Sets & Set Operations

What a set is

A set is an unordered collection of unique items. It is great for removing duplicates and for fast membership checks.

Creating sets

Use braces for non-empty sets or set() for empty sets.

python
nums = {1, 2, 3}
words = set(["a", "b", "a"])  # {'a', 'b'}
empty = set()  # {} is a dict, not a set

Membership tests are fast

Sets are optimized for membership checks.

python
print("a" in words)  # True

Adding and removing items

Use add, remove, or discard.

python
s = {1, 2, 3}
s.add(4)
s.remove(2)   # error if missing
s.discard(9)  # no error if missing

Set operations

Union, intersection, difference, and symmetric difference are built in.

python
a = {1, 2, 3}
b = {3, 4, 5}

print(a | b)  # union
print(a & b)  # intersection
print(a - b)  # difference
print(a ^ b)  # symmetric difference

Subsets and supersets

You can test relationships with <= and >= or with methods.

python
small = {1, 2}
large = {1, 2, 3}
print(small <= large)
print(large >= small)

When order matters

Sets do not keep order. If you need order, convert to a list and sort it.

python
ordered = sorted(list(a))

Frozen sets (immutable sets)

If you need a set as a dictionary key, use frozenset().

python
f = frozenset([1, 2, 3])

Common mistakes to avoid

• Using {} for an empty set.
• Expecting sets to keep order.
• Putting a list or dict inside a set.

What you should understand after this lesson

• How to create sets and remove duplicates.
• How to use union, intersection, difference, and symmetric difference.
• How to check subset and superset relationships.
• When to convert a set back to a list.