code: def swaplevel(self, i=-2, j=-1, copy=True):
          
          new_index = self.index.swaplevel(i, j)
          return self._constructor(self._values, index=new_index,
                                   copy=copy).__finalize__(self)
docs: Swap levels i and j in a MultiIndex.
  
          Parameters
          ----------
          i, j : int, str (can be mixed)
              Level of index to be swapped. Can pass level name as string.
  
          Returns
          -------
          Series
              Series with levels swapped in MultiIndex.
  
          .. versionchanged:: 0.18.1
  
             The indexes ``i`` and ``j`` are now optional, and default to
             the two innermost levels of the index.