使用 .xs 选择横截面

In [1]:
import pandas as pd
import numpy as np
arrays = [['bar', 'bar', 'baz', 'baz', 'foo', 'foo', 'qux', 'qux'],
          ['one', 'two', 'one', 'two', 'one', 'two', 'one', 'two']]
idx_row = pd.MultiIndex.from_arrays(arrays, names=['Row_First', 'Row_Second'])
idx_col = pd.MultiIndex.from_product([['A','B'], ['i', 'ii']], names=['Col_First','Col_Second'])
df = pd.DataFrame(np.random.randn(8,4), index=idx_row, columns=idx_col)

Out[1]:
Col_First                    A                   B          
Col_Second                   i        ii         i        ii
Row_First Row_Second                                        
bar       one        -0.452982 -1.872641  0.248450 -0.319433
          two        -0.460388 -0.136089 -0.408048  0.998774
baz       one         0.358206 -0.319344 -2.052081 -0.424957
          two        -0.823811 -0.302336  1.158968  0.272881
foo       one        -0.098048 -0.799666  0.969043 -0.595635
          two        -0.358485  0.412011 -0.667167  1.010457
qux       one         1.176911  1.578676  0.350719  0.093351
          two         0.241956  1.082138 -0.516898 -0.196605

.xs 接受 level（所述级别的名称或整数），axis：0 表示行，1 表示列。

.xs 适用于 pandas.Series 和 pandas.DataFrame。

行选择：

In [2]: df.xs('two', level='Row_Second', axis=0)
Out[2]:  
Col_First          A                   B          
Col_Second         i        ii         i        ii
Row_First                                         
bar        -0.460388 -0.136089 -0.408048  0.998774
baz        -0.823811 -0.302336  1.158968  0.272881
foo        -0.358485  0.412011 -0.667167  1.010457
qux         0.241956  1.082138 -0.516898 -0.196605

列上的选择：

In [3]: df.xs('ii', level=1, axis=1)
Out[3]:
Col_First                    A         B
Row_First Row_Second                    
bar       one        -1.872641 -0.319433
          two        -0.136089  0.998774
baz       one        -0.319344 -0.424957
          two        -0.302336  0.272881
foo       one        -0.799666 -0.595635
          two         0.412011  1.010457
qux       one         1.578676  0.093351
          two         1.082138 -0.196605

.xs 仅适用于选择，分配是不可能的（获取，而不是设置）：¨

In [4]: df.xs('ii', level='Col_Second', axis=1) = 0
  File "<ipython-input-10-92e0785187ba>", line 1
    df.xs('ii', level='Col_Second', axis=1) = 0
                                               ^
SyntaxError: can't assign to function call