Python Set

Summary: in this tutorial, you’ll learn about Python Set type and how to use it effectively.

Introduction to the Python Set type #

A Python set is an unordered list of immutable elements. It means:

• Elements in a set are unordered.
• Elements in a set are unique. A set doesn’t allow duplicate elements.
• Elements in a set cannot be changed. For example, they can be numbers, strings, and tuples, but cannot be lists or dictionaries.

To define a set in Python, you use the curly brace {}. For example:

skills = {'Python programming', 'Databases', 'Software design'}Copy

Note a dictionary also uses curly braces, but its elements are key-value pairs.

To define an empty set, you cannot use the curly braces like this:

empty_set = {}Copy

…because it defines an empty dictionary.

Therefore, you need to use the built-in set() function:

empty_set = set()Copy

An empty set evaluates to False in Boolean context. For example:

skills = set()

if not skills:
    print('Empty sets are falsy')Copy

Try it

Output:

Empty sets are falsyCopy

In fact, you can pass an iterable to the set() function to create a set. For example, you can pass a list, which is an iterable, to the set() function like this:

skills = set(['Problem solving','Critical Thinking'])
print(skills)Copy

Try it

Output:

{'Critical Thinking', 'Problem solving'}Copy

Note that the original order of elements may not be preserved.

If an iterable has duplicate elements, the set() function will remove them. For example:

characters = set('letter')
print(characters)Copy

Try it

Output:

{'r', 'l', 't', 'e'}Copy

In this example, the string 'letter' has two e and t characters and the set() removes each of them.

Getting sizes of a set #

To get the number of elements in a set, you use the built-in len() function.

len(set)Copy

For example:

ratings = {1, 2, 3, 4, 5}
size = len(ratings)

print(size)Copy

Try it

Output:

5Copy

Checking if an element is in a set #

To check if a set contains an element, you use the in operator:

element in setCopy

The in operator returns True if the set contains the element. Otherwise, it returns False. For example:

ratings = {1, 2, 3, 4, 5}
rating = 1

if rating in ratings:
    print(f'The set contains {rating}')Copy

Try it

Output:

The set contains 1Copy

To negate the in operator, you use the not operator. For example:

ratings = {1, 2, 3, 4, 5}
rating = 6

if rating not in ratings:
    print(f'The set does not contain {rating}')Copy

Try it

Output:

The set does not contain 6Copy

Adding elements to a set #

To add an element to a set, you use the add() method:

set.add(element)Copy

For example:

skills = {'Python programming', 'Software design'}
skills.add('Problem solving')

print(skills)Copy

Try it

Output:

{'Problem solving', 'Software design', 'Python programming'}Copy

Removing an element from a set #

To remove an element from a set, you use the remove() method:

set.remove(element)Copy

For example:

skills = {'Problem solving', 'Software design', 'Python programming'}
skills.remove('Software design')

print(skills)Copy

Try it

Output:

{'Problem solving', 'Python programming'}Copy

If you remove an element that doesn’t exist in a set, you’ll get an error (KeyError). For example:

skills = {'Problem solving', 'Software design', 'Python programming'}
skills.remove('Java')Copy

Try it

Error:

KeyError: 'Java'Copy

To avoid the error, you should use the in operator to check if an element is in the set before removing it:

skills = {'Problem solving', 'Software design', 'Python programming'}
if 'Java' in skills:
    skills.remove('Java')Copy

To make it more convenient, the set has the discard() method that allows you to remove an element. And it doesn’t raise an error if the element is not in the list:

set.discard(element)Copy

For example:

skills = {'Problem solving', 'Software design', 'Python programming'}
skills.discard('Java')

print(skills)Copy

Try it

Output:

{'Problem solving', 'Python programming', 'Software design'}Copy

Returning an element from a set #

To remove and return an element from a set, you use the pop() method.

Since the elements in a set have no specific order, the pop() method removes an unspecified element from a set.

If you execute the following code multiple times, it’ll show a different value each time:

skills = {'Problem solving', 'Software design', 'Python programming'}
skill = skills.pop()

print(skill)Copy

Try it

Output:

Problem solvingCopy

Removing all elements from a set #

To remove all elements from a set, you use the clear() method:

set.clear()Copy

For example:

skills = {'Problem solving', 'Software design', 'Python programming'}
skills.clear()

print(skills)Copy

Try it

Output:

set()Copy

Frozen a set #

To make a set immutable, you use the built-in function called frozenset(). The frozenset() returns a new immutable set from an existing one. For example:

skills = {'Problem solving', 'Software design', 'Python programming'}
skills = frozenset(skills)Copy

After that, if you attempt to modify elements of the set, you’ll get an error:

skills = {'Problem solving', 'Software design', 'Python programming'}
skills = frozenset(skills)

skills.add('Django')Copy

Try it

Error:

AttributeError: 'frozenset' object has no attribute 'add'Copy

Looping through set elements #

Since a set is an iterable, you can use a for loop to iterate over its elements. For example:

skills = {'Problem solving', 'Software design', 'Python programming'}

for skill in skills:
    print(skill)Copy

Try it

Output:

Software design
Python programming
Problem solvingCopy

To access the index of the current element inside the loop, you can use the built-in enumerate() function:

skills = {'Problem solving', 'Software design', 'Python programming'}

for index, skill in enumerate(skills):
    print(f"{index}.{skill}")Copy

Try it

Output:

0.Software design
1.Python programming
2.Problem solvingCopy

By default, the index starts at zero. To change this, you pass the starting value to the second argument of the enumerate() function. For example:

skills = {'Problem solving', 'Software design', 'Python programming'}

for index, skill in enumerate(skills, 1):
    print(f"{index}.{skill}")Copy

Try it

Output:

1.Python programming
2.Problem solving
3.Software designCopy

Notice that every time you run the code, you’ll get the set elements in a different order.

In the next tutorial, you’ll learn how to perform useful operations on sets such as:

• Union of Sets
• Intersection of Sets
• Difference between Sets
• Symmetric Difference of Sets

Summary #

• A set is an unordered collection of immutable elements.

Quiz #