BUG: groupby.agg should always agg

pandas/core/groupby/generic.py

@@ -373,6 +373,61 @@ def aggregate(self, func=None, *args, engine=None, engine_kwargs=None, **kwargs)
 
     agg = aggregate
 
+    def _agg_for_resample(
+        self, func=None, *args, engine=None, engine_kwargs=None, **kwargs
+    ):
+        relabeling = func is None
+        columns = None
+        if relabeling:
+            columns, func = validate_func_kwargs(kwargs)
+            kwargs = {}
+
+        if isinstance(func, str):
+            if maybe_use_numba(engine) and engine is not None:
+                # Not all agg functions support numba, only propagate numba kwargs
+                # if user asks for numba, and engine is not None
+                # (if engine is None, the called function will handle the case where
+                # numba is requested via the global option)
+                kwargs["engine"] = engine
+            if engine_kwargs is not None:
+                kwargs["engine_kwargs"] = engine_kwargs
+            return getattr(self, func)(*args, **kwargs)
+
+        elif isinstance(func, abc.Iterable):
+            # Catch instances of lists / tuples
+            # but not the class list / tuple itself.
+            func = maybe_mangle_lambdas(func)
+            kwargs["engine"] = engine
+            kwargs["engine_kwargs"] = engine_kwargs
+            ret = self._aggregate_multiple_funcs(func, *args, **kwargs)
+            if relabeling:
+                # columns is not narrowed by mypy from relabeling flag
+                assert columns is not None  # for mypy
+                ret.columns = columns
+            if not self.as_index:
+                ret = ret.reset_index()
+            return ret
+
+        else:
+            if maybe_use_numba(engine):
+                return self._aggregate_with_numba(
+                    func, *args, engine_kwargs=engine_kwargs, **kwargs
+                )
+
+            if self.ngroups == 0:
+                # e.g. test_evaluate_with_empty_groups without any groups to
+                #  iterate over, we have no output on which to do dtype
+                #  inference. We default to using the existing dtype.
+                #  xref GH#51445
+                obj = self._obj_with_exclusions
+                return self.obj._constructor(
+                    [],
+                    name=self.obj.name,
+                    index=self._grouper.result_index,
+                    dtype=obj.dtype,
+                )
+            return self._python_agg_general(func, *args, **kwargs)
+
     def _python_agg_general(self, func, *args, **kwargs):
         f = lambda x: func(x, *args, **kwargs)
 
@@ -1501,6 +1556,61 @@ def aggregate(self, func=None, *args, engine=None, engine_kwargs=None, **kwargs)
         relabeling, func, columns, order = reconstruct_func(func, **kwargs)
         func = maybe_mangle_lambdas(func)
 
+        if maybe_use_numba(engine):
+            # Not all agg functions support numba, only propagate numba kwargs
+            # if user asks for numba
+            kwargs["engine"] = engine
+            kwargs["engine_kwargs"] = engine_kwargs
+
+        op = GroupByApply(self, func, args=args, kwargs=kwargs)
+        result = op.agg()
+        if not is_dict_like(func) and result is not None:
+            # GH #52849
+            if not self.as_index and is_list_like(func):
+                return result.reset_index()
+            else:
+                return result
+        elif relabeling:
+            # this should be the only (non-raising) case with relabeling
+            # used reordered index of columns
+            result = cast(DataFrame, result)
+            result = result.iloc[:, order]
+            result = cast(DataFrame, result)
+            # error: Incompatible types in assignment (expression has type
+            # "Optional[List[str]]", variable has type
+            # "Union[Union[Union[ExtensionArray, ndarray[Any, Any]],
+            # Index, Series], Sequence[Any]]")
+            result.columns = columns  # type: ignore[assignment]
+
+        if result is None:
+            # Remove the kwargs we inserted
+            # (already stored in engine, engine_kwargs arguments)
+            if "engine" in kwargs:
+                del kwargs["engine"]
+                del kwargs["engine_kwargs"]
+            # at this point func is not a str, list-like, dict-like,
+            # or a known callable(e.g. sum)
+            if maybe_use_numba(engine):
+                return self._aggregate_with_numba(
+                    func, *args, engine_kwargs=engine_kwargs, **kwargs
+                )
+            # grouper specific aggregations
+            result = self._python_agg_general(func, *args, **kwargs)
+
+        if not self.as_index:
+            result = self._insert_inaxis_grouper(result)
+            result.index = default_index(len(result))
+
+        return result
+
+    agg = aggregate
+
+    def _agg_for_resample(
+        self, func=None, *args, engine=None, engine_kwargs=None, **kwargs
+    ):
+        relabeling, func, columns, order = reconstruct_func(func, **kwargs)
+        func = maybe_mangle_lambdas(func)
+
         if maybe_use_numba(engine):
             # Not all agg functions support numba, only propagate numba kwargs
             # if user asks for numba
@@ -1577,8 +1687,6 @@ def aggregate(self, func=None, *args, engine=None, engine_kwargs=None, **kwargs)
 
         return result
 
-    agg = aggregate
-
     def _python_agg_general(self, func, *args, **kwargs):
         f = lambda x: func(x, *args, **kwargs)
 
