code: def fillna(self, column_name, value):
          
          # Normal error checking
          if type(column_name) is not str:
              raise TypeError("column_name must be a str")
          ret = self[self.column_names()]
          ret[column_name] = ret[column_name].fillna(value)
          return ret
docs: Fill all missing values with a given value in a given column. If the
          ``value`` is not the same type as the values in ``column_name``, this method
          attempts to convert the value to the original column's type. If this
          fails, an error is raised.
  
          Parameters
          ----------
          column_name : str
              The name of the column to modify.
  
          value : type convertible to SArray's type
              The value used to replace all missing values.
  
          Returns
          -------
          out : SFrame
              A new SFrame with the specified value in place of missing values.
  
          See Also
          --------
          dropna
  
          Examples
          --------
          >>> sf = turicreate.SFrame({'a':[1, None, None],
          ...                       'b':['13.1', '17.2', None]})
          >>> sf = sf.fillna('a', 0)
          >>> sf
          +---+------+
          | a |  b   |
          +---+------+
          | 1 | 13.1 |
          | 0 | 17.2 |
          | 0 | None |
          +---+------+
          [3 rows x 2 columns]