Group List by Values in Python (3 Examples)


In this Python tutorial, we will explain how to group a list by values.

Here you have an overview:

Let’s start right away!


Create Demo Python List

At the start, we will create a demo Python list that we will use in the examples below. Take a look at how to create my_list.

my_list = [["John", 25], ["Alice", 32], 
 ["Bob", 25], ["Emma", 32], ["Chris", 28]]
# [['John', 25], ['Alice', 32], 
# ['Bob', 25], ['Emma', 32], ['Chris', 28]]

As you can see, my_list is a list that contains several sublists. Each sublist represents a person’s name and age. Some of the age values are repeated. Now, let’s see some examples of how to group my_list by the different age values!


Example 1: Group List by Values Using List Comprehension

In this first example, we will use a combination of list comprehension, map(), and lambda() functions to categorize my_list by values. Take a look at the code below.

values = set(map(lambda x:x[1], my_list))
grouped_values = [[y[0] for y in my_list if y[1]==x] for x in values]
# [['Alice', 'Emma'], ['John', 'Bob'], ['Chris']]

In this code, values is a set containing the unique age values extracted from the sublists in my_list. The map() function is used with a lambda function to iterate over my_list and extract the second element (age) from each sublist.

The resulting set values is then used to group the sublists in my_list by age. The outer list comprehension iterates over each value in the values list. For each of these values, it constructs a new list using another list comprehension.

The inner list comprehension extracts the first element (name) from the sublists in my_list where the second element (age) matches the current value.

The result of this operation is a list of lists, with each inner list containing the first elements from the lists in my_list that have their second element equal to each corresponding value in values.


Example 2: Group List by Values Using for Loop

This example uses an empty dictionary and a for loop to store the categorized elements in my_list as follows.

grouped_values = {}
for sublist in my_list:
    key = sublist[1] 
    value = sublist
    if key not in grouped_values:
        grouped_values[key] = []
# {25: [['John', 25], ['Bob', 25]], 
# 32: [['Alice', 32], ['Emma', 32]], 28: [['Chris', 28]]}

As shown in the previous output, here we have first created an empty dictionary to store the grouped values. Then, we iterated over the elements in my_list and extracted the names and ages from each sublist in my_list.

In the for loop, an if statement checks if the name is already in the dictionary grouped_values. If not, it’s added as a new key with an empty list as the value. Then, the current value is appended to the list associated with the corresponding key.

Finally, we get a dictionary where the keys are the grouping ages, and the values are the list of names representing the people at that age.

But if you’re looking for a similar output to the one in Example 1, you can variate the code and obtain a similar output, as shown below.

grouped_values = {}
for sublist in my_list:
    key = sublist[1]
    value = sublist[0]
    if key not in grouped_values:
        grouped_values[key] = []
grouped_values = list(grouped_values.values())
# [['John', 'Bob'], ['Alice', 'Emma'], ['Chris']]

In the modified code, we use sublist[1] to group the sublists based on the age value. The value sublist[0] (name) is appended to the corresponding key in the grouped_values dictionary.

Then, the grouped_values variable is updated using the result of converting the dictionary values to a list using the list() function.

Example 3: Group List by Values Using collections Module

In this third example, we will explore the defaultdict class from the collections module to group my_list by values. First, we will import the collections module using the code below.

import collections

Now, we can use the defaultdict class from this module to group my_list by values.

grouped_dict = collections.defaultdict(list)
for sublist in my_list:
grouped_values = grouped_dict.values()
# dict_values([['John', 'Bob'], ['Alice', 'Emma'], ['Chris']])

This first line creates a new defaultdict where each value is a list. If a key is accessed in the dictionary and it doesn’t exist, instead of raising a KeyError, the dictionary automatically creates that key with a default value, in this case, an empty list.

For each sublist in my_list, it appends the first element sublist[0] to the list in grouped_dict that is associated with the key equal to the second element of the sublist sublist[1]. If the key doesn’t already exist in grouped_dict, it is automatically created with a default value of an empty list due to the behavior of defaultdict.

Then the values from grouped_dict are stored in grouped_values. Note that grouped_values is a dict_values object, which is a view on the dictionary’s values.


Video, Further Resources & Summary

Do you need more explanations on how to group a list by values in Python? Then you should have a look at the following YouTube video of the Statistics Globe YouTube channel.


The YouTube video will be added soon.


Furthermore, you could have a look at some of the related on Statistics Globe:

This post has shown how to group a Python list by values. In case you have additional questions, please let me know in the comments section below.


Paula Villasante Soriano Statistician & R Programmer

This page was created in collaboration with Paula Villasante Soriano. Please have a look at Paula’s author page to get more information about her academic background and the other articles she has written for Statistics Globe.


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