cleanup

xarray/core/groupby.py

@@ -3,7 +3,7 @@
 import copy
 import datetime
 import warnings
-from abc import ABC
+from abc import ABC, abstractmethod
 from collections.abc import Hashable, Iterator, Mapping, Sequence
 from dataclasses import dataclass, field
 from typing import (
@@ -407,11 +407,8 @@ def factorize(self) -> None:
 
 
 class Grouper(ABC):
-    def _post_resolve(self, rgrouper: ResolvedGrouper) -> None:
-        """
-        This hook is intended for internal use to preserve
-        existing performance optimizations for UniqueGrouper and BinGrouper.
-        """
+    @abstractmethod
+    def factorize(self, group) -> T_FactorizeOut:
         pass