先从原dataframe取出一个子dataframe，然后再对其中的元素赋值，例如
s = d[d['col_1'] == 0]
s.loc[:, 'col_2'] = 1就会出现报错:
SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead
解决方法:
- 使用推荐的 .loc[row_indexer,col_indexer] = value
- 如果不知道，就先copy，再赋值。
s = d[d['col_1'] == 0].copy()
s.loc[:, 'col_2'] = 1