Set Basics — Uniqueness, Creation, and Limitations

What you will learn

Unique Values · Hashable · Membership · Deduplication

A set is an unordered collection that stores only unique values. The biggest use cases are removing duplicates and performing fast membership checks.

Property	Set behavior
Duplicates	Removed automatically
Order	Not guaranteed
Index / slice	Not supported
Membership	Fast with in
Elements	Must be hashable

Memory formula: Add → Unique → Check → No Index

After File 07 — Tuple Methods, you move from ordered sequences to unordered unique collections. Intro: set — Unique Unordered Collection.

Learning roadmap

1. What is a set?

A set automatically removes duplicate values.

s = {10, "RS", 10, 20}

print(s)      # {10, 'RS', 20} — order may vary
print(len(s)) # 3

Note: Order is not guaranteed, so your output may appear differently.

2. Set vs list vs tuple

Feature	List	Tuple	Set
Ordered	Yes	Yes	No
Duplicates	Yes	Yes	No
Mutable	Yes	No	Yes
Indexing	Yes	Yes	No
Best for	Dynamic data	Fixed data	Unique values

Visual memory card

3. Creating sets

Different ways

s1 = {10, 20, 30}
s2 = set()
s3 = set([10, 20, 10, 30])
s4 = set("MISSISSIPPI")

From tuple:

print(set((1, 2, 2, 3)))  # {1, 2, 3}

4. The empty set trap

This is another famous Python interview question.

not_set = {}    # dict!
real_set = set()

print(type(not_set))   # dict
print(type(real_set))  # set

5. Remove duplicates instantly

One of the most practical uses of sets.

marks = [85, 90, 85, 78, 90, 92]
unique = set(marks)
print(unique)  # {78, 85, 90, 92} — order may vary

If needed again as a list:

back = list(unique)

6. Sets have no index

Sets are unordered, so indexing makes no sense.

Wrong

s = {10, 20, 30}
s[0]  # TypeError: 'set' object is not subscriptable

Correct

print(10 in s)  # True
print(50 in s)  # False

Use membership (in) instead of indexing.

Item assignment and del s[i] also fail — only del s removes the whole variable.

7. Create sets from strings

Every character becomes one unique element.

letters = set("MISSISSIPPI")
print(letters)  # {'M', 'I', 'S', 'P'} — order may vary

8. Hashable vs unhashable

Only hashable (generally immutable) objects can be stored inside a set.

Allowed	Not allowed
int, float, str	list
tuple (if items hashable)	dict
frozenset	set (mutable)

s = {10, "RS", (1, 2)}
print(s)  # OK

Invalid:

s = {[1, 2]}  # TypeError: unhashable type: 'list'

See Hashability and Mixed Collections for the full rules.

Mini project — website unique visitors

Imagine you are tracking users visiting a webpage.

visitors = set()

visitors.add("user_42")
visitors.add("user_17")
visitors.add("user_42")  # duplicate ignored

print(len(visitors))  # 2
print(visitors)

Visual dashboard

Notice how adding "user_42" twice does not increase the count. Full mutating methods come in File 09 — Set Methods.

Set category

Mutable	Immutable
set	frozenset

One-page revision sheet

Task	Syntax
Create set	{1, 2, 3}
Empty set	set()
From list	set(lst)
From string	set("HELLO")
Remove duplicates	set(data)
Membership	x in s
Size	len(s)

Common interview mistakes

Mistake 1 — using {} for an empty set:

s = {}       # creates dict — wrong
s = set()    # correct

Mistake 2 — trying to access s[0]:

s[0]         # TypeError
10 in s      # correct — membership test

Practice questions

Test yourself before File 09 — Set Methods:

1. Why does set([1, 2, 2, 3]) contain only three values?
2. What is the difference between {} and set()?
3. Why can't you use s[0] on a set?
4. What is the output of set("BANANA")?
5. Why is a list not allowed inside a set?

Knowledge check

Q1: Why only three values in set([1, 2, 2, 3])?
Answer: Sets store unique values — duplicate 2 is removed.

Q2: {} vs set()?
Answer: {} is an empty dict; set() is an empty set.

Q3: Why no s[0]?
Answer: Sets are unordered — there is no index position.

Q4: set("BANANA")?
Answer: {'B', 'A', 'N'} — three unique letters (order may vary).

Q5: Why no list inside a set?
Answer: Lists are mutable/unhashable — set elements must be hashable.