code: def get_years(months=0, refer=None):
      
      if refer is None:
          refer = datetime.now()
      # 计算当前总月份数
      try:
          months_count = refer.year * 12 + refer.month
      except Exception:
          raise TypeError('refer except {}, got an {}'.format(type(datetime.now()), type(refer)))
      
      # 计算结果总月分数
      months_count += months
      
      y, m = divmod(months_count, 12)
      
      # 将m的值转换为1-12
      if m == 0:
          y -= 1
          m = 12
      
      return ''.join(['%04d' % y, '%02d' % m])
docs: 获取基准时月份增量的年月
  
      :param:
          * months: (int) 月份增量，正数为往后年月，整数为往前年月
          * refer: (datetime obj) datetime 对象，或者有 month 和 year 属性的实例，默认为当前时间
      :return:
          * result: (string) 年月字符串
  
      举例如下::
  
          print('--- get_years demo ---')
          print(get_years(-5))
          print(get_years(7, datetime.now()))
          print('---')
  
      执行结果::
  
          --- get_years demo ---
          201801
          201901
          ---