python dataframe基于另一列提取对应列值

In [2]: df
Out[2]:
A B
0 p1 1
1 p1 2
2 p3 3
3 p2 4

In [3]: df.loc[df['B'] == 3, 'A']
Out[3]:
2 p3
Name: A, dtype: object

In [4]: df.loc[df['B'] == 3, 'A'].iloc[0]
Out[4]: 'p3'