Python Basics — Creating Sets

Creating Sets

A set is an unordered collection where every element is unique. Duplicate values are automatically removed.

Set Literal Syntax

fruits = {"apple", "banana", "cherry"}
print(fruits)  # {'cherry', 'banana', 'apple'} — order may vary
print(type(fruits))  # <class 'set'>

Automatic Deduplication

numbers = {1, 2, 3, 2, 1, 4, 3}
print(numbers)  # {1, 2, 3, 4}

set() Constructor

# From a list
from_list = set([1, 2, 3, 2, 1])
print(from_list)  # {1, 2, 3}

# From a string
from_string = set("hello")
print(from_string)  # {'h', 'e', 'l', 'o'}

# From a tuple
from_tuple = set((1, 2, 3))
print(from_tuple)  # {1, 2, 3}

# From a range
from_range = set(range(5))
print(from_range)  # {0, 1, 2, 3, 4}

Empty Set

This is a common gotcha — {} creates an empty dict, not a set:

# ❌ This is a dict!
empty_dict = {}
print(type(empty_dict))  # <class 'dict'>

# ✅ Use set() for empty sets
empty_set = set()
print(type(empty_set))  # <class 'set'>
print(len(empty_set))   # 0
print(bool(empty_set))  # False (empty = falsy)

Set Properties

Unordered

Sets have no index — you cannot access elements by position:

s = {3, 1, 2}
# print(s[0])  # TypeError: 'set' object is not subscriptable

Unique

Every element appears at most once:

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

Mutable

You can add and remove elements:

s = {1, 2, 3}
s.add(4)
s.remove(1)
print(s)  # {2, 3, 4}

Hashable Elements Requirement

Set elements must be hashable (immutable):

# ✅ Valid elements
s = {1, 2.5, "hello", (1, 2), True, None}

# ❌ Invalid — mutable types
# s = {[1, 2]}     # TypeError: unhashable type: 'list'
# s = {{"a": 1}}   # TypeError: unhashable type: 'dict'
# s = {{1, 2}}     # TypeError: unhashable type: 'set'

Deduplicating a List

One of the most common uses for sets:

names = ["Alice", "Bob", "Alice", "Charlie", "Bob", "Diana"]
unique = list(set(names))
print(unique)  # ['Diana', 'Charlie', 'Alice', 'Bob'] — order not preserved

To preserve order:

names = ["Alice", "Bob", "Alice", "Charlie", "Bob", "Diana"]
unique = list(dict.fromkeys(names))
print(unique)  # ['Alice', 'Bob', 'Charlie', 'Diana'] — order preserved

Membership Testing (Why Sets Are Fast)

# Set lookup is O(1) — constant time
large_set = set(range(1_000_000))
print(999_999 in large_set)  # True (instant)

# List lookup is O(n) — linear time
large_list = list(range(1_000_000))
print(999_999 in large_list)  # True (much slower for large lists)

Summary

• Create with {1, 2, 3} or set(iterable)
• Empty set: set() (not {})
• Unordered, unique, mutable
• Elements must be hashable (immutable types)
• Duplicate values are automatically removed
• O(1) membership testing — much faster than lists

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