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Python Sets

Sets are one of Python’s built-in data types used to store multiple unique values in a single variable. They are unordered, mutable, and unindexed, making them ideal for fast membership testing and removing duplicates.
This topic covers sets from basics to practical usage.

What Is a Set?

A set is a collection that:
  • Stores unique elements only
  • Is unordered
  • Is mutable
  • Does not allow indexing
python
my_set = {1, 2, 3, 4}
print(my_set)
python
duplicates = {1, 2, 2, 3, 3}
print(duplicates)

Set vs List vs Tuple

FeatureListTupleSet
OrderedYesYesNo
Allows duplicatesYesYesNo
MutableYesNoYes
IndexingYesYesNo

Creating Sets

Using Curly Braces {}

python
fruits = {"apple", "banana", "cherry"}
print(fruits)

Using set() Constructor

python
numbers = set([1, 2, 3, 3])
print(numbers)
python
letters = set("Python")
print(letters)

Empty Set (Important)

python
empty_set = set()
print(type(empty_set))
This is NOT a set:
python
empty = {}
print(type(empty))  # dict

Access Set Items

Sets are unordered, so you cannot access items using indexes.
python
fruits = {"apple", "banana", "cherry"}

for item in fruits:
    print(item)

Check If Item Exists

python
fruits = {"apple", "banana", "cherry"}
print("apple" in fruits)
python
print("mango" not in fruits)

Add Items to a Set

Add One Item – add()

python
fruits = {"apple", "banana"}
fruits.add("cherry")
print(fruits)

Add Multiple Items – update()

python
fruits.update(["orange", "mango"])
print(fruits)
python
fruits.update({"grape", "kiwi"})

Remove Items from a Set

remove() – Raises Error if Not Found

python
fruits.remove("banana")
print(fruits)

discard() – No Error if Not Found

python
fruits.discard("pineapple")
print(fruits)

pop() – Removes Random Item

python
item = fruits.pop()
print(item)
print(fruits)

clear() – Remove All Items

python
fruits.clear()
print(fruits)

Set Length

python
numbers = {1, 2, 3, 4}
print(len(numbers))

Join Sets (Set Operations)

Union (| or union())

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

print(a | b)
python
print(a.union(b))

Intersection (& or intersection())

python
print(a & b)
python
print(a.intersection(b))

Difference (- or difference())

python
print(a - b)
python
print(a.difference(b))

Symmetric Difference (^)

python
print(a ^ b)

Set Comprehension

python
numbers = [1, 2, 3, 4, 5]
even_set = {x for x in numbers if x % 2 == 0}
print(even_set)

Frozen Set (Immutable Set)

python
fs = frozenset([1, 2, 3])
print(fs)
Cannot modify:
python
# fs.add(4)  # AttributeError

Common Mistakes

Expecting Order in Sets

python
s = {1, 2, 3}
print(s)
Order may change.

Using Mutable Items in Set

python
# s = {[1, 2], 3}  # TypeError

When to Use Sets

Use sets when:
  • You need unique values
  • Membership checking is frequent
  • Order does not matter
  • Removing duplicates is required

Summary

  • Sets store unique values
  • Unordered and unindexed
  • Mutable and dynamic
  • Very fast membership testing
  • Support powerful mathematical operations
  • Ideal for deduplication and comparisons