python - Pandas :根据其他列和值本身更改单元格值

我有以下熊猫df：

category1   category2
A           B
A           C
B           NaN
A           NaN

如果要满足以下条件，我想将category2中的值更改为D：


category1 == A
category2 == NaN


因此，我的预期输出将是：

category1   category2
A           B
A           C
B           NaN
A           D

我尝试了两种方法：

df.loc[((df.category1 == "A") & (df.category2 == 'nan')), "category2"] = "D" # doesn't change anything

和

import numpy as np
df['category2'] = np.where(((df['category1'] == 'A') & (df['category2'] == "")), "D")
# ValueError: either both or neither of x and y should be given

为什么这两条线都不起作用？

最佳答案

使用以下内容：

df.loc[df.category2.isna()&df.category1.eq('A'),'category2']='D'
print(df)

  category1 category2
0         A         B
1         A         C
2         B       NaN
3         A         D