Move .rolling_exp functions from reduce to apply_ufunc

doc/whats-new.rst

@@ -84,6 +84,9 @@ Bug fixes
   issues (:issue:`7817`, :issue:`7942`, :issue:`7790`, :issue:`6191`, :issue:`7096`,
   :issue:`1064`, :pull:`7827`).
   By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_.
+- ``.rolling_exp`` functions no longer mistakenly lose non-dimensioned coords
+  (:issue:`6528`, :pull:`8114`)
+  By `Maximilian Roos <https://github.com/max-sixty>`_.
 
 Documentation
 ~~~~~~~~~~~~~
@@ -96,6 +99,10 @@ Internal Changes
   By `András Gunyhó <https://github.com/mgunyho>`_.
 - Refactor of encoding and decoding times/timedeltas to preserve nanosecond resolution in arrays that contain missing values (:pull:`7827`).
   By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_.
+- Transition ``.rolling_exp`` functions to use `.apply_ufunc` internally rather
+  than `.reduce`, as the start of a broader effort to move non-reducing
+  functions away from ```.reduce``, (:pull:`8114`).
+  By `Maximilian Roos <https://github.com/max-sixty>`_.
 
 .. _whats-new.2023.08.0:
 

xarray/core/computation.py

@@ -433,6 +433,7 @@ def apply_dict_of_variables_vfunc(
     join="inner",
     fill_value=None,
     on_missing_core_dim: MissingCoreDimOptions = "raise",
+    keep_attrs="override",
 ):
     """Apply a variable level function over dicts of DataArray, DataArray,
     Variable and ndarray objects.
@@ -445,7 +446,7 @@ def apply_dict_of_variables_vfunc(
     for name, variable_args in zip(names, grouped_by_name):
         core_dim_present = _check_core_dims(signature, variable_args, name)
         if core_dim_present is True:
-            result_vars[name] = func(*variable_args)
+            result_vars[name] = func(*variable_args, keep_attrs=keep_attrs)
         else:
             if on_missing_core_dim == "raise":
                 raise ValueError(core_dim_present)
@@ -522,6 +523,7 @@ def apply_dataset_vfunc(
         join=dataset_join,
         fill_value=fill_value,
         on_missing_core_dim=on_missing_core_dim,
+        keep_attrs=keep_attrs,
     )
 
     out: Dataset | tuple[Dataset, ...]

xarray/core/rolling_exp.py

@@ -5,6 +5,7 @@
 
 import numpy as np
 
+from xarray.core.computation import apply_ufunc
 from xarray.core.options import _get_keep_attrs
 from xarray.core.pdcompat import count_not_none
 from xarray.core.pycompat import is_duck_dask_array
@@ -128,9 +129,18 @@ def mean(self, keep_attrs: bool | None = None) -> T_DataWithCoords:
         if keep_attrs is None:
             keep_attrs = _get_keep_attrs(default=True)
 
-        return self.obj.reduce(
-            move_exp_nanmean, dim=self.dim, alpha=self.alpha, keep_attrs=keep_attrs
-        )
+        dim_order = self.obj.dims
+
+        return apply_ufunc(
+            move_exp_nanmean,
+            self.obj,
+            input_core_dims=[[self.dim]],
+            kwargs=dict(alpha=self.alpha, axis=-1),
+            output_core_dims=[[self.dim]],
+            exclude_dims={self.dim},
+            keep_attrs=keep_attrs,
+            on_missing_core_dim="copy",
+        ).transpose(*dim_order)
 
     def sum(self, keep_attrs: bool | None = None) -> T_DataWithCoords:
         """
@@ -155,6 +165,15 @@ def sum(self, keep_attrs: bool | None = None) -> T_DataWithCoords:
         if keep_attrs is None:
             keep_attrs = _get_keep_attrs(default=True)
 
-        return self.obj.reduce(
-            move_exp_nansum, dim=self.dim, alpha=self.alpha, keep_attrs=keep_attrs
-        )
+        dim_order = self.obj.dims
+
+        return apply_ufunc(
+            move_exp_nansum,
+            self.obj,
+            input_core_dims=[[self.dim]],
+            kwargs=dict(alpha=self.alpha, axis=-1),
+            output_core_dims=[[self.dim]],
+            exclude_dims={self.dim},
+            keep_attrs=keep_attrs,
+            on_missing_core_dim="copy",
+        ).transpose(*dim_order)