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Sets

Here is a clear, complete, and exam-oriented explanation of Sets in Python, perfect for BCA/MCA/B.Tech students.

Sets in Python

A set in Python is an unordered, mutable, and collection of unique elements.
Sets are used when:

  • You want to store unique values
  • You don’t care about order
  • You need fast membership testing

Sets are implemented using hash tables, which makes them very fast for operations like searching.

1. Creating a Set

Sets are created using curly braces { } or the set() function.

Example:

s = {1, 2, 3, 4}
print(s)

Mixed data types:

s = {"apple", 10, 3.14}

Creating an empty set:

s = set()      # Correct

❌ Wrong:

s = {}         # This creates an empty dictionary, not a set

2. Characteristics of Sets

✔ Unordered → no index, no slicing
✔ Mutable → can add/remove elements
✔ Unique → duplicates automatically removed
✔ Does not support indexing → s[0] ❌ error

Example:

s = {1, 2, 2, 3}
print(s)   # Output: {1, 2, 3}

3. Accessing Set Elements

Because sets are unordered, you cannot access elements by index.

You can access elements using a loop:

s = {10, 20, 30}
for x in s:
    print(x)

4. Adding Elements

add() → add single item

s = {1, 2}
s.add(3)

update() → add multiple items

s.update([4, 5], {6, 7})

5. Removing Elements

remove(x) → removes item; gives error if not found

s.remove(3)

discard(x) → removes item; no error if item not found

s.discard(10)

pop() → removes a random element

s.pop()

clear() → remove all elements

s.clear()

6. Set Operations (Very Important for Exams)

Python supports mathematical set operations.

1. Union (| or union())

Combines all unique elements from two sets.

a = {1, 2, 3}
b = {3, 4, 5}

print(a | b)            # {1, 2, 3, 4, 5}
print(a.union(b))       # same

2. Intersection (& or intersection())

Common elements.

print(a & b)            # {3}
print(a.intersection(b))

3. Difference (- or difference())

Elements in a but not in b.

print(a - b)            # {1, 2}
print(a.difference(b))

4. Symmetric Difference (^ or symmetric_difference())

Elements not common to both sets.

print(a ^ b)            # {1, 2, 4, 5}

7. Set Membership

s = {1, 2, 3}
print(2 in s)          # True
print(5 not in s)      # True

8. Frozenset (Immutable Set)

frozenset is an immutable version of a set.

fs = frozenset([1, 2, 3])
  • Cannot add/delete elements
  • Can be used as dictionary keys
  • Supports all set operations

9. Built-in Set Methods

MethodDescription
add()Add element
update()Add multiple elements
remove()Remove item (error if not found)
discard()Remove item (no error)
pop()Remove random item
clear()Remove all items
union()Set union
intersection()Common items
difference()Items in one set only
symmetric_difference()Uncommon items
issubset()Check if a set is subset
issuperset()Check if superset
isdisjoint()True if no common elements

10. Practical Examples

Example 1: Remove duplicates from a list

lst = [1, 2, 2, 3, 4, 4]
unique = list(set(lst))
print(unique)

Example 2: Testing subset

a = {1, 2}
b = {1, 2, 3, 4}
print(a.issubset(b))    # True

Example 3: Finding common characters

set("python") & set("typing")

Summary Table: Sets

PropertyDetail
Ordered❌ No
Mutable✔ Yes
Allow duplicates❌ No
Indexing allowed❌ No
Syntax{} or set()
Used forFast lookup, unique items, mathematical operations