How to use Sets in Python

John on September 15, 2020

In Python, a set is a data type used to store an unordered collection of unique values. If you need to keep a collection of data that must never have any duplicate values, sets are designed for this task.

In this tutorial, we will learn how to create and use sets in Python using examples.

Create a Set

Firstly, let's create an empty set. Create a new variable called x and set its value to set followed by () (parenthesis).

x = set()

Create a Set from a List, Tuple, String or Object

To create a new set using a list, tuple, string or object pass it inside the () (parenthesis) of set().

x = set([1,2,2,4,5])
print(x)

{1, 2, 4, 5}

Adding Elements to a Set

Now we have defined a new set in the variable x, we can add data to it. Type x followed by . (dot) followed by add(). The value to add is put inside the () (parenthesis) of add().

x.add(1)
print(x)

{1}

If we tried to add another 1 to the list, nothing will be added, but other unique values will be added.

x.add(1)
x.add(1)
x.add(2)
print(x)

{1, 2}

Adding Multiple Items to a Set

To add multiple items to a set use the update() method. This will accept any iterable data type. These data types are strings, lists, objects and tuples. in the example below, we are adding a list of numbers to a set.

x = set()
x.update([1,2,3,4])
print(x)

{1, 2, 3, 4}

Iterate over a Set

We can iterate over a set using a for loop, which is in fact in the same way we would iterate over a list.

for v in x:
    print(v)

1
2

Remove Elements from a Set

To remove elements from a list use either the remove() or discard() method, passing the value to remove inside the () (parenthesis). The only difference between the two methods is that remove() will throw an error when trying to remove a value that doesn't exist on the set.

x.remove(1)
print(x)

{2, 3, 4}

Remove the Last Element from a Set

To remove the last element from a set use the pop() method. Since sets are an unordered data type you won't know what element is at the end of the object making pop() a little redundant.

x.pop()

Remove All Elements from a Set

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

x.clear()
print(x)

set()

Remove Duplicates from a List

We can convert a list with duplicate values into a set to remove duplicates then loop through the set and append the values to a new list.

dups = [1,1,2,2,3,3]

unique = set(dups)

new = []

for e in unique:
    new.append(e)
    
print(new)

[1, 2, 3]

Check if a Value Exists in a Set

To check if a value exists in a set type the value followed by in then the name of the set. It will return True or False.

x = set([1,2,2,4,5])
print(1 in x)

True

Join Sets Together using union()

To join sets together use either a | (pipe) or use the built-in union() method.

x = set([1,2,2,4,5])
y = set([1,2,2,6,6])

z = x | y
print(z)

{1, 2, 4, 5, 6}

Or with union():

x.union(y)

Intersect Sets

An intersection will return values that are found in both sets. This is done using an & (ampersand) or the intersection() method.

x = set([1,2,2,4,5])
y = set([1,2,2,6,6])

z = x & y;
print(z)

{1, 2}

Or with intersection():

z = x.intersection(y)

Set Difference

The difference is the values from the first set that are in the first set and not second. This is done using a - (dash) or with the difference() method.

x = set([1,2,2,4,5])
y = set([1,2,2,6,6])

z = x - y
print(z)

{4, 5}

Or with difference():

z = x.difference(y)

Symmetric Set Difference

The symmetric difference is the values from both sets that are not in both sets. This can be done using a ^ (caret) or the symmetric_difference() method.

x = set([1,2,2,4,5])
y = set([1,2,2,6,6])

z = x ^ y
print(z)

Or using the symmetric_difference() method:

z = x.symmetric_difference(y)

How to Create an Immutable Set

To create an immutable set in Python, create the set using the frozenset() method. The list it creates can still be used with methods that don't change the values on the set.

x = frozenset([1,2,2,4,5])
print(x)

frozenset({1, 2, 4, 5})

All the Set Methods

For reference here are all the methods available to use with a set.

• add() - add an element to the set
• copy() - copy the whole set
• pop() - remove the last item
• remove() - remove a value
• discard() - remove a value
• difference() - returns a new set from the difference of two sets
• clear() - remove all elements
• difference_update() - remove all elements from another set from a set
• intersection() - intersect two sets
• intersection_update() - update set with intersection with another set
• isdisjoint() - returns True if intersection result is null
• issubset() - returns True if set is contained in the other
• issuperset() - returns True if the set contains nested sets
• symmetric_difference() - returns symmetric difference of sets
• symmetric_difference_update() - update symmetric difference to a set
• union() - create a union of sets
• update() - add one of many elements to a set

Built-in Set Functions

Here are all the native functions that can be used with sets.

• all() - returns True if the set is empty or all its values are true
• any() - if any value is True
• len() - get the number of elements in a set
• max() - get the largest value
• min() - get the smallest value
• sorted() - create a sorted list based on a set
• sum() - sum of all the values in a set
• enumerate() - get an enumerable object containing indexes and values

Conclusion

You now know how to use sets in Python and how they can be used to remove duplicates from lists.