Combining Multi-Index Data Frames on Certain Index

In this article, we will explore how to combine multi-index data frames in pandas. We will first look at an example of what the problem is and then discuss possible solutions.

Problem Statement

We have a list of multi-index data frames, each with its own index. The index levels are named ‘0’, ‘1’, and so on. For this article, we’ll assume that the only level that changes between data frames is the ‘0’ level. We want to combine these data frames, but we only want to combine them based on the ‘0’ level.

Example Data Frames

import pandas as pd
import numpy as np

# Create a random multi-index data frame
df1 = pd.DataFrame(np.random.randn(4, 4), 
                      index=[np.array(['bar', 'baz', 'foo', 'qux']), np.array(['one','one','one','one'])])

# Create another random multi-index data frame
df2 = pd.DataFrame(np.random.randn(4, 4), 
                      index=[np.array(['bar', 'baz', 'foo', 'qux']), np.array(['two','two','two','two'])])

# List of the two data frames
l = [df1, df2]

We can see that df1 and df2 both have a different value for the first element in their second index.

Solution

One way to combine these data frames is by using the concat function along with the sort_index function. Here’s an example:

# Concatenate the two data frames
combined_df = pd.concat([df1, df2]).sort_index(level=0,axis=0)

# Print the combined data frame
print(combined_df)

However, this will not give us the desired output because concat and sort_index do not know that we want to combine based on only the ‘0’ level.

Another solution is by using the join function with a multi-index:

# Join the two data frames based on their first index
combined_df = df1.join(df2, lsuffix='_df1', suffix='_df2')

# Print the combined data frame
print(combined_df)

In this solution, we are telling pandas to join the ‘0’ level of df1 with the ‘0’ level of df2. This will give us the desired output.

Conclusion

We have seen that combining multi-index data frames in pandas is a bit tricky. By using either the concat function along with the sort_index function or the join function, we can combine these data frames based on only certain levels of their index.

Last modified on 2025-03-20