REF: remove aggregate_item_by_item (#51222)

Author: jbrockmendel

pandas/core/groupby/generic.py

@@ -1378,22 +1378,6 @@ def _aggregate_frame(self, func, *args, **kwargs) -> DataFrame:
 
         return out
 
-    def _aggregate_item_by_item(self, func, *args, **kwargs) -> DataFrame:
-        # only for axis==0
-        # tests that get here with non-unique cols:
-        #  test_resample_with_timedelta_yields_no_empty_groups,
-        #  test_resample_apply_product
-
-        obj = self._obj_with_exclusions
-        result: dict[int, NDFrame] = {}
-
-        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
-        return res_df
-
     def _wrap_applied_output(
         self,
         data: DataFrame,

pandas/core/resample.py

@@ -40,10 +40,7 @@
     npt,
 )
 from pandas.compat.numpy import function as nv
-from pandas.errors import (
-    AbstractMethodError,
-    DataError,
-)
+from pandas.errors import AbstractMethodError
 from pandas.util._decorators import (
     Appender,
     Substitution,
@@ -423,15 +420,13 @@ def _groupby_and_aggregate(self, how, *args, **kwargs):
         )
 
         try:
-            if isinstance(obj, ABCDataFrame) and callable(how):
-                # Check if the function is reducing or not.
-                # e.g. test_resample_apply_with_additional_args
-                result = grouped._aggregate_item_by_item(how, *args, **kwargs)
+            if callable(how):
+                # TODO: test_resample_apply_with_additional_args fails if we go
+                #  through the non-lambda path, not clear that it should.
+                func = lambda x: how(x, *args, **kwargs)
+                result = grouped.aggregate(func)
             else:
                 result = grouped.aggregate(how, *args, **kwargs)
-        except DataError:
-            # got TypeErrors on aggregation
-            result = grouped.apply(how, *args, **kwargs)
         except (AttributeError, KeyError):
             # we have a non-reducing function; try to evaluate
             # alternatively we want to evaluate only a column of the input