Python Sets

What We Cover in This Tutorial

We will go through everything you need to know about Python sets, from zero to hero. Here’s the complete list:

• What is a Set and why it is useful
• How to create a Set
• Important properties of Sets
• Adding elements to a Set
• Removing elements from a Set
• Checking if an item exists
• Looping through a Set
• All built-in Set methods (with examples)
• Mathematical Set Operations (Union, Intersection, Difference, etc.)
• Set Comprehensions (a shortcut way to create sets)
• Frozen Sets (the “read-only” version of sets)
• Real-world examples and common mistakes
• Practice exercises with solutions

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Table of Contents

1. What is a Python Set?
2. Creating a Set
3. Properties of a Set
4. Adding Elements to a Set
5. Removing Elements from a Set
6. Checking Membership (Is an item in the set?)
7. Iterating (Looping) Through a Set
8. Built-in Set Methods
9. Set Operations (Mathematics)
10. Set Comprehensions
11. Frozen Sets
12. Real-World Examples
13. Common Mistakes & Tips
14. Practice Exercises

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1. What is a Python Set?

A Set in Python is a collection of items where:

• Every item is unique (no duplicates allowed)
• The order of items does not matter (unordered)
• You can add or remove items later (it is mutable)

Real-life analogy:
Think of a set like a bag of marbles where:

• You cannot put two identical marbles (duplicates are automatically removed)
• You don’t care which marble comes out first
• You can add or remove marbles anytime

Why use Sets?

• Automatically remove duplicates
• Very fast to check if something exists (much faster than lists)
• Perfect for finding common or different items between groups

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2. Creating a Set

There are two easy ways to create a set:

Method 1: Using curly braces {}

# Creating a set of fruits
fruits = {"apple", "banana", "cherry"}
print(fruits)

Method 2: Using the set() function

numbers = set([1, 2, 3, 4])          # from a list
letters = set("hello")               # from a string
empty_set = set()                    # empty set (IMPORTANT: {} creates a dictionary!)

Output example:

{'apple', 'banana', 'cherry'}

Pro Tip:
{} creates an empty dictionary, not a set. Always use set() for an empty set.

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3. Properties of a Set

Property	Description	Example
Unordered	No guaranteed order	Items can print in any order
Unique elements	Duplicates are automatically removed	{"a", "a", "b"} → {"a", "b"}
Mutable	You can add/remove items after creation	Yes
Elements must be immutable	Only numbers, strings, tuples allowed (not lists)	{"apple", 42, (1,2)} works
Cannot be indexed	No set[0] because order doesn’t exist	Error if you try

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4. Adding Elements to a Set

add() – Add one item

colors = {"red", "blue"}
colors.add("green")
print(colors)        # {'red', 'blue', 'green'}

update() – Add multiple items (from list, tuple, or another set)

colors.update(["yellow", "purple", "red"])   # "red" already exists → ignored
print(colors)

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5. Removing Elements from a Set

Method	What it does	Error if item not found?
remove()	Removes item	Yes (raises error)
discard()	Removes item (safer)	No
pop()	Removes and returns one random item	No (works on empty too)
clear()	Removes all items	No

Examples:

fruits = {"apple", "banana", "cherry"}

fruits.remove("banana")      # works
# fruits.remove("mango")     # ← would give KeyError!

fruits.discard("mango")      # safe, does nothing

item = fruits.pop()          # removes one random item
print("Removed:", item)

fruits.clear()               # now empty

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6. Checking Membership (Is an item in the set?)

Use the in keyword — super fast!

fruits = {"apple", "banana", "cherry"}

print("apple" in fruits)     # True
print("mango" in fruits)     # False

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7. Iterating (Looping) Through a Set

fruits = {"apple", "banana", "cherry"}

for fruit in fruits:
    print(fruit)

Note: Order may be different every time you run it — that’s normal for sets!

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8. Built-in Set Methods

Here’s a quick reference table of all important set methods:

Method	Description	Example
add()	Add one item	s.add(5)
update()	Add multiple items	s.update([6,7])
remove()	Remove item (error if missing)	s.remove(5)
discard()	Remove item (no error)	s.discard(99)
pop()	Remove & return random item	x = s.pop()
clear()	Remove all items	s.clear()
copy()	Make a copy	new = s.copy()
len()	Count items	len(s)

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9. Set Operations (Mathematics)

Sets shine when you combine them! Here are the 4 most useful operations:

Operation	Symbol	Method	Meaning	Example Result
Union (combine)	|	union()	All items from both sets	A ∪ B
Intersection	&	intersection()	Common items only	A ∩ B
Difference	-	difference()	Items in A but not in B	A – B
Symmetric Difference	^	symmetric_difference()	Items in either but not both	A △ B

Code Example:

A = {1, 2, 3, 4}
B = {3, 4, 5, 6}

print(A | B)      # Union: {1,2,3,4,5,6}
print(A & B)      # Intersection: {3,4}
print(A - B)      # Difference: {1,2}
print(A ^ B)      # Symmetric difference: {1,2,5,6}

You can also use the full method names:

print(A.union(B))
print(A.intersection(B))

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Union

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Intersection

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Difference

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Symmetric difference

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10. Set Comprehensions

A short and beautiful way to create sets (just like list comprehensions):

# Create a set of squares
squares = {x**2 for x in range(1, 6)}
print(squares)          # {1, 4, 9, 16, 25}

# Set of even numbers from a list
numbers = [1, 2, 3, 4, 5, 6]
evens = {x for x in numbers if x % 2 == 0}
print(evens)            # {2, 4, 6}

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11. Frozen Sets

A frozen set is like a set that cannot be changed (immutable).

frozen = frozenset(["apple", "banana"])
# frozen.add("cherry")   ← This will give an error!

When to use?
When you want to use a set as a key in a dictionary or inside another set.

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12. Real-World Examples

Example 1: Remove duplicates from a list

names = ["Alice", "Bob", "Alice", "Charlie"]
unique_names = set(names)
print(unique_names)      # {'Alice', 'Bob', 'Charlie'}

Example 2: Find common friends

my_friends = {"Alice", "Bob", "David"}
your_friends = {"Bob", "Charlie", "David"}

common = my_friends & your_friends
print("Common friends:", common)

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13. Common Mistakes & Tips

• Mistake: Trying set[0] → Sets have no index!
• Tip: Use set() for empty set, not {}
• Tip: Sets are fastest for membership testing (in)
• Tip: You cannot put a list or another set inside a set (they are mutable)

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14. Practice Exercises

Exercise 1: Create a set of your favorite movies. Add 2 more, then remove one.

Exercise 2: Given two sets A = {10, 20, 30, 40} and B = {30, 40, 50, 60}, print:

• Union
• Intersection
• Items only in A

Exercise 3: Write a set comprehension to create a set of all odd numbers from 1 to 20.

Solutions (try first, then check):

# Exercise 2 solution
A = {10, 20, 30, 40}
B = {30, 40, 50, 60}
print(A | B)
print(A & B)
print(A - B)

# Exercise 3 solution
odds = {x for x in range(1, 21) if x % 2 == 1}
print(odds)

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