From 850e68906e410a1053c1ada552b297ae45e1f373 Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Fri, 10 Sep 2021 13:10:35 -0700
Subject: [PATCH 1/2] REF: de-duplicate operate-column-wise code

---
 pandas/core/groupby/generic.py | 19 ++++---------------
 1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/pandas/core/groupby/generic.py b/pandas/core/groupby/generic.py
index 1cbfcff5e94f1..dbb640a5bdf19 100644
--- a/pandas/core/groupby/generic.py
+++ b/pandas/core/groupby/generic.py
@@ -1084,14 +1084,10 @@ def _aggregate_item_by_item(self, func, *args, **kwargs) -> DataFrame:
         #  test_resample_apply_product
 
         obj = self._obj_with_exclusions
-        result: dict[int | str, NDFrame] = {}
-        for i, item in enumerate(obj):
-            ser = obj.iloc[:, i]
-            colg = SeriesGroupBy(
-                ser, selection=item, grouper=self.grouper, exclusions=self.exclusions
-            )
+        result: dict[int, NDFrame] = {}
 
-            result[i] = colg.aggregate(func, *args, **kwargs)
+        for i, (item, sgb) in enumerate(self._iterate_column_groupbys(obj)):
+            result[i] = sgb.aggregate(func, *args, **kwargs)
 
         res_df = self.obj._constructor(result)
         res_df.columns = obj.columns
@@ -1370,14 +1366,7 @@ def _transform_item_by_item(self, obj: DataFrame, wrapper) -> DataFrame:
         #  gets here with non-unique columns
         output = {}
         inds = []
-        for i, col in enumerate(obj):
-            subset = obj.iloc[:, i]
-            sgb = SeriesGroupBy(
-                subset,
-                selection=col,
-                grouper=self.grouper,
-                exclusions=self.exclusions,
-            )
+        for i, (colname, sgb) in enumerate(self._iterate_column_groupbys(obj)):
             try:
                 output[i] = sgb.transform(wrapper)
             except TypeError:

From 7765bea40c4e5862d3133b968d7d5e57300c2d10 Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Fri, 10 Sep 2021 16:50:02 -0700
Subject: [PATCH 2/2] REF: remove redundant pct_change

---
 pandas/core/groupby/generic.py | 18 ------------------
 pandas/core/groupby/groupby.py |  2 ++
 2 files changed, 2 insertions(+), 18 deletions(-)

diff --git a/pandas/core/groupby/generic.py b/pandas/core/groupby/generic.py
index dbb640a5bdf19..a3d749bdbe5cb 100644
--- a/pandas/core/groupby/generic.py
+++ b/pandas/core/groupby/generic.py
@@ -791,24 +791,6 @@ def count(self) -> Series:
         )
         return self._reindex_output(result, fill_value=0)
 
-    def pct_change(self, periods=1, fill_method="pad", limit=None, freq=None):
-        """Calculate pct_change of each value to previous entry in group"""
-        # TODO: Remove this conditional when #23918 is fixed
-        if freq:
-            return self.apply(
-                lambda x: x.pct_change(
-                    periods=periods, fill_method=fill_method, limit=limit, freq=freq
-                )
-            )
-        if fill_method is None:  # GH30463
-            fill_method = "pad"
-            limit = 0
-        filled = getattr(self, fill_method)(limit=limit)
-        fill_grp = filled.groupby(self.grouper.codes)
-        shifted = fill_grp.shift(periods=periods, freq=freq)
-
-        return (filled / shifted) - 1
-
     @doc(Series.nlargest)
     def nlargest(self, n: int = 5, keep: str = "first"):
         f = partial(Series.nlargest, n=n, keep=keep)
diff --git a/pandas/core/groupby/groupby.py b/pandas/core/groupby/groupby.py
index 85e7c9a62b2d4..20a9f7d088ae9 100644
--- a/pandas/core/groupby/groupby.py
+++ b/pandas/core/groupby/groupby.py
@@ -3210,6 +3210,7 @@ def shift(self, periods=1, freq=None, axis=0, fill_value=None):
         )
         return res
 
+    @final
     @Substitution(name="groupby")
     @Appender(_common_see_also)
     def pct_change(self, periods=1, fill_method="pad", limit=None, freq=None, axis=0):
@@ -3221,6 +3222,7 @@ def pct_change(self, periods=1, fill_method="pad", limit=None, freq=None, axis=0
         Series or DataFrame
             Percentage changes within each group.
         """
+        # TODO: Remove this conditional for SeriesGroupBy when GH#23918 is fixed
         if freq is not None or axis != 0:
             return self.apply(
                 lambda x: x.pct_change(