REF: re-use _get_data_to_aggregate in GroupBy methods (#43571)

Author: jbrockmendel

Diff for: pandas/core/groupby/generic.py

@@ -610,10 +610,7 @@ def nunique(self, dropna: bool = True) -> Series:
 
     @doc(Series.describe)
     def describe(self, **kwargs):
-        result = self.apply(lambda x: x.describe(**kwargs))
-        if self.axis == 1:
-            return result.T
-        return result.unstack()
+        return super().describe(**kwargs)
 
     def value_counts(
         self,

Diff for: pandas/core/groupby/groupby.py

@@ -2693,11 +2693,7 @@ def blk_func(values: ArrayLike) -> ArrayLike:
 
         obj = self._obj_with_exclusions
         is_ser = obj.ndim == 1
-        if is_ser:
-            # i.e. SeriesGroupBy
-            mgr = obj.to_frame()._mgr
-        else:
-            mgr = self._get_data_to_aggregate()
+        mgr = self._get_data_to_aggregate()
 
         res_mgr = mgr.grouped_reduce(blk_func, ignore_failures=True)
         if len(res_mgr.items) != len(mgr.items):
@@ -3135,13 +3131,7 @@ def blk_func(values: ArrayLike) -> ArrayLike:
 
         # Operate block-wise instead of column-by-column
         orig_ndim = obj.ndim
-        if orig_ndim == 1:
-            # Operate on DataFrame, then squeeze below
-            obj = obj.to_frame()
-
-        mgr = obj._mgr
-        if self.axis == 1:
-            mgr = obj.T._mgr
+        mgr = self._get_data_to_aggregate()
 
         if numeric_only:
             mgr = mgr.get_numeric_data()
@@ -3168,14 +3158,10 @@ def blk_func(values: ArrayLike) -> ArrayLike:
                 # We should never get here
                 raise TypeError("All columns were dropped in grouped_reduce")
 
-        out = type(obj)(res_mgr)
-
         if orig_ndim == 1:
-            assert out.ndim == 2
-            assert out.shape[1] == 1
-            out = out.iloc[:, 0]
-            # restore name=None in case to_frame set columns to [0]
-            out.name = self.obj.name
+            out = self._wrap_agged_manager(res_mgr)
+        else:
+            out = type(obj)(res_mgr)
 
         return self._wrap_aggregated_output(out)