code: def _open_sheet(self):
          
          if self.sheet_name and not self.header:
              self._sheet = self._file.worksheet(self.sheet_name.title)
              self.ncols = self._sheet.col_count
              self.nrows = self._sheet.row_count
              for i in range(1, self.ncols+1):
                  self.header = self.header + [self._sheet.cell(1, i).value]
docs: Read the sheet, get value the header, get number columns and rows
          :return: