Use RangeIndex placeholder for DataFrame.reduce

Author: mroeschke

pandas/core/frame.py

@@ -12180,6 +12180,7 @@ def _get_data() -> DataFrame:
         #  simple case where we can use BlockManager.reduce
         res = df._mgr.reduce(blk_func)
         out = df._constructor_from_mgr(res, axes=res.axes).iloc[0]
+        out.name = None
         if out_dtype is not None and out.dtype != "boolean":
             out = out.astype(out_dtype)
         elif (df._mgr.get_dtypes() == object).any() and name not in ["any", "all"]:

pandas/core/internals/managers.py

@@ -1656,7 +1656,7 @@ def reduce(self, func: Callable) -> Self:
             nbs = blk.reduce(func)
             res_blocks.extend(nbs)
 
-        index = Index([None])  # placeholder
+        index = default_index(1)  # placeholder
         new_mgr = type(self).from_blocks(res_blocks, [self.items, index])
         return new_mgr