Python frozenset() Function

The frozenset() function in Python returns an immutable version of a set. Once created, you cannot add or remove elements from a frozenset. It is often used when you need a constant set that won’t change and can be used as a dictionary key or set element.

Syntax

frozenset([iterable])

Parameters:

• iterable – Optional. An iterable like list, set, tuple, or string. If no argument is passed, it returns an empty frozenset.

Returns:

• An immutable frozenset object containing the unique elements from the iterable.

Example 1: Create a Frozenset from a List

nums = [1, 2, 3, 2]
fset = frozenset(nums)
print(fset)

frozenset({1, 2, 3})

Note: Duplicate elements are automatically removed, just like a normal set.

Example 2: Frozenset with Strings

fset = frozenset("hello")
print(fset)

frozenset({'e', 'h', 'l', 'o'})

Use Case: Frozenset as Dictionary Keys

fset1 = frozenset([1, 2])
fset2 = frozenset([3, 4])
my_dict = {fset1: "Group A", fset2: "Group B"}
print(my_dict)

{frozenset({1, 2}): 'Group A', frozenset({3, 4}): 'Group B'}

Why? Because frozensets are immutable, they can be used as dictionary keys or elements of another set, unlike normal sets.

Common Mistakes

• Trying to modify a frozenset using add() or remove() – will raise an AttributeError.
• Assuming it behaves exactly like a set in all situations — remember it is immutable!

Interview Tip

Use frozenset when you want to store a group of items that must remain constant and be hashable (usable in keys, nested sets, etc).

Summary

• frozenset() creates an immutable set.
• It removes duplicates like a normal set.
• It can be used in places where a hashable type is required (e.g., dict keys).
• Does not support add(), remove(), or any modification methods.

Practice Problem

Create a frozenset from a tuple and try to add an element to it. What happens?

colors = ("red", "blue", "red", "green")
fcolors = frozenset(colors)
print(fcolors)

# Try this (will raise an error)
# fcolors.add("yellow")

frozenset({'green', 'blue', 'red'})