Python Set

In Python, a set is a collection of unordered items. Every item of a set is unique (no duplicates) and immutable. However, the set itself is mutable; we can add or remove set items.

Creating Set

A set is created by placing items inside curly braces, {}, separated by commas.

animals = {'dog', 'cat', 'tiger'}
print(animals)    # {'tiger', 'dog', 'cat'}

Note: The order of items in output may be different than how we assigned them. It's because sets are unordered.

If we add duplicate items to a set, duplicates will be removed

animals = {'dog', 'cat', 'tiger', 'dog', 'tiger'}
print(animals)    # {'tiger', 'cat', 'dog'}

Here, we can see 'dog' and 'tiger' only once in the output because sets don't allow duplicate values.

Creating an Empty Set

We cannot use {} to create an empty set. It's because this will create an empty dictionary. To create an empty set, we use the set() function.

# creating empty set
animals = set()
print(animals)    # set()

Add Items to a Set

Sets in Python are mutable. We can add and remove items from them.

To add a single item to a set, we use the add() method.

animals = {'dog', 'cat'}

# adding 'monkey' to the set
animals.add('monkey')

print(animals)    # {'monkey', 'cat', 'dog'}

Add Items of Iterables (Lists, Tuples etc.)

We can use the update() method to add items from other iterables to a set.

animals = {'dog', 'cat'}
wild_animals = {'tiger', 'lion'}

# adding items from the wild_animals iterable
animals.update(wild_animals) 

print(animals)    # {'dog', 'lion', 'cat', 'tiger'}

Remove Items from a Set

We can use either the discard() method or the remove() method to remove an item from a set.

The difference between discard() and remove() is that

• discard() returns None if the item is not in a set
• remove() throws an error if the item is not in a set

Using discard()

animals = {'dog', 'cat', 'tiger'}

# removing 'tiger'
animals.discard('tiger')

print(animals)    # {'dog', 'cat'}

# trying to remove 'ferret'
# doesn't throw error
animals.discard('ferret')

print(animals)    # {'cat', 'dog'}

Using remove()

animals = {'dog', 'cat', 'tiger'}

# removing 'tiger'
animals.remove('tiger')

print(animals)    # {'dog', 'cat'}

# trying to remove 'ferret'
# throws an error
animals.remove('ferret')    KeyError: 'ferret'

print(animals)    # {'cat', 'dog'}

Remove all Items from a Set

We can use the clear() method to remove all items from a set.

animals = {'dog', 'cat', 'tiger'}

# remove all items
animals.clear()

print(animals)    # set()

Python Set Operations

Python sets have the same properties as that of sets in mathematics. Let's look at some set operations.

Union of Sets

The union of two sets is a set of all items in both sets. To find the union of sets, we can either use the union() method or the pipe symbol, |.

domestic_animals = {'dog', 'cat', 'elephant'}
wild_animals = {'lion', 'tiger', 'elephant'}

# finding the union of sets
animals = domestic_animals | wild_animals
print(animals)

# Output
# {'lion', 'elephant', 'cat', 'dog', 'tiger'}

Intersection of Sets

The intersection of two sets is a set of items that are common in both sets.

To find the intersection of sets, we can either use the intersection() method or the ampersand symbol, &.

domestic_animals = {'dog', 'cat', 'elephant'}
wild_animals = {'lion', 'tiger', 'elephant'}

# finding the intersection of sets
common_animals = domestic_animals & wild_animals

print(common_animals)    # {'elephant'}

Other Set Operations

We can find

• the difference of sets using either the - operator or the difference() method.
• the symmetric difference of sets using either the ^ operator or the symmetric_difference() method.

Python Set Methods

Python has many useful methods that make it really easy to work with sets. Here are the commonly used set methods.

Python Set Length

We can find the number of items in a set using the len() function. For example,

animals = {'dog', 'cat', 'tiger'}
print(len(animals))    # 3

Iterating Through a Set

We can iterate through elements of a set using the for loop.

animals = {'dog', 'cat', 'tiger'}

for animal in animals:
    print(animal)

Output

tiger
cat
dog

Note: Since sets are unordered, the order in which the items are printed is random.

Python Set Summary

In Python, a set has the following features:

• unordered - items are in random order
• unique - items of a set are unique
• mutable - can add or remove items

Recommended Reading: Python Dictionary