Use cumsum from flox

xarray/core/_aggregations.py

@@ -3655,6 +3655,17 @@ def _flox_reduce(
     ) -> Dataset:
         raise NotImplementedError()
 
+    def _flox_scan(
+        self,
+        dim: Dims,
+        *,
+        func: str,
+        skipna: bool | None = None,
+        keep_attrs: bool | None = None,
+        **kwargs: Any,
+    ) -> Dataset:
+        raise NotImplementedError()
+
     def count(
         self,
         dim: Dims = None,
@@ -5015,14 +5026,28 @@ def cumsum(
         Data variables:
             da       (time) float64 48B 1.0 2.0 3.0 3.0 4.0 nan
         """
-        return self.reduce(
-            duck_array_ops.cumsum,
-            dim=dim,
-            skipna=skipna,
-            numeric_only=True,
-            keep_attrs=keep_attrs,
-            **kwargs,
-        )
+        if (
+            flox_available
+            and OPTIONS["use_flox"]
+            and module_available("flox", minversion="0.10.5")
+            and contains_only_chunked_or_numpy(self._obj)
+        ):
+            return self._flox_scan(
+                func="cumsum",
+                dim=dim,
+                skipna=skipna,
+                # fill_value=fill_value,
+                keep_attrs=keep_attrs,
+                **kwargs,
+            )
+        else:
+            return self.reduce(
+                duck_array_ops.cumsum,
+                dim=dim,
+                skipna=skipna,
+                keep_attrs=keep_attrs,
+                **kwargs,
+            )
 
     def cumprod(
         self,
@@ -6647,6 +6672,17 @@ def _flox_reduce(
     ) -> DataArray:
         raise NotImplementedError()
 
+    def _flox_scan(
+        self,
+        dim: Dims,
+        *,
+        func: str,
+        skipna: bool | None = None,
+        keep_attrs: bool | None = None,
+        **kwargs: Any,
+    ) -> DataArray:
+        raise NotImplementedError()
+
     def count(
         self,
         dim: Dims = None,
@@ -7904,13 +7940,28 @@ def cumsum(
           * time     (time) datetime64[ns] 48B 2001-01-31 2001-02-28 ... 2001-06-30
             labels   (time) <U1 24B 'a' 'b' 'c' 'c' 'b' 'a'
         """
-        return self.reduce(
-            duck_array_ops.cumsum,
-            dim=dim,
-            skipna=skipna,
-            keep_attrs=keep_attrs,
-            **kwargs,
-        )
+        if (
+            flox_available
+            and OPTIONS["use_flox"]
+            and module_available("flox", minversion="0.10.5")
+            and contains_only_chunked_or_numpy(self._obj)
+        ):
+            return self._flox_scan(
+                func="cumsum",
+                dim=dim,
+                skipna=skipna,
+                # fill_value=fill_value,
+                keep_attrs=keep_attrs,
+                **kwargs,
+            )
+        else:
+            return self.reduce(
+                duck_array_ops.cumsum,
+                dim=dim,
+                skipna=skipna,
+                keep_attrs=keep_attrs,
+                **kwargs,
+            )
 
     def cumprod(
         self,

xarray/core/groupby.py

@@ -13,6 +13,7 @@
 from packaging.version import Version
 
 from xarray.computation import ops
+from xarray.computation.apply_ufunc import apply_ufunc
 from xarray.computation.arithmetic import (
     DataArrayGroupbyArithmetic,
     DatasetGroupbyArithmetic,
@@ -1028,6 +1029,26 @@ def _maybe_unstack(self, obj):
 
         return obj
 
+    def _parse_dim(self, dim: Dims) -> tuple[Hashable, ...]:
+        parsed_dim: tuple[Hashable, ...]
+        if isinstance(dim, str):
+            parsed_dim = (dim,)
+        elif dim is None:
+            parsed_dim_list = list()
+            # preserve order
+            for dim_ in itertools.chain(
+                *(grouper.codes.dims for grouper in self.groupers)
+            ):
+                if dim_ not in parsed_dim_list:
+                    parsed_dim_list.append(dim_)
+            parsed_dim = tuple(parsed_dim_list)
+        elif dim is ...:
+            parsed_dim = tuple(self._original_obj.dims)
+        else:
+            parsed_dim = tuple(dim)
+
+        return parsed_dim
+
     def _flox_reduce(
         self,
         dim: Dims,
@@ -1088,22 +1109,7 @@ def _flox_reduce(
                 # set explicitly to avoid unnecessarily accumulating count
                 kwargs["min_count"] = 0
 
-        parsed_dim: tuple[Hashable, ...]
-        if isinstance(dim, str):
-            parsed_dim = (dim,)
-        elif dim is None:
-            parsed_dim_list = list()
-            # preserve order
-            for dim_ in itertools.chain(
-                *(grouper.codes.dims for grouper in self.groupers)
-            ):
-                if dim_ not in parsed_dim_list:
-                    parsed_dim_list.append(dim_)
-            parsed_dim = tuple(parsed_dim_list)
-        elif dim is ...:
-            parsed_dim = tuple(obj.dims)
-        else:
-            parsed_dim = tuple(dim)
+        parsed_dim = self._parse_dim(dim)
 
