How to Use xs for Cross Section in pandas DataFrames

Use the xs() method in pandas to select a cross section from a DataFrame by specifying a label along a particular axis. It works well for selecting rows or columns by label, especially in multi-index DataFrames.

Syntax

The xs() method syntax is:

• DataFrame.xs(key, axis=0, level=None, drop_level=True)

Where:

• key: The label to select.
• axis: 0 for rows (default), 1 for columns.
• level: For MultiIndex, specify which level to select from.
• drop_level: Whether to drop the level from the result (default True).

python

df.xs(key, axis=0, level=None, drop_level=True)

Example

This example shows how to use xs() to select a row by label and a cross section from a MultiIndex DataFrame.

python

import pandas as pd

# Simple DataFrame
data = {'A': [1, 2, 3], 'B': [4, 5, 6]}
df = pd.DataFrame(data, index=['x', 'y', 'z'])

# Select row 'y'
row_y = df.xs('y')

# MultiIndex DataFrame
arrays = [['bar', 'bar', 'baz', 'baz'], ['one', 'two', 'one', 'two']]
index = pd.MultiIndex.from_arrays(arrays, names=['first', 'second'])
df_multi = pd.DataFrame({'A': [1, 2, 3, 4], 'B': [5, 6, 7, 8]}, index=index)

# Select cross section where first level is 'bar'
cross_section = df_multi.xs('bar', level='first')

print('Single index selection:\n', row_y)
print('\nMultiIndex cross section:\n', cross_section)

Output

Single index selection:
 A    2
B    5
Name: y, dtype: int64

MultiIndex cross section:
        A  B
second      
one   1  5
two   2  6

Common Pitfalls

Common mistakes when using xs() include:

• Not specifying the level parameter for MultiIndex, which can cause errors or unexpected results.
• Using axis=1 incorrectly when trying to select columns by label.
• Forgetting that xs() returns a reduced dimension, so the result may be a Series instead of a DataFrame.

Example of a wrong and right usage:

python

import pandas as pd

# MultiIndex DataFrame
arrays = [['a', 'a', 'b', 'b'], ['x', 'y', 'x', 'y']]
index = pd.MultiIndex.from_arrays(arrays, names=['level1', 'level2'])
df = pd.DataFrame({'val': [10, 20, 30, 40]}, index=index)

# Wrong: missing level parameter
try:
    wrong = df.xs('a')
except Exception as e:
    print('Error:', e)

# Right: specify level
right = df.xs('a', level='level1')
print('\nCorrect cross section:\n', right)

Output

Error: 'a' is not in list

Correct cross section:
         val
level2      
x        10
y        20

Quick Reference

Parameter	Description	Default
key	Label to select	Required
axis	0 for rows, 1 for columns	0
level	Level name or number for MultiIndex	None
drop_level	Drop the level from result	True

Key Takeaways

Use xs() to select a cross section by label from rows or columns in a DataFrame.

Specify the level parameter when working with MultiIndex to avoid errors.

xs() returns a reduced dimension, often a Series when selecting a single label.

Use axis=1 to select columns by label with xs().

Common errors come from missing level or wrong axis parameters.