Returns the super key value for the row.

def KeyValue(self, row=None):
        """Returns the super key value for the row."""
        if not row:
            if self._iterator:
                # If we are inside an iterator use current row iteration.
                row = self[self._iterator]
            else:
                row = self.row
        # If no superkey then use row number.
        if not self.superkey:
            return ["%s" % row.row]

        sorted_list = []
        for header in self.header:
            if header in self.superkey:
                sorted_list.append(row[header])
        return sorted_list