Pandas DataFrame sort_index() Method

import pandas as pd 

df = pd.DataFrame({'VAL': [10, 20, 30]}, index=['b', 'c', 'a'])
print(df.sort_index())

Expected Output:

   VAL
a   30
b   10
c   20

1. Overview and Syntax

The DataFrame.sort_index() method sorts DataFrame objects by row index labels or column names. It is essential after operations like GroupBy, slicing, or appending that leave index labels unaligned or unsorted.

2. Descending Sort & Column Sorting (`axis=1`)

Sort row labels descending using ascending=False, or sort column names alphabetically across columns using axis=1.

import pandas as pd 

df_data = pd.DataFrame({'Z_COL': [1, 2], 'A_COL': [3, 4]})

# Alphabetize column headers
df_sorted_cols = df_data.sort_index(axis=1)
print(df_sorted_cols)

3. Multi-Index Sorting

In MultiIndex DataFrames, sort_index() sorts hierarchically across index levels.

import pandas as pd 

tuples = [('b', 2), ('a', 2), ('a', 1)]
index = pd.MultiIndex.from_tuples(tuples, names=['LETTER', 'NUM'])
df_multi = pd.DataFrame({'SCORE': [10, 20, 30]}, index=index)

print(df_multi.sort_index())

Expected Output:

            SCORE
LETTER NUM       
a      1       30
       2       20
b      2       10

4. Frequently Asked Questions (FAQs)

Q: What is the difference between sort_values() and sort_index()?
sort_values() sorts DataFrames based on data values inside specified columns. sort_index() sorts based on row index labels or column headers.

Q: Does sort_index() alter the original DataFrame in-place?
No, by default it returns a sorted copy. Pass inplace=True to modify the original object directly.

5. Common Errors & Troubleshooting

Interactive Companion Jupyter Notebook

Explore interactive code cells, dynamically generate classes, test type vs isinstance comparisons, and run self-validating assertions.

View on GitHub

6. Related Tutorials

• Pandas sort_values() Method - Sort DataFrames by values inside columns.
• Pandas set_index() Method - Set existing columns as DataFrame index.
• Pandas DataFrame Methods Directory - Searchable directory of DataFrame operations.

Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

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