default_index inherits index.name

Author: minggli

pandas/core/frame.py

@@ -4198,7 +4198,7 @@ def _maybe_casted_values(index, labels=None):
                             values, mask, np.nan)
             return values
 
-        new_index = ibase.default_index(len(new_obj))
+        new_index = ibase.default_index(len(new_obj), name=self.index.name)
         if level is not None:
             if not isinstance(level, (tuple, list)):
                 level = [level]