Sets

Sets in Python are an unordered collection of objects that are used in situations where membership and uniqueness to the set are the most important information of the object. The in operator runs faster with sets than with Lists:

set

>>> x = set([1, 2, 3, 2, 4])
>>> x
{1, 2, 3, 4}
>>> 1 in x
True
>>> 5 in x
False
>>> x.add(0)
>>> x
{0, 1, 2, 3, 4}
>>> x.remove(4)
>>> x
{0, 1, 2, 3}
>>> y = set([3, 4, 5])
>>> x | y
{0, 1, 2, 3, 4, 5}
>>> x & y
{3}
>>> x ^ y
{0, 1, 2, 4, 5}
>>> x.update(y)
>>> x
{0, 1, 2, 3, 4, 5}

Line 1

You can create a set by applying set to a sequence, for example to a list.

Line 3

When a sequence is made into a set, duplicates are removed.

Lines 4–7

The keyword in is used to check whether an object belongs to a set.

Lines 8–13

With add and remove you can change the elements in set.

Line 15

| is used to get the union or combination of two sets.

Line 17

& is used to get the intersection.

Line 19

^ is used to find the symmetrical difference, meaning elements that are contained in one or the other set, but not in both.

frozenset

In addition to set, there is also frozenset, which is immutable. This means that they can also be members of other sets:

>>> x = set([4, 2, 3, 2, 1])
>>> z = frozenset(x)
>>> z
frozenset({1, 2, 3, 4})
>>> z.add(5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'frozenset' object has no attribute 'add'
>>> x.add(z)
>>> x
{1, 2, 3, 4, frozenset({1, 2, 3, 4})}

Summary

However, the speed advantage also comes at a price: sets do not keep the elements elements in the correct order, whereas Lists and Tuples do. If the order is important to you, you should use a data structure that remembers the order.

data type	mutable	ordered	indexed	duplicates
Sets	✅	❌	❌	❌
Frozensets	❌	❌	❌	❌

Checks

• How many elements does a set have if it is formed from the following list [4, 2, 3, 2, 1]?