Programmer's Picnic · Python Foundations · 0 → ∞

Python Sets: From 0 to Infinity

Remove duplicates and solve membership and comparison problems with set algebra.

Free course · 55–75 minutes reading and practice · No installation beyond Python 3

Start at Level 0 ↓Try code in Python Editor ↗Collections overview

How to learn

Read one level, type the code into the editor, predict the output, then run it. Change one line and observe the result before proceeding. All code examples work with standard Python 3 (the dictionary union example requires 3.9 or newer).

The course moves from first principles to realistic code, common mistakes, performance and a mini-project.

Level 0 / 10

0 · A set stores unique values

A set is a mutable, unordered collection of distinct hashable elements. Display order is not guaranteed.

Python 3 · runnable example

numbers = {2, 3, 2, 5}
print(len(numbers))
print(3 in numbers)
print(type(numbers).__name__)

Expected output

3
True
set

Remember: Do not assume the order printed by a set; use sorted() for a predictable display.

Level 1 / 10

1 · Creating an empty set

{} makes an empty dictionary, not a set. Write set(). Convert an iterable to a set to discard duplicates.

Python 3 · runnable example

empty = set()
unique = set([4, 4, 2, 1])
print(type(empty).__name__)
print(sorted(unique))

Expected output

set
[1, 2, 4]

Remember: Converting to a set does not retain original order. dict.fromkeys(items) can deduplicate while preserving order.

Level 2 / 10

2 · Adding and removing

add() inserts one value; update() inserts from an iterable; discard() is safe for absent members, whereas remove() raises KeyError.

Python 3 · runnable example

skills = {'Python'}
skills.add('SQL')
skills.update(['HTML', 'CSS'])
skills.discard('Java')
print(sorted(skills))

Expected output

['CSS', 'HTML', 'Python', 'SQL']

Remember: pop() removes an arbitrary element; do not use it to retrieve a supposed first member.

Level 3 / 10

3 · Union and intersection

Union collects all distinct members. Intersection keeps only members shared by both sets. Operators | and & provide concise syntax.

Python 3 · runnable example

a = {'Python', 'SQL'}
b = {'SQL', 'Java'}
print(sorted(a | b))
print(sorted(a & b))

Expected output

['Java', 'Python', 'SQL']
['SQL']

Remember: a.union(b) and a.intersection(b) have the same meanings as | and &.

Level 4 / 10

4 · Difference and symmetric difference

Difference a-b keeps members only in a. Symmetric difference a^b keeps members in exactly one of the sets.

Python 3 · runnable example

a = {1, 2, 3}
b = {3, 4}
print(sorted(a - b))
print(sorted(b - a))
print(sorted(a ^ b))

Expected output

[1, 2]
[4]
[1, 2, 4]

Remember: Difference is directional: a-b is generally not the same as b-a.

Level 5 / 10

5 · Subsets and disjoint sets

A <= B checks whether A is a subset of B. isdisjoint() checks whether two sets share no elements.

Python 3 · runnable example

required = {'HTML', 'CSS'}
student = {'HTML', 'CSS', 'Python'}
print(required <= student)
print(required < student)
print(required.isdisjoint({'SQL'}))

Expected output

True
True
True

Remember: < tests a proper subset; <= allows equality.

Level 6 / 10

6 · Set comprehensions

Like list comprehensions, set comprehensions transform and filter values but produce unique results.

Python 3 · runnable example

words = ['cat', 'dog', 'cat', 'owl']
lengths = {len(word) for word in words}
initials = {word[0] for word in words}
print(sorted(lengths))
print(sorted(initials))

Expected output

[3]
['c', 'd', 'o']

Remember: Use generator expressions if you do not need to store the complete set.

Level 7 / 10

7 · Hashability and nested data

Set members must be hashable: numbers, strings and tuples of hashable members work; lists and dictionaries do not.

Python 3 · runnable example

coordinates = {(2, 3), (4, 5), (2, 3)}
print(len(coordinates))
print((4, 5) in coordinates)

Expected output

2
True

Remember: A tuple containing an unhashable list is also unhashable.

Level 8 / 10

8 · frozenset for immutable sets

frozenset cannot be modified after creation and can be used as a dictionary key if its elements are hashable.

Python 3 · runnable example

pair = frozenset({'Asha', 'Meera'})
team_scores = {pair: 10}
print(team_scores[frozenset({'Meera', 'Asha'})])

Expected output

Remember: A frozenset has no add() or remove() methods.

Level 9 / 10

9 · Efficiency and order pitfalls

Average-case membership tests are O(1), although pathological hashing can degrade performance. A set has no meaningful position, so never index or slice it.

Python 3 · runnable example

seen = set()
for code in ['A', 'B', 'A', 'C']:
    if code not in seen:
        seen.add(code)
        print('New:', code)

Expected output

New: A
New: B
New: C

Remember: The example prints in input order because we iterate a list, not because sets are ordered.

Level 10 / 10

10 · Mini-project: club comparison

Find students who belong to both clubs, just one club, or all clubs combined.

Python 3 · runnable example

coding = {'Asha', 'Ravi', 'Meera'}
robotics = {'Meera', 'Kabir'}
print('Both:', sorted(coding & robotics))
print('Only coding:', sorted(coding - robotics))
print('Either:', sorted(coding | robotics))

Expected output

Both: ['Meera']
Only coding: ['Asha', 'Ravi']
Either: ['Asha', 'Kabir', 'Meera', 'Ravi']

Remember: Extension: show students in exactly one of the clubs with coding ^ robotics.

Practice lab

Test your understanding

Predict or explain the answer before opening the solution. Try rewriting one of the examples without copying it.

1. What is the result of type({}).__name__?

Answer: dict

Use set() for an empty set.

2. Which operation finds common elements?

Answer: a & b or a.intersection(b)

Intersection is the overlap.

3. Can you use my_set[0]?

Answer: No; sets have no positional indexing.

Sort when you need a predictable sequence.

Take it further: expand the mini-project with an input() prompt, a reusable function, input validation and three tests (normal, empty and boundary cases).

When to use each Python collection

Comparison of Python built-in data types
Type	Ordered?	Mutable?	Duplicates?	Best for
str	Yes	No	Yes	Text
list	Yes	Yes	Yes	Changing sequences
tuple	Yes	No*	Yes	Fixed records
set	No guaranteed order	Yes	No	Unique membership
dict	Insertion order	Yes	Keys unique	Named lookups

* A tuple itself cannot be reassigned, but a mutable item stored inside it can still change.

Python official built-in types reference ↗

Programmer's Picnic · Developed and maintained by Champak Roy · Practice with real Python, one step at a time.