Sets

Think of a Set as a bag of unique items. Unlike a list, where things are lined up in a specific order, a set is just a collection where:

Unordered: Items don’t have a specific “seat” or index.
No Duplicates: If you try to add the same thing twice, the set will just ignore the second one.
Unchangeable (mostly): You cannot change an item once it’s in the bag, but you can add new items or take old ones out.

Creating a Set

Sets are written with curly braces {}.

# Creating a set
fruits = {"apple", "banana", "cherry"}

# Sets ignore duplicates automatically
numbers = {1, 2, 2, 3, 3, 3}
print(numbers) # Output: {1, 2, 3}

Note: To create an empty set, you must use set(), because {} is used to create an empty Dictionary.

Access Set Items

Because sets are unordered, they do not have an index. You cannot ask for my_set[0].

To see if an item is there, use the in keyword.
To see all items, you must loop through them.

colors = {"red", "green", "blue"}
# Check if 'red' is present
print("red" in colors) # Output: True

Adding and Removing Item

You can grow your “bag” of items using these methods:

.add(): To add one item.
.update(): To add multiple items (like another list or set).

Following are main ways to take things out:

.remove(): Removes an item. Warning: If the item isn’t there, it will crash your program.
.discard(): Removes an item. Safe: If the item isn’t there, it does nothing (no crash).
.pop(): Removes a random item (since there is no order, you never know which one it will pick).

numbers = {1, 2, 3}
numbers.add(4)        # {1, 2, 3, 4}
numbers.update([2, 5, 6])  # {1, 2, 3, 4, 5, 6}
numbers.remove(2)     # {1, 3, 4, 5, 6}
numbers.discard(10)   # No error even if 10 is not present
numbers.pop()         # Removes something randomly

Set Operations

Sets are powerful because they let you do operations similar to math:

Union (| or .union()) → Combines items from both sets.
Intersection (& or .intersection()) → Keeps only common items.
Difference (- or .difference()) → Items in one set but not the other.
Symmetric Difference (^) → Items that are in either set, but not both

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

print(a | b)   # {1, 2, 3, 4, 5}  (union)
print(a & b)   # {3}              (intersection)
print(a - b)   # {1, 2}           (difference)
print(a ^ b)   # {1, 2, 4, 5}     (symmetric difference)

Frozenset

A Frozenset is exactly like a regular Set (it’s unordered and contains unique items), but with one major twist: it is immutable.
If a Set is like a dry-erase board where you can add and erase names, a Frozenset is like a stone monument—once the names are carved, they cannot be changed.

# Starting with a list
vowels_list = ['a', 'e', 'i', 'o', 'u']

# Freezing it
frozen_vowels = frozenset(vowels_list)
print(frozen_vowels)  # Output: frozenset({'e', 'o', 'a', 'i', 'u'})

Why Use a Frozenset?

You might wonder, “Why lock a set?” There are two main reasons:

Data Integrity: It ensures that a collection of data (like a list of restricted usernames or country codes) cannot be accidentally modified by another part of your program.
Using Sets as Keys: In Python, you cannot use a regular Set as a key in a Dictionary because sets can change. However, because a Frozenset never changes, Python allows you to use it as a key.

Quick Summary Table

Feature	set {}	frozenset()
Unique Items?	Yes	Yes
Unordered?	Yes	Yes
Changeable?	Yes (Mutable)	No (Immutable)
Can use as Dict Key?	No	Yes