code: def combine_first(self, other):
          
          new_index = self.index.union(other.index)
          this = self.reindex(new_index, copy=False)
          other = other.reindex(new_index, copy=False)
          if is_datetimelike(this) and not is_datetimelike(other):
              other = to_datetime(other)
  
          return this.where(notna(this), other)
docs: Combine Series values, choosing the calling Series's values first.
  
          Parameters
          ----------
          other : Series
              The value(s) to be combined with the `Series`.
  
          Returns
          -------
          Series
              The result of combining the Series with the other object.
  
          See Also
          --------
          Series.combine : Perform elementwise operation on two Series
              using a given function.
  
          Notes
          -----
          Result index will be the union of the two indexes.
  
          Examples
          --------
          >>> s1 = pd.Series([1, np.nan])
          >>> s2 = pd.Series([3, 4])
          >>> s1.combine_first(s2)
          0    1.0
          1    4.0
          dtype: float64