code: def get_column_index(self, header):
          
          try:
              index = self._column_headers.index(header)
              return index
          except ValueError:
              raise_suppressed(KeyError(("'{}' is not a header for any "
                                         "column").format(header)))
docs: Get index of a column from it's header.
  
          Parameters
          ----------
          header: str
              header of the column.
  
          Raises
          ------
          ValueError:
              If no column could be found corresponding to `header`.