         # Do this so we raise the same error message whether flox is present or not.
         # Better to control it here than in flox.
@@ -1202,6 +1208,61 @@ def _flox_reduce(
 
         return result
 
+    def _flox_scan(
+        self,
+        dim: Dims,
+        *,
+        func: str,
+        skipna: bool | None = None,
+        keep_attrs: bool | None = None,
+        **kwargs: Any,
+    ) -> DataArray:
+        from flox import groupby_scan
+
+        obj = self._original_obj
+
+        parsed_dim = self._parse_dim(dim)
+
+        axis = obj.get_axis_num(parsed_dim)
+        # axis = (axis_,) if isinstance(axis_, int) else axis_
+        codes = tuple(g.codes for g in self.groupers)
+
+        def wrapper(array, *by, func: str, skipna: bool | None, **kwargs):
+            if skipna or (skipna is None and obj.dtype.kind in "cfO"):
+                if "nan" not in func:
+                    func = f"nan{func}"
+
+            return groupby_scan(array, *codes, func=func, **kwargs)
+
+        actual = apply_ufunc(
+            wrapper,
+            obj,
+            *codes,
+            # input_core_dims=input_core_dims,
+            # for xarray's test_groupby_duplicate_coordinate_labels
+            # exclude_dims=set(dim_tuple),
+            # output_core_dims=[output_core_dims],
+            dask="allowed",
+            # dask_gufunc_kwargs=dict(
+            #     output_sizes=output_sizes,
+            #     output_dtypes=[dtype] if dtype is not None else None,
+            # ),
+            keep_attrs=(
+                _get_keep_attrs(default=True) if keep_attrs is None else keep_attrs
+            ),
+            kwargs=dict(
+                func=func,
+                skipna=skipna,
+                expected_groups=None,
+                axis=axis,
+                dtype=None,
+                method=None,
+                engine=None,
+            ),
+        )
+
+        return actual
+
     def fillna(self, value: Any) -> T_Xarray:
         """Fill missing values in this object by group.
 

xarray/util/generate_aggregations.py

@@ -15,7 +15,7 @@
 
 import textwrap
 from dataclasses import dataclass, field
-from typing import NamedTuple
+from typing import Literal, NamedTuple
 
 MODULE_PREAMBLE = '''\
 """Mixin classes with reduction operations."""
@@ -132,6 +132,17 @@ def _flox_reduce(
         dim: Dims,
         **kwargs: Any,
     ) -> {obj}:
+        raise NotImplementedError()
+
+    def _flox_scan(
+        self,
+        dim: Dims,
+        *,
+        func: str,
+        skipna: bool | None = None,
+        keep_attrs: bool | None = None,
+        **kwargs: Any,
+    ) -> DataArray:
         raise NotImplementedError()"""
 
 TEMPLATE_REDUCTION_SIGNATURE = '''
@@ -284,6 +295,7 @@ def __init__(
         see_also_methods=(),
         min_flox_version=None,
         additional_notes="",
+        flox_aggregation_type: Literal["reduce", "scan"] = "reduce",
     ):
         self.name = name
         self.extra_kwargs = extra_kwargs
@@ -292,6 +304,7 @@ def __init__(
         self.see_also_methods = see_also_methods
         self.min_flox_version = min_flox_version
         self.additional_notes = additional_notes
+        self.flox_aggregation_type = flox_aggregation_type
         if bool_reduce:
             self.array_method = f"array_{name}"
             self.np_example_array = (
@@ -444,7 +457,7 @@ def generate_code(self, method, has_keep_attrs):
 
         # median isn't enabled yet, because it would break if a single group was present in multiple
         # chunks. The non-flox code path will just rechunk every group to a single chunk and execute the median
-        method_is_not_flox_supported = method.name in ("median", "cumsum", "cumprod")
+        method_is_not_flox_supported = method.name in ("median", "cumprod")
         if method_is_not_flox_supported:
             indent = 12
         else:
@@ -476,7 +489,7 @@ def generate_code(self, method, has_keep_attrs):
             + f"""
             and contains_only_chunked_or_numpy(self._obj)
         ):
-            return self._flox_reduce(
+            return self._flox_{method.flox_aggregation_type}(
                 func="{method.name}",
                 dim=dim,{extra_kwargs}
                 # fill_value=fill_value,
@@ -537,6 +550,8 @@ def generate_code(self, method, has_keep_attrs):
         numeric_only=True,
         see_also_methods=("cumulative",),
         additional_notes=_CUM_NOTES,
+        min_flox_version="0.10.5",
+        flox_aggregation_type="scan",
     ),
     Method(
         "cumprod",