Fixes

xarray/core/groupby.py

@@ -29,7 +29,13 @@
     safe_cast_to_index,
 )
 from xarray.core.options import _get_keep_attrs
-from xarray.core.types import Dims, QuantileMethods, T_DataArray, T_Xarray
+from xarray.core.types import (
+    Dims,
+    QuantileMethods,
+    T_DataArray,
+    T_DataWithCoords,
+    T_Xarray,
+)
 from xarray.core.utils import (
     FrozenMappingWarningOnValuesAccess,
     either_dict_or_kwargs,
@@ -276,9 +282,9 @@ def to_array(self) -> DataArray:
 T_Group = Union["T_DataArray", "IndexVariable", _DummyGroup]
 
 
-def _ensure_1d(group: T_Group, obj: T_Xarray) -> tuple[
+def _ensure_1d(group: T_Group, obj: T_DataWithCoords) -> tuple[
     T_Group,
-    T_Xarray,
+    T_DataWithCoords,
     Hashable | None,
     list[Hashable],
 ]:
@@ -341,10 +347,10 @@ def _apply_loffset(
 
 
 @dataclass
-class ResolvedGrouper:
+class ResolvedGrouper(Generic[T_DataWithCoords]):
     grouper: Grouper
     group: T_Group
-    obj: T_Xarray
+    obj: T_DataWithCoords
 
     # Defined by factorize:
     codes: DataArray = field(init=False)
@@ -354,7 +360,7 @@ class ResolvedGrouper:
 
     # _ensure_1d:
     group1d: T_Group = field(init=False)
-    stacked_obj: T_Xarray = field(init=False)
+    stacked_obj: T_DataWithCoords = field(init=False)
     stacked_dim: Hashable | None = field(init=False)
     inserted_dims: list[Hashable] = field(init=False)
 
@@ -660,7 +666,7 @@ def _validate_groupby_squeeze(squeeze: bool | None) -> None:
         )
 
 
-def _resolve_group(obj: T_Xarray, group: T_Group | Hashable) -> T_Group:
+def _resolve_group(obj: T_DataWithCoords, group: T_Group | Hashable) -> T_Group:
     from xarray.core.dataarray import DataArray
 
     error_msg = (
@@ -698,12 +704,12 @@ def _resolve_group(obj: T_Xarray, group: T_Group | Hashable) -> T_Group:
                 "name of an xarray variable or dimension. "
                 f"Received {group!r} instead."
             )
-        group = obj[group]
-        if group.name not in obj._indexes and group.name in obj.dims:
+        group_da: DataArray = obj[group]
+        if group_da.name not in obj._indexes and group_da.name in obj.dims:
             # DummyGroups should not appear on groupby results
-            newgroup = _DummyGroup(obj, group.name, group.coords)
+            newgroup = _DummyGroup(obj, group_da.name, group_da.coords)
         else:
-            newgroup = group
+            newgroup = group_da
 
     if newgroup.size == 0:
         raise ValueError(f"{newgroup.name} must not be empty")