CLN: Don't modify state in FrameApply.agg (#40428)

Author: rhshadrach

pandas/core/apply.py

@@ -633,28 +633,28 @@ 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()
+            if axis == 1:
+                result = FrameRowApply(
+                    obj.T,
+                    self.orig_f,
+                    self.raw,
+                    self.result_type,
+                    self.args,
+                    self.kwargs,
+                ).agg()
+                result = result.T if result is not None else result
+            else:
+                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)
+            result = obj.apply(self.orig_f, axis, args=self.args, **self.kwargs)
 
         return result