Sets

Sets are much like lists, but they can only contain unique values. Also, sets are unordered, so you cannot access them by index.

Creating a set

To create a set you can use the {} syntax.

fruits = {"apple", "banana", "cherry"}

Another way would be to use the set() function. When using the set() function, you can pass in an iterable, like a list, as an argument.

fruits_from_list = set(["apple", "banana", "cherry"])

In both examples we will get the same result:

print(fruits == fruits_from_list)
# => True

The set function can also be used to create an empty set:

empty_set = set()
print(len(empty_set))
# => 0

We canno use the {} syntax to create an empty set, because it would create an empty dictionary instead.

Accessing set items

We cannot access set items by index, because sets are unordered.

But we can loop over the set and print each item:

for fruit in fruits:
    print(fruit)

which produces (not necessarily in this order):

0
apple
banana

We can also test if an item is in a set:

print("banana" in fruits)
# => True

print("pineapple" in fruits)
# => False

Add items to a set

To add items to a set, we can use the add() method:

fruits.add("pineapple")
# => {"apple", "banana", "cherry", "pineapple"}

print("pineapple" in fruits)
# => True

Another way would be to use the update() method, which takes an iterable as an argument:

fruits.update(["orange", "mango", "grapes"])
# => {"apple", "banana", "cherry", "pineapple", "orange", "mango", "grapes"}

Remove items from a set

You can use the remove() method to remove an item from a set:

fruits.remove("banana")
# => {"apple", "cherry", "pineapple", "orange", "mango", "grapes"}

If you need to remove any item from a set, you can use the pop() method:

fruit = fruits.pop()
print(fruit)
# => "pineapple" or any other element

Set operations

Sets have a lot of useful methods to perform set operations.

Union

To get the union of two sets, you can use the union() method:

{1, 2, 3}.union({3, 4, 5})
# => {1, 2, 3, 4, 5}

Or, alternatively, you can use the | operator:

{1, 2, 3} | {3, 4, 5}
# => {1, 2, 3, 4, 5}

Intersection

To get the intersection of two sets, you can use the intersection() method:

{1, 2, 3}.intersection({3, 4, 5})
# => {3}

Or, alternatively, you can use the & operator:

{1, 2, 3} & {3, 4, 5}
# => {3}

Related to intersection are the isdisjoint() and issubset() methods. For example,

{1, 2, 3}.isdisjoint({10, 11, 12})
# => True (because there are no common elements)

{1, 2, 3}.isdisjoint({3, 4, 5})
# => False (because 3 is in both sets)

{1, 2, 3}.issubset({3, 4, 5})
# => False (because not all elements of the first set are in the second set)

{1, 2, 3}.issubset({1, 2, 3, 4, 5})
# => True (because all elements of the first set are in the second set)

Difference

You can get difference of two sets using the difference() method:

{1, 2, 3}.difference({3, 4, 5})
# => {1, 2}

Note that the operation is not commutative:

{3, 4, 5}.difference({1, 2, 3})
# => {4, 5}

Alternatively, you can use the - operator:

{1, 2, 3} - {3, 4, 5}
# => {1, 2}