pandas对指定列进行填充的方法

> import pandas as pd
> import numpy as np
> ts1 = [0, 1, np.nan, np.nan, np.nan, np.nan]
> ts2 = [0, 2, np.nan, 3, np.nan, np.nan]
> d = {'X': ts1, 'Y': ts2, 'Z': ts2}
> df = pd.DataFrame(data=d)
> df
   X  Y  Z
0 0.0 0.0 0.0
1 1.0 2.0 2.0
2 NaN NaN NaN
3 NaN 3.0 3.0
4 NaN NaN NaN
5 NaN NaN NaN
> col =['X','Y']
> df[col] = df[col].ffill()
> df
   X  Y  Z
0 0.0 0.0 0.0
1 1.0 2.0 2.0
2 1.0 2.0 NaN
3 1.0 3.0 3.0
4 1.0 3.0 NaN
5 1.0 3.0 NaN
>