Sets

Sets

In this tutorial, we'll store unique values and compare groups.

A set stores unique values. If you add the same value more than once, it only appears once.

tags = {"python", "beginner", "python"}
print(tags)

Output may be:

{'beginner', 'python'}

Sets do not keep a reliable order, so do not use them when positions matter.

Why Sets Are Useful

Sets are useful for uniqueness and membership checks:

allowed_users = {"ada", "grace", "linus"}

print("ada" in allowed_users)
print("sam" in allowed_users)

Output:

True
False

Adding and Removing

skills = {"Python", "HTML"}
skills.add("CSS")
skills.discard("HTML")

print(skills)

discard() is safe if the item is not there. remove() gives an error if the item is missing.

Set Operations

Sets can compare groups:

python_students = {"Ada", "Ben", "Cara"}
javascript_students = {"Ben", "Dan"}

print(python_students & javascript_students)  # both
print(python_students | javascript_students)  # either
print(python_students - javascript_students)  # only Python

Common Mistake

This creates an empty dictionary, not an empty set:

empty = {}

Use:

empty_set = set()

When Not to Use a Set

Do not use a set if you care about the order of items or need duplicate values.

For example, scores in a game should probably be a list:

scores = [10, 10, 8, 6]

The repeated 10 matters. A set would remove the duplicate, which would change the meaning of the data.

Practice

Start with a list containing repeated names. Convert it to a set and print how many unique names there are.