Python Set Comprehension

Summary: in this tutorial, you’ll learn how to use Python set comprehension to create a new set based on an existing one.

Introduction to Python Set comprehension

Suppose that you have the following set that consists of three tags:

tags = {'Django', 'Pandas', 'Numpy'}Code language: Python (python)

To convert the tags in the set to another set of tags in lowercase, you may use the following for loop:

tags = {'Django', 'Pandas', 'Numpy'}

lowercase_tags = set()
for tag in tags:
    lowercase_tags.add(tag.lower())

print(lowercase_tags)Code language: Python (python)

Output:

{'django', 'numpy', 'pandas'}Code language: Python (python)

How it works:

• First, iterate over each element of the tags set.
• Second, convert each tag to lowercase and add it the new set (lowercase_tags)

Or you can use the built-in map() function with a lambda expression:

tags = {'Django', 'Pandas', 'Numpy'}
lowercase_tags = set(map(lambda tag: tag.lower(), tags))

print(lowercase_tags)Code language: Python (python)

The map() function returns a map object so you need to use the set() function to convert it to a set.

To make the code more concise, Python provides you with the set comprehension syntax as follows:

{expression for element in set if condition}Code language: Python (python)

The set comprehension allows you to create a new set based on an existing set.

A set comprehension carries the following steps:

• First, iterate over the elements of a set.
• Second, apply an expression to each element
• Third, create a new set of elements resulting from the expression.

In addition, the set comprehension allows you to select which element to apply the expression via a condition in the if clause.

Note that the set comprehension returns a new set, it doesn’t modify the original set.

Back to the previous example, you can convert all the tags in the tags set by using the following set comprehension:

tags = {'Django', 'Pandas', 'Numpy'}
lowercase_tags = {tag.lower() for tag in tags}

print(lowercase_tags)Code language: Python (python)

This syntax definitely looks more concise than a for loop and more elegant than the map() function.

Python Set comprehension with an if clause example

Suppose you want to convert all elements of the tags set to lowercase except for the Numpy.

To do it, you can add a condition to the set comprehension like this:

tags = {'Django', 'Pandas', 'Numpy'}
new_tags = {tag.lower() for tag in tags if tag != 'Numpy'}

print(new_tags)
Code language: Python (python)

Output:

{'django', 'pandas'}Code language: Python (python)

Summary

• Use Python set comprehension to create a new set based on an existing set by applying an expression to each element of the existing set.