pandas 数据处理遇到的问题

数据为DataFrame格式，如下：

1.对每一行，FirstCab的值为空时，Weight的值乘以0.8

方法一（可行）：df.loc[df[‘FirstCab‘].isnull(),‘Weight‘] *= 0.8

方法二（可行）：df[‘Weight‘] = np.where(df[‘FirstCab‘].isnull(),df[‘Weight‘]*0.8,df[‘Weight‘])

方法三（不可行）：df[df[‘FirstCab‘].isnull()][‘Weight‘]  *= 0.8 或者 df.loc[df[‘FirstCab‘].isnull(),:][‘Weight‘]  *= 0.8

错误提示：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.

方法四（不可行）：df.assign(Weight=lambda x: x[‘Weight‘]*0.8 if x[‘FirstCab‘].isnull() else x[‘Weight‘])

　　　　　　　　或者df.assign(Weight=lambda x: x[‘Weight‘]*0.8 if x[‘FirstCab‘] == np.nan else x[‘Weight‘])

错误提示：ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().