@@ -1589,9 +1697,11 @@ def _python_agg_general(self, func, *args, **kwargs):
 
         obj = self._obj_with_exclusions
 
-        if not len(obj.columns):
+        if self._grouper._is_resample and not len(obj.columns):
             # e.g. test_margins_no_values_no_cols
-            return self._python_apply_general(f, self._selected_obj)
+            return obj._constructor(
+                index=self._grouper.result_index, columns=obj.columns
+            )
 
         output: dict[int, ArrayLike] = {}
         for idx, (name, ser) in enumerate(obj.items()):

pandas/core/groupby/ops.py

@@ -589,6 +589,7 @@ def __init__(
         self._groupings: list[grouper.Grouping] = list(groupings)
         self._sort = sort
         self.dropna = dropna
+        self._is_resample = False
 
     @property
     def groupings(self) -> list[grouper.Grouping]:
@@ -940,12 +941,14 @@ def _aggregate_series_pure_python(
 
         for i, group in enumerate(splitter):
             res = func(group)
-            res = extract_result(res)
 
-            if not initialized:
-                # We only do this validation on the first iteration
-                check_result_array(res, group.dtype)
-                initialized = True
+            if self._is_resample:
+                res = extract_result(res)
+
+                if not initialized:
+                    # We only do this validation on the first iteration
+                    check_result_array(res, group.dtype)
+                    initialized = True
 
             result[i] = res
 

pandas/core/resample.py

@@ -249,6 +249,7 @@ def _get_binner(self):
         binner, bins, binlabels = self._get_binner_for_time()
         assert len(bins) == len(binlabels)
         bin_grouper = BinGrouper(bins, binlabels, indexer=self._indexer)
+        bin_grouper._is_resample = True
         return binner, bin_grouper
 
     @overload
@@ -370,7 +371,21 @@ def aggregate(self, func=None, *args, **kwargs):
         return result
 
     agg = aggregate
-    apply = aggregate
+
+    @final
+    @doc(
+        _shared_docs["aggregate"],
+        see_also=_agg_see_also_doc,
+        examples=_agg_examples_doc,
+        klass="DataFrame",
+        axis="",
+    )
+    def apply(self, func=None, *args, **kwargs):
+        result = ResamplerWindowApply(self, func, args=args, kwargs=kwargs).agg()
+        if result is None:
+            how = func
+            result = self._groupby_and_apply(how, *args, **kwargs)
+        return result
 
     @final
     def transform(self, arg, *args, **kwargs):
@@ -487,6 +502,53 @@ def _groupby_and_aggregate(self, how, *args, **kwargs):
 
         return self._wrap_result(result)
 
+    def _groupby_and_apply(self, how, *args, **kwargs):
+        """
+        Re-evaluate the obj with a groupby aggregation.
+        """
+        grouper = self._grouper
+
+        # Excludes `on` column when provided
+        obj = self._obj_with_exclusions
+
+        grouped = get_groupby(obj, by=None, grouper=grouper, group_keys=self.group_keys)
+
+        try:
+            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._agg_for_resample(func)
+            else:
+                result = grouped._agg_for_resample(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
+
+            # test_apply_to_one_column_of_df the function being applied references
+            #  a DataFrame column, but aggregate_item_by_item operates column-wise
+            #  on Series, raising AttributeError or KeyError
+            #  (depending on whether the column lookup uses getattr/__getitem__)
+            result = _apply(
+                grouped, how, *args, include_groups=self.include_groups, **kwargs
+            )
+
+        except ValueError as err:
+            if "Must produce aggregated value" in str(err):
+                # raised in _aggregate_named
+                # see test_apply_without_aggregation, test_apply_with_mutated_index
+                pass
+            else:
+                raise
+
+            # we have a non-reducing function
+            # try to evaluate
+            result = _apply(
+                grouped, how, *args, include_groups=self.include_groups, **kwargs
+            )
+
+        return self._wrap_result(result)
+
     @final
     def _get_resampler_for_grouping(
         self, groupby: GroupBy, key, include_groups: bool = True
@@ -1530,6 +1592,7 @@ def func(x):
     _upsample = _apply
     _downsample = _apply
     _groupby_and_aggregate = _apply
+    _groupby_and_apply = _apply
 
     @final
     def _gotitem(self, key, ndim, subset=None):

pandas/core/reshape/pivot.py

@@ -442,7 +442,7 @@ def _generate_marginal_results_without_values(
     margins_name: Hashable = "All",
 ):
     margin_keys: list | Index
-    if len(cols) > 0:
+    if len(table.columns) > 0:
         # need to "interleave" the margins
         margin_keys = []