In xarray too

flox/xarray.py

@@ -10,6 +10,7 @@
 
 from .aggregations import Aggregation, _atleast_1d
 from .core import (
+    _choose_engine,
     _convert_expected_groups_to_index,
     _get_expected_groups,
     _validate_expected_groups,
@@ -72,7 +73,7 @@ def xarray_reduce(
     fill_value=None,
     dtype: np.typing.DTypeLike = None,
     method: str = "map-reduce",
-    engine: str = None,
+    engine: str | None = None,
     keep_attrs: bool | None = True,
     skipna: bool | None = None,
     min_count: int | None = None,
@@ -362,9 +363,12 @@ def wrapper(array, *by, func, skipna, core_dims, **kwargs):
             if "nan" not in func and func not in ["all", "any", "count"]:
                 func = f"nan{func}"
 
+        if kwargs.get("engine", None) is None:
+            kwargs["engine"] = _choose_engine(by, func)
+
         # Flox's count works with non-numeric and its faster than converting.
         requires_numeric = func not in ["count", "any", "all"] or (
-            func == "count" and engine != "flox"
+            func == "count" and kwargs["engine"] != "flox"
         )
         if requires_numeric:
             is_npdatetime = array.dtype.kind in "Mm"