 
 
-class DataArrayGroupBy(DataArrayGroupByBase, DataArrayGroupByReductions):
+class DataArrayGroupBy(DataArrayGroupByBase, DataArrayGroupByReductions, IncludeCumMethods):
     __slots__ = ()
 
 
@@ -1108,5 +1109,5 @@ def assign(self, **kwargs):
         return self.map(lambda ds: ds.assign(**kwargs))
 
 
-class DatasetGroupBy(DatasetGroupByBase, DatasetGroupByReductions):
+class DatasetGroupBy(DatasetGroupByBase, DatasetGroupByReductions, IncludeCumMethods):
     __slots__ = ()

xarray/tests/test_groupby.py

@@ -1918,4 +1918,23 @@ def func(arg1, arg2, arg3=0.0):
         assert_identical(expected, actual)
 
 
+def test_groupby_cumsum():
+    ds = xr.Dataset(
+        {"foo": (("x",), [7, 3, 1, 1, 1, 1, 1])},
+        coords={"x": [0, 1, 2, 3, 4, 5, 6], "group_id": ("x", [0, 0, 1, 1, 2, 2, 2])},
+    )
+    actual = ds.groupby("group_id").cumsum(dim="x")
+    expected = xr.Dataset(
+        {
+            "foo": (("x",), [7, 10, 1, 2, 1, 2, 3]),
+            "group_id": (("x",), [0, 0, 1, 1, 2, 2, 2]),
+        },
+        coords={"x": [0, 1, 2, 3, 4, 5, 6]},
+    )
+    assert_identical(expected, actual)
+
+    actual = ds.foo.groupby(ds.group_id).cumsum(dim="x")
+    assert_identical(expected.foo, actual)
+
+
 # TODO: move other groupby tests from test_dataset and test_dataarray over here