Pandas: Find the most frequent value in each group of a DataFrame

Updated: February 23, 2024 By: Guest Contributor Post a comment


Pandas is a Python library that provides extensive means for data analysis. Data scientists and analysts use it extensively to manipulate large datasets and make sense of them. One common task when working with datasets is grouping data based on some criteria and then finding the most frequent occurrence (mode) within each group. This tutorial will guide you through different methods to accomplish this task, starting from basic to more advanced techniques.

Getting Started with GroupBy

Before diving into finding the most frequent values, let’s understand the groupby operation. Grouping in pandas is akin to the SQL group by statement. It involves some combination of splitting the object, applying a function, and combining the results. Here’s a basic example to demonstrate grouping:

import pandas as pd

# Sample DataFrame
data = {
    'A': ['foo', 'bar', 'foo', 'bar',
         'foo', 'bar', 'foo', 'foo'],
    'B': ['one', 'one', 'two', 'three',
         'two', 'two', 'one', 'three'],
    'C': np.random.randn(8),
    'D': np.random.randn(8)
df = pd.DataFrame(data)

# Group by column 'A'
grouped = df.groupby('A')

Now that we have grouped our DataFrame, let’s move on to finding the most frequent value in each group.

Finding the Most Frequent Value

The simplest method to find the most frequent value or mode in each group is using the agg (aggregate) function along with the pd.Series.mode as follows:

freq_vals = grouped.agg(lambda x: x.mode().values[0])

This code returns the first mode of each column for the grouped object. Note that if a group has more than one mode, this method will only return the first one.

Dealing with Multiple Modes

When a group has multiple modes, you might want to list all of them. Here’s how you can extract all modes:

all_modes = grouped.agg(lambda x: list(x.mode()))

This approach will give you a list of all modes for each column in the group. It’s quite handy for more detailed analysis.

Advanced Grouping with Custom Functions

Let’s move to more advanced scenarios. Suppose you want more control over how the frequent values are selected, especially when dealing with numeric data. Here’s how you can use custom functions to find the most frequent value in a specific way:

def most_frequent(x):
    values, counts = np.unique(x, return_counts=True)
    max_count_index = np.argmax(counts)
    return values[max_count_index]

advanced_freq_vals = grouped.agg(most_frequent)

This function calculates the frequency of each unique value using np.unique, then selects the value with the highest frequency. This method gives you full control over the aggregation process and can be particularly useful in complex datasets.

Visualizing Frequent Values

Visualizing the results can often provide more insight into the data. Here’s a basic example using the matplotlib and seaborn libraries to plot the frequency of the most common value within each group:

import matplotlib.pyplot as plt
import seaborn as sns

df['MostFrequent'] = df.groupby('A')['B'].transform(lambda x: x.mode()[0])
sns.countplot(x='MostFrequent', data=df)

This will create a plot showing the frequency of the most common value in the ‘B’ column grouped by the ‘A’ column. Such visualizations can be incredibly informative in understanding the distribution of modes in your dataset.


Finding the most frequent value in each group of a DataFrame is a common task that can be solved in various ways using pandas. Starting from basic groupby and aggregate functions to more complex custom aggregations, pandas provides a robust set of tools for handling this analysis. Understanding these tools and methods is crucial for anyone looking to perform data analysis with Python. With practice, these techniques will become an indispensable part of your data manipulation toolkit.