Master Python Sets: Creation, Operations, and Quick Reference Guide

What is a Python set?

In Python, a set is an unordered, mutable collection of unique hashable elements. It is ideal for membership testing, deduplication, and performing mathematical operations such as union, intersection, and difference.

Key characteristics

Unordered : Elements have no fixed position.

Mutable : Elements can be added or removed.

No duplicates : Duplicate entries are automatically eliminated.

Elements must be immutable (e.g., int, str, tuple).

Creating sets

# Using curly braces
my_set = {1, 2, 3}
# Using the set() constructor
my_set = set([1, 2, 3, 3, 4])  # {1, 2, 3, 4}

To create an empty set you must use set(); {} creates an empty dictionary.

Set comprehension

squares = {x*x for x in range(5)}  # {0, 1, 4, 9, 16}

Basic set methods

add(elem)

– add a single element. update(iterable) – add multiple elements. remove(elem) – remove element, raise KeyError if absent. discard(elem) – remove element if present, no error otherwise. pop() – remove and return an arbitrary element. clear() – empty the set. in / not in – membership test.

Set algebra

union()

or a | b – union of two sets. intersection() or a & b – intersection. difference() or a - b – elements in a but not in b. symmetric_difference() or a ^ b – elements in either set but not both. issubset() – true if all elements of a are in b. issuperset() – true if b is subset of a. isdisjoint() – true if sets share no elements.

Practical examples

# Remove duplicates from a list
unique = set([1, 2, 2, 3, 4, 4])  # {1, 2, 3, 4}
# Fast membership test
if 3 in unique:
    print("3 is in the set")
# Find common and unique elements
a = {1, 2, 3}
b = {2, 3, 4}
print(a & b)  # {2, 3}
print(a | b)  # {1, 2, 3, 4}
print(a - b)  # {1}
print(a ^ b)  # {1, 4}

Limitations

Sets cannot contain mutable (unhashable) types such as lists or other sets.

Because sets are unordered, they do not support indexing or slicing.

Quick reference

Creating a set: {1, 2, 3} or set([1, 2, 3]) Adding an element: s.add(x) Batch update: s.update([x, y]) Removing an element: s.remove(x) (error if missing) or s.discard(x) (no error).

Random pop: s.pop() Clear: s.clear() Union: s | t or s.union(t) Intersection: s & t or s.intersection(t) Difference: s - t or s.difference(t) Symmetric difference: s ^ t or s.symmetric_difference(t) Subset test: s <= t or s.issubset(t) Superset test: s >= t or s.issuperset(t) Disjoint test:

s.isdisjoint(t)