REF: Move the rest of DataFrame.agg into apply (pandas-dev#39955)

pandas/core/apply.py

@@ -151,6 +151,7 @@ def f(x):
         else:
             f = func
 
+        self.orig_f: AggFuncType = func
         self.f: AggFuncType = f
 
     @property
@@ -527,6 +528,35 @@ def apply(self) -> FrameOrSeriesUnion:
 
         return self.apply_standard()
 
+    def agg(self):
+        obj = self.obj
+        axis = self.axis
+
+        # TODO: Avoid having to change state
+        self.obj = self.obj if self.axis == 0 else self.obj.T
+        self.axis = 0
+
+        result = None
+        try:
+            result = super().agg()
+        except TypeError as err:
+            exc = TypeError(
+                "DataFrame constructor called with "
+                f"incompatible data and dtype: {err}"
+            )
+            raise exc from err
+        finally:
+            self.obj = obj
+            self.axis = axis
+
+        if axis == 1:
+            result = result.T if result is not None else result
+
+        if result is None:
+            result = self.obj.apply(self.orig_f, axis, args=self.args, **self.kwargs)
+
+        return result
+
     def apply_empty_result(self):
         """
         we have an empty result; at least 1 axis is 0

pandas/core/frame.py

@@ -7728,21 +7728,14 @@ def _gotitem(
         examples=_agg_examples_doc,
     )
     def aggregate(self, func=None, axis: Axis = 0, *args, **kwargs):
+        from pandas.core.apply import frame_apply
+
         axis = self._get_axis_number(axis)
 
         relabeling, func, columns, order = reconstruct_func(func, **kwargs)
 
-        result = None
-        try:
-            result = self._aggregate(func, axis, *args, **kwargs)
-        except TypeError as err:
-            exc = TypeError(
-                "DataFrame constructor called with "
-                f"incompatible data and dtype: {err}"
-            )
-            raise exc from err
-        if result is None:
-            return self.apply(func, axis=axis, args=args, **kwargs)
+        op = frame_apply(self, func=func, axis=axis, args=args, kwargs=kwargs)
+        result = op.agg()
 
         if relabeling:
             # This is to keep the order to columns occurrence unchanged, and also
@@ -7758,25 +7751,6 @@ def aggregate(self, func=None, axis: Axis = 0, *args, **kwargs):
 
         return result
 
-    def _aggregate(self, arg, axis: Axis = 0, *args, **kwargs):
-        from pandas.core.apply import frame_apply
-
-        op = frame_apply(
-            self if axis == 0 else self.T,
-            func=arg,
-            axis=0,
-            args=args,
-            kwargs=kwargs,
-        )
-        result = op.agg()
-
-        if axis == 1:
-            # NDFrame.aggregate returns a tuple, and we need to transpose
-            # only result
-            result = result.T if result is not None else result
-
-        return result
-
     agg = aggregate
 
     @doc(