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How to groupby in Pandas

Understanding GroupBy in Pandas

Imagine you're at a farmer's market, and you've got a basket full of different kinds of fruits. To make sense of what you have, you start sorting them out. You put all the apples together, all the oranges together, and so on. This is essentially what the groupby operation in Pandas allows you to do with your data.

Pandas is a powerful Python library that provides easy-to-use data structures and data analysis tools. One of the key functions in Pandas is groupby, which enables you to organize and summarize data in a meaningful way.

What is GroupBy?

In the Pandas context, groupby refers to a process involving one or more of the following steps:

  1. Splitting the data into groups based on some criteria.
  2. Applying a function to each group independently.
  3. Combining the results into a data structure.

The analogy of sorting fruits is similar to the splitting step. When you're applying a function, it's like deciding what to do with each type of fruit (maybe you want to count them, or find the heaviest one). Finally, combining the results is akin to putting these insights into a basket labeled with summaries like "15 apples" or "heaviest orange: 250 grams".

How to Use GroupBy in Pandas

Let's dive into some actual code examples to see how this works in practice. We'll start with a simple dataset that we'll create using Pandas. This dataset will have two columns: 'Fruit' and 'Weight'.

import pandas as pd

# Create a simple dataset
data = {
    'Fruit': ['Apple', 'Orange', 'Banana', 'Apple', 'Banana', 'Orange'],
    'Weight': [150, 250, 100, 130, 90, 260]

df = pd.DataFrame(data)

This will give us the following DataFrame:

    Fruit  Weight
0   Apple     150
1  Orange     250
2  Banana     100
3   Apple     130
4  Banana      90
5  Orange     260

Grouping Data

Now, let's group this data by the 'Fruit' column.

grouped = df.groupby('Fruit')

What we have now is not a DataFrame, but a DataFrameGroupBy object. This object is ready for us to apply a function to each of the groups.

Applying Functions

To get a sense of what we can do with our grouped data, let's apply the sum function to combine the weights of the same fruits.

grouped_sum = grouped.sum()

The output will be:

Apple      280
Banana     190
Orange     510

We can see that the weights of the apples and bananas have been added together. This is the applying step.

Other Aggregate Functions

The sum function is just one example of an aggregate function that can be applied to grouped data. Others include:

  • mean: Calculates the average of a group.
  • max: Finds the maximum value in each group.
  • min: Finds the minimum value in each group.
  • count: Counts the number of occurrences in each group.

Let's try the mean function to find the average weight of each type of fruit.

grouped_mean = grouped.mean()

This will output:

Apple    140.0
Banana    95.0
Orange   255.0

More Complex Grouping

You can also group by multiple columns. Let's add another column to our dataset to see this in action.

data['Color'] = ['Red', 'Orange', 'Yellow', 'Green', 'Green', 'Orange']
df = pd.DataFrame(data)
grouped = df.groupby(['Fruit', 'Color'])
grouped_sum = grouped.sum()

Now our output looks like this:

Fruit  Color        
Apple  Green      130
       Red        150
Banana Green       90
       Yellow     100
Orange Orange     510

We have grouped by both 'Fruit' and 'Color', and summed the weights within these groups.

Transform and Filter with GroupBy

Apart from aggregation, groupby can also be used for transformation and filtering. Transformation might involve standardizing data within groups, while filtering could mean removing data that doesn't meet certain criteria.


For example, if you wanted to subtract the mean weight from each fruit's weight to see the difference from the average, you could use the transform function.

grouped_transform = grouped['Weight'].transform(lambda x: x - x.mean())


If you only want to keep groups with a total weight greater than 200, you could use the filter function.

grouped_filter = grouped.filter(lambda x: x['Weight'].sum() > 200)

Intuition and Analogies

Think of groupby as a way of creating buckets of your data based on a key (or keys) that you provide. Once your data is in these buckets, you can then decide what to do with it, whether that's summing it up, finding averages, or applying more complex transformations.


Mastering the groupby operation in Pandas can feel like learning to sort and summarize a market's worth of data produce. It's a powerful tool that, once understood, can provide deep insights into the patterns and relationships within your data. Just as a well-organized fruit stand can quickly inform customers of what's available, a well-grouped dataset can inform data scientists and analysts about the underlying structure and trends. So, next time you find yourself with a complex dataset, remember the simplicity of sorting fruits, and let Pandas' groupby help you make sense of your data harvest.