Cache observed grouping/grouper instead of self obj

Author: Kei

pandas/core/groupby/grouper.py

@@ -668,11 +668,15 @@ def groups(self) -> dict[Hashable, Index]:
         cats = Categorical.from_codes(codes, uniques, validate=False)
         return self._index.groupby(cats)
 
-    @cache_readonly
+    @property
     def observed_grouping(self) -> Grouping:
         if self._observed:
             return self
 
+        return self._observed_grouping
+
+    @cache_readonly
+    def _observed_grouping(self) -> Grouping:
         grouping = Grouping(
             self._index,
             self._orig_grouper,

pandas/core/groupby/ops.py

@@ -823,11 +823,15 @@ def result_index_and_ids(self) -> tuple[Index, npt.NDArray[np.intp]]:
 
         return result_index, ids
 
-    @cache_readonly
+    @property
     def observed_grouper(self) -> BaseGrouper:
         if all(ping._observed for ping in self.groupings):
             return self
 
+        return self._observed_grouper
+
+    @cache_readonly
+    def _observed_grouper(self) -> BaseGrouper:
         groupings = [ping.observed_grouping for ping in self.groupings]
         grouper = BaseGrouper(self.axis, groupings, sort=self._sort, dropna=self.dropna)
         return grouper
@@ -1163,7 +1167,7 @@ def groupings(self) -> list[grouper.Grouping]:
         )
         return [ping]
 
-    @cache_readonly
+    @property
     def observed_grouper(self) -> BinGrouper:
         return self