code: def save_book(self, f_buf=None):
          
          if f_buf is None:
              f_buf = StringIO.StringIO()
          f_buf.write(openpyxl.writer.excel.save_virtual_workbook(self.book))
          f_buf.seek(0)
          return f_buf
docs: Return a file buffer containing the resulting xls
  
          :param obj f_buf: A file buffer supporting the write and seek
          methods