作为对象构造函数的一部分，我想按元素减去两个 pandas 数据帧的值：
self.dfload=pd.read_csv(self.name +'/' + 'load.csv')
self.dfload.set_index('snapshot', inplace=True)
load_1 load_2 load_3 load_4
snapshot
2018-01-01 00:00:00 68.248569 91.998188 64.988923 139.535086
2018-01-01 00:15:00 138.274243 127.186259 80.769227 33.509007
2018-01-01 00:30:00 129.824298 56.706114 75.234845 138.610287
2018-01-01 00:45:00 51.754610 45.703056 73.060490 36.913774
2018-01-01 01:00:00 52.129775 139.315283 67.093788 60.488806
和
self.dfsupply=pd.read_csv(self.name + '/' + 'supply.csv')
self.dfsupply.set_index('snapshot', inplace=True)
supply_1 supply_2 supply_3 supply_4
snapshot
2018-01-01 00:00:00 28.448017 45.383377 56.626144 40.044848
2018-01-01 00:15:00 37.534878 29.094980 67.722537 15.002448
2018-01-01 00:30:00 46.163805 28.324557 26.322953 23.250904
2018-01-01 00:45:00 48.192774 55.049855 72.872200 …