code: def combine_first(self, other):
          
          if isinstance(other, SparseSeries):
              other = other.to_dense()
  
          dense_combined = self.to_dense().combine_first(other)
          return dense_combined.to_sparse(fill_value=self.fill_value)
docs: Combine Series values, choosing the calling Series's values
          first. Result index will be the union of the two indexes
  
          Parameters
          ----------
          other : Series
  
          Returns
          -------
          y : Series