A detailed guide to pandas.Series.reindex() method

Updated: February 18, 2024 By: Guest Contributor Post a comment


Pandas is one of the most popular data manipulation and analysis libraries for Python, known for its powerful and flexible data structures. This tutorial focuses on one of the library’s more subtle yet powerful methods: Series.reindex(). Whether you’re reshuffling your data, aligning it with another series, or preparing it for further analysis, understanding how to properly use the reindex() method is crucial. This guide walks you through the basics to more advanced uses of Series.reindex(), complete with multiple code examples.

Introduction to pandas.Series.reindex()

The reindex() method in pandas allows for the conforming of a Series to a new index. This can involve changing the order of the existing data, introducing missing values for new index labels not previously in the series, or even filling in missing values with a specified method.

Basic Usage

Let’s start with a basic example:

import pandas as pd 

# Creating a series
s = pd.Series([10, 20, 30, 40], index=['a', 'b', 'c', 'd'])

# Reindexing
new_s = s.reindex(['d', 'c', 'b', 'a'])

# Output

This simple reordering demonstrates the straightforward use of reindexing. Here’s what you see when you run this script:

d    40
c    30
b    20
a    10
dtype: int64

Introducing Missing Values

Expanding the index introduces null values where data wasn’t previously:

import pandas as pd

# Creating a series
s = pd.Series([10, 20, 30, 40], index=['a', 'b', 'c', 'd'])

# Expanding the index
s_expanded = s.reindex(['a', 'b', 'c', 'd', 'e', 'f'])

# Output shows missing values for 'e' and 'f'

Missing values (NaN) for the new indexes ‘e’ and ‘f’ clearly demonstrate the flexibility of reindex(), especially in preparing data for analysis where indexes represent specific entities or time periods that may be incomplete.

Filling Missing Values

Pandas not only allows introducing missing values but also offers methods to fill these. This is particularly useful in time-series data. For filling in missing values while reindexing, we can use the fill_value argument.

import pandas as pd

# Example with fill_value
s_filled = s.reindex(['a', 'b', 'c', 'd', 'e', 'f'], fill_value=0)

# Observing filled values for 'e' and 'f'

In the above code, the missing values for new indexes ‘e’ and ‘f’ are filled with 0. This can be incredibly useful for maintaining data integrity and avoiding errors in analysis caused by missing data.

Advanced Indexing: Using Reindex with a method

For sequential data, like time series, reindex() can also be paired with a method argument to interpolate missing data. Let’s explore using the ‘ffill’ method (forward fill), where missing values are filled with the last observed point.

import pandas as pd

# Series with missing dates
dates = pd.date_range('2021-01-01', periods=5)
s = pd.Series([1, None, 3, None, 5], index=dates)

# Reindex with forward fill
s_reindexed = s.reindex(dates, method='ffill')

# Display the result

This method ensures that gaps in data are filled sensibly, given the sequential nature of the data. Forward fill is just one of the methods available; ‘bfill’ (backward fill) is another option that fills missing values with the next known value.


In conclusion, the Series.reindex() method in pandas is a powerful tool for data manipulation, offering versatility in handling data index changes. From basic reordering to advanced techniques for dealing with missing data, understanding how to utilize this function effectively can enhance your data analysis and manipulation capabilities significantly. As demonstrated, the application of reindex() caters to a wide range of scenarios, making it an invaluable part of any data scientist’s arsenal.