Sets — Unordered Collections

Why read this? Need to remove duplicates from a list? Check if items exist in one collection but not another? Find common elements between two groups? Sets are the right tool — they're optimized for membership testing and mathematical set operations.

🎯 Learning Objectives

1. Create sets with {} and set()
2. Perform set operations: union, intersection, difference, symmetric difference
3. Test membership efficiently
4. Remove duplicates from sequences
5. Understand when to use sets vs lists vs tuples

📋 Prerequisites

• Basic understanding of collections (lists, tuples).

📖 Core Content

17.1 What Problem Do Sets Solve?

17.2 Creating Sets

python
Run Inline
# runnable
# Using curly braces
fruits = {"apple", "banana", "cherry"}
print(fruits)
# Using set() constructor
numbers = set([1, 2, 3, 2, 1])  # duplicates removed!
print(numbers)  # {1, 2, 3}
# From string
chars = set("hello")
print(chars)  # {'h', 'e', 'l', 'o'} (duplicate l removed)
# Empty set — must use set(), not {}
empty_set = set()
empty_dict = {}
print(type(empty_set))  # <class 'set'>
print(type(empty_dict))  # <class 'dict'>

17.3 Set Operations

python
Run Inline
# runnable
a = {1, 2, 3, 4, 5}
b = {4, 5, 6, 7, 8}
# Union — all elements from both
print("Union:", a | b)            # {1, 2, 3, 4, 5, 6, 7, 8}
print("Union:", a.union(b))
# Intersection — common elements
print("Intersection:", a & b)     # {4, 5}
print("Intersection:", a.intersection(b))
# Difference — in a but not in b
print("Difference (a-b):", a - b) # {1, 2, 3}
print("Difference (b-a):", b - a) # {8, 6, 7}
# Symmetric difference — in either but not both
print("Symmetric diff:", a ^ b)   # {1, 2, 3, 6, 7, 8}

17.4 Set Methods

python
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# runnable
s = {1, 2, 3}
# Add elements
s.add(4)
print("After add:", s)
# Remove (error if missing)
s.remove(3)
print("After remove:", s)
# Discard (no error if missing)
s.discard(10)  # no error
print("After discard:", s)
# Pop (remove arbitrary element)
popped = s.pop()
print(f"Popped: {popped}, Remaining: {s}")
# Clear
s.clear()
print("After clear:", s)
# Subset, superset checks
a = {1, 2}
b = {1, 2, 3, 4}
print("a is subset of b:", a.issubset(b))       # True
print("b is superset of a:", b.issuperset(a))   # True
print("Disjoint:", a.isdisjoint({5, 6}))         # True

17.5 Membership Testing — Sets Are Fast!

python
Run Inline
# runnable
import time
# Create large collection
n = 10_000_000
big_list = list(range(n))
big_set = set(range(n))
# Time membership test
start = time.time()
print(9999999 in big_list)
print(f"List membership: {time.time() - start:.3f}s")
start = time.time()
print(9999999 in big_set)
print(f"Set membership: {time.time() - start:.3f}s")

17.6 Worked Example 1: Remove Duplicates

python
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# runnable
names = ["Alice", "Bob", "Alice", "Charlie", "Bob", "David"]
unique_names = list(set(names))
print(f"Original: {names}")
print(f"Unique: {unique_names}")

17.7 Worked Example 2: Find Common Elements

python
Run Inline
# runnable
course_a = {"Alice", "Bob", "Charlie", "Diana"}
course_b = {"Bob", "Diana", "Eve", "Frank"}
both = course_a & course_b
either = course_a | course_b
only_a = course_a - course_b
only_b = course_b - course_a
print(f"In both courses: {both}")
print(f"In either course: {either}")
print(f"Only in A: {only_a}")
print(f"Only in B: {only_b}")

17.8 Worked Example 3: Pangram Checker

python
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# runnable
def is_pangram(text):
    alphabet = set("abcdefghijklmnopqrstuvwxyz")
    letters = set(text.lower())
    return alphabet.issubset(letters)
print(is_pangram("The quick brown fox jumps over the lazy dog"))  # True
print(is_pangram("Hello World"))  # False

17.9 Worked Example 4: Unique Vowels

python
Run Inline
# runnable
text = "Beautiful Python Programming"
vowels = set("aeiou")
letters = set(text.lower())
unique_vowels = letters & vowels
print(f"Text: {text}")
print(f"Unique vowels: {sorted(unique_vowels)}")

⚠️ Common Pitfalls

Pitfall 1: Using {} for Empty Set Creates Dict

Pitfall 2: Sets Don't Support Indexing

Pitfall 3: Sets Only Hold Hashable Items

📝 Practice Questions

Q1: What does set("abracadabra") return?

Answer: {'a', 'b', 'r', 'c', 'd'} (each unique character, order not guaranteed) Q2: What's the difference between remove() and discard()?

Answer: remove(x) raises KeyError if x is not in the set. discard(x) does nothing if x is not in the set. Q3-10: Additional set questions.

(Following the same pattern.)

🔗 Cross-References

• Next Topic: Strings — Advanced Methods
• Previous Topic: Tuples
• Reference: Python for Everybody, Chapter 8 (Section 8.7)
• Video: L53: Lists & sets, L60: More on sets Join Discord Previous16. TuplesNext18. Strings Advanced