Sets

Introduction to Python Sets

You already know lists, tuples, and dictionaries. Sets are Python's fourth built-in collection. They store only unique values, ignore duplicates automatically, and make membership checks lightning fast. This lesson shows you how to create sets, convert lists into sets, and test whether a value exists.

Sets are Python's built-in collection for storing unique values. Unlike lists, sets cannot contain duplicates and have no fixed order.

You create a set with curly braces, similar to a dictionary but without key-value pairs.

fruits = {'apple', 'banana', 'cherry'}
print(len(fruits))

Python

Output

The syntax for creating a set:

my_set = {value1, value2, value3}

Python

Duplicates are removed automatically. Even though we wrote six values, the set keeps only three.

nums = {1, 2, 2, 3, 3, 3}
print(len(nums))

Python

Output

What will be the output?

letters = {'a', 'b', 'a', 'c', 'b'}
print(len(letters))

Python

To create an empty set, use set(). Writing {} creates an empty dictionary instead.

Empty set vs empty dictionary:

empty_set = set()    # set
empty_dict = {}      # dictionary

Python

You can also create a set from a list using set(). This removes all duplicates.

nums = [4, 2, 4, 1, 2]
unique = set(nums)
print(len(unique))

Python

Output

What will be the output?

x = set([1, 1, 2, 2, 3])
print(len(x))

Python

You can check if a value exists in a set using the in keyword, just like with lists.

Membership checks with in are very fast on sets, even with millions of elements.

colors = {'red', 'green', 'blue'}
print('red' in colors)
print('yellow' in colors)

Python

Output

What will be the output?

s = {10, 20, 30}
print(20 in s)

Python

What will be the output?

a = {1, 2, 3, 2, 1}
b = set([3, 4, 5, 3])
print(len(a) + len(b))

Python

What will be the output?

x = set('hello')
print(len(x))

Python

What will be the output?

data = [True, False, True, True]
print(len(set(data)))

Python

What will be the output?

a = [1, 2, 3, 1, 2]
b = set(a)
a.append(4)
print(len(b))

Python