按行值筛选pandas数据帧

我知道如何按列值筛选数据帧:

import pandas as pd
import numpy as np
from numpy.random import randn
np.random.seed(101)
df = pd.DataFrame(randn(5,4),index='A B C D E'.split(),columns='W X Y Z'.split())
print(df)
# show only rows where 'W' is positive
# here, the row for 'C' will be deleted, since df['W']['C']<0
df[df['W']>0]

但如何按行值筛选,例如“b”>0?

由于df['x']['b']<=0和df['y']['b']<=0,我想删除列x和y。我尝试了以下代码,但它报告了一个错误:

df.loc[df.loc['B']>0]