熊猫：根据索引列获取值

WeN*_*Ben 5

在你的情况下

df['new'] = df.val
df.loc[df.new.isna(),'new'] = df.loc[df.parent.dropna().values,'val'].values
df
Out[289]: 
   val  parent  new
0  0.1     NaN  0.1
1  0.2     NaN  0.2
2  0.3     NaN  0.3
3  NaN     0.0  0.1
4  NaN     2.0  0.3

或尝试fillna用replace

df['new'] = df.val.fillna(df.parent.replace(df.val))
Out[290]: 
0    0.1
1    0.2
2    0.3
3    0.1
4    0.3
Name: val, dtype: float64