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)

Try it

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)

Try it

Output:

{'numpy', 'pandas', 'django'}Code language: JavaScript (javascript)

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)

Try it

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.

Quiz #