Sets (unique items)
📚 What are Sets? A set is an unordered collection that stores only unique items - duplicates are automatically removed. If you add the same item twice, the set only keeps one copy. Sets are extremely fast for checking membership (if x in my_set) which is far quicker than searching a list.
Appy Says…
Need to find every unique visitor to your app? Remove duplicate entries from a list? Check if two groups share any members? Python's set does all three instantly — and it's way faster than a loop.
What is a Set?
A set is an unordered collection of unique items. Duplicate values are automatically ignored. Sets support fast membership testing (in) and powerful mathematical operations like union, intersection, and difference.
- •Create:
my_set = {1, 2, 3}orset([1, 2, 2, 3])→{1, 2, 3} - •Empty set:
set()— NOT{}(that creates an empty dict!) - •
.add(item)— add one item;.remove(item)— remove (error if missing);.discard(item)— remove silently - •
a | b— union (all items from both) - •
a & b— intersection (items in both) - •
a - b— difference (items in a but not b) - •
a ^ b— symmetric difference (items in one but not both)
Think of it like a Minecraft inventory with unique items only
Imagine an inventory where each item type can only appear once — no stacks. If you try to add a second diamond sword, nothing changes. That's a set. And you can instantly compare two inventories to find what's common or what's exclusive to each.
How It Works
- •1. Sets use a hash table internally — membership check is O(1), regardless of set size
- •2. When you add a duplicate, Python silently ignores it — no error
- •3. Order is not guaranteed — don't rely on iteration order
- •4. Only hashable (immutable) items can be in a set: ints, strings, tuples — not lists or dicts
- •5. Convert a list to a set to remove duplicates:
unique = list(set(my_list)) - •6.
item in my_setis much faster thanitem in my_listfor large collections
Real-World Examples
- •Count unique daily users:
len(set(user_ids_today)) - •Find common friends:
friends_of_alice & friends_of_bob - •Find songs in one playlist but not another:
playlist_a - playlist_b - •Remove duplicate tags from a blog post:
tags = list(set(tags)) - •Check if a username has been taken:
if username in registered_users_set:
Key Facts
- •Set membership (
in) is O(1) average — checking a million-item set is as fast as checking a 10-item set - •Lists have O(n) membership — a set is ~100,000× faster for large collections
- •Python also has
frozenset— an immutable set that can be used as a dictionary key - •Set comprehensions work just like list comprehensions:
{x*2 for x in range(5)}
Watch Out!
{} creates an empty dictionary, NOT an empty set. For an empty set you must write set(). This trips up almost every Python beginner at least once.
Remember
Use a set whenever you need uniqueness or fast membership checking. It's the right tool for 'have I seen this before?' logic.
What You Learned
- •Sets store unique items; duplicates are silently ignored
- •Operations:
|union,&intersection,-difference — all very fast - •Unlocks: deduplication, fast lookups, comparing groups, unique visitor counting
Key Facts
- →Set membership (
in) is O(1) average — checking a million-item set is as fast as checking a 10-item set - →Lists have O(n) membership — a set is ~100,000× faster for large collections
- →Python also has
frozenset— an immutable set that can be used as a dictionary key - →Set comprehensions work just like list comprehensions:
{x*2 for x in range(5)}
Real-World Examples
Remember
Use a set whenever you need uniqueness or fast membership checking. It's the right tool for 'have I seen this before?' logic.
Quick Quiz
Sets contain?