code: def _convert_to_style(cls, style_dict, num_format_str=None):
          
          import xlwt
  
          if style_dict:
              xlwt_stylestr = cls._style_to_xlwt(style_dict)
              style = xlwt.easyxf(xlwt_stylestr, field_sep=',', line_sep=';')
          else:
              style = xlwt.XFStyle()
          if num_format_str is not None:
              style.num_format_str = num_format_str
  
          return style
docs: converts a style_dict to an xlwt style object
          Parameters
          ----------
          style_dict : style dictionary to convert
          num_format_str : optional number format string