What is the difference between "set" and "frozenset" in Python?

Question

Jayant Kumar · Accepted Answer

Table of Contents
Introduction
In Python, both set and frozenset are used to store collections of unique elements. While they share some similarities, they also have key differences in terms of mutability and usage. This article explores these differences, detailing their properties, use cases, and providing practical examples to illustrate their distinct characteristics.
Key Differences Between set and frozenset
1. Mutability

**set**: A set is a mutable collection of unique elements. This means that you can add, remove, or update elements within a set after it has been created. Because of its mutability, set objects are not hashable and cannot be used as dictionary keys or elements of other sets.
**frozenset**: A frozenset is an immutable version of a set. Once created, its elements cannot be changed, added, or removed. frozenset objects are hashable, which allows them to be used as keys in dictionaries or elements in other sets.

Example:
# Creating and modifying a set
my_set = {1, 2, 3}
my_set.add(4)        # Add element
my_set.remove(2)     # Remove element
print(my_set)        # Output: {1, 3, 4}

# Creating a frozenset
my_frozenset = frozenset([1, 2, 3])
# my_frozenset.add(4)  # This will raise an AttributeError
print(my_frozenset)  # Output: frozenset({1, 2, 3})

2. Usage and Performance

**set**: Suitable for scenarios where you need a collection of unique elements and require operations like adding or removing elements. Sets are typically used for membership testing, eliminating duplicates from a sequence, and performing set operations like union, intersection, and difference.
**frozenset**: Useful when you need a fixed collection of unique elements that should not be modified. Because frozenset is immutable, it is ideal for situations where you need a hashable collection to use as a dictionary key or an element of another set.

Example:
# Using set operations
set1 = {1, 2, 3}
set2 = {3, 4, 5}
print(set1.union(set2))       # Output: {1, 2, 3, 4, 5}
print(set1.intersection(set2)) # Output: {3}
print(set1.difference(set2))  # Output: {1, 2}

# Using frozenset as a dictionary key
my_dict = {frozenset([1, 2, 3]): 'value'}
print(my_dict[frozenset([1, 2, 3])])  # Output: value

3. Methods and Operations

**set**: Supports a wide range of methods for modifying the collection, including add(), remove(), discard(), and pop(). It also supports various set operations like union, intersection, difference, and symmetric difference.
**frozenset**: Supports similar set operations but does not include methods for modifying the collection, such as add() or remove(). You can use frozenset methods for operations like union, intersection, difference, and symmetric difference, but the collection remains immutable.

Example:
# Using set methods
my_set = {1, 2, 3}
my_set.add(4)          # Add an element
my_set.discard(2)      # Remove an element without raising an error if not present
print(my_set)          # Output: {1, 3, 4}

# Using frozenset methods
my_frozenset = frozenset([1, 2, 3])
new_frozenset = my_frozenset.union(frozenset([3, 4]))
print(new_frozenset)   # Output: frozenset({1, 2, 3, 4})

Practical Examples
Example : Using set for Dynamic Collections
# Dynamic collection with set
elements = {1, 2, 3}
elements.add(4)         # Add an element
print(elements)        # Output: {1, 2, 3, 4}

Example : Using frozenset as a Dictionary Key
# Immutable collection used as a dictionary key
def describe_elements(elements):
    return f"Elements are: {elements}"

element_set = frozenset([10, 20, 30])
description = describe_elements(element_set)
print(description)    # Output: Elements are: frozenset({10, 20, 30})

Conclusion
In Python, set and frozenset both represent collections of unique elements, but they differ in terms of mutability and use cases. set is mutable and suitable for scenarios where elements need to be added or removed, while frozenset is immutable and ideal for fixed collections that require hashability. Understanding these differences helps in selecting the appropriate data structure based on your requirements for mutability and performance.

What is the difference between "set" and "frozenset" in Python?

Table of Contents

Introduction

Key Differences Between `set` and `frozenset`

1. Mutability

Example:

2. Usage and Performance

Example:

3. Methods and Operations

Example:

Practical Examples

Example : Using `set` for Dynamic Collections

Example : Using `frozenset` as a Dictionary Key

Conclusion

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