Pandas DataFrame Reindex Method

Introduction

In this tutorial, we will learn about the Python pandas DataFrame.reindex() method. We will explore how to change the index and columns of a DataFrame using this method. The DataFrame.reindex() method allows us to reconcile the DataFrame with a new index by filling null values in the locations that have no value in the previous index.

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Skills Graph

Import the pandas library and create a DataFrame

Let's start by importing the pandas library and creating a DataFrame. We will use this DataFrame to demonstrate the DataFrame.reindex() method.

import pandas as pd

## Create a DataFrame
df = pd.DataFrame([[1, 6, 2], [3, 4, 6], [12, 1, 0]], columns=['A', 'B', 'C'], index=['index_1', 'index_2', 'index_3'])

Reindex the DataFrame using the index parameter

To reindex the DataFrame by changing the index, pass a list of new index labels to the DataFrame.reindex() method. Any index labels that are not present in the original DataFrame will be filled with NaN values.

## Reindex the DataFrame with a new index
new_index = ['index_1', 'index_2', 'index_4']
reindexed_df = df.reindex(index=new_index)

print(reindexed_df)

Output:

         A    B    C
index_1  1.0  6.0  2.0
index_2  3.0  4.0  6.0
index_4  NaN  NaN  NaN

Reindex the DataFrame using the columns parameter

Similarly, you can reindex the DataFrame by changing the columns using the DataFrame.reindex() method. Provide a list of new column labels to the columns parameter. Any columns that are not present in the original DataFrame will be filled with NaN values.

## Reindex the DataFrame with new columns
new_columns = ['A', 'C', 'D']
reindexed_df = df.reindex(columns=new_columns)

print(reindexed_df)

Output:

         A    C    D
index_1  1.0  2.0  NaN
index_2  3.0  6.0  NaN
index_3  12.0  0.0  NaN

Fill null values using the fill_value parameter

If you want to fill the null values with a specific value, you can use the fill_value parameter of the DataFrame.reindex() method. Provide the desired value to be used for filling null values.

## Reindex the DataFrame and fill null values with 2
new_index = ['index_1', 'index_2', 'index_4']
reindexed_df = df.reindex(index=new_index, fill_value=2)

print(reindexed_df)

Output:

         A  B  C
index_1  1  6  2
index_2  3  4  6
index_4  2  2  2

Summary

In this tutorial, we learned how to reindex a DataFrame using the DataFrame.reindex() method in pandas. We explored how to change the index and columns of a DataFrame, fill null values, and specify a fill value for the null values. Knowing how to properly reindex a DataFrame is useful for manipulating and aligning data in pandas.