Python frozenset(): Immutable and Hashable Set

frozenset() is a built-in that creates an immutable, hashable set-like object from an iterable. It keeps the uniqueness and mathematical set operations of a regular set, but it does not provide methods that modify its contents.

Because a frozenset is immutable and hashable, it can be used where a regular mutable set cannot, such as a dictionary key or an element inside another set.

Syntax

frozenset(iterable)

The iterable can be a list, tuple, set, string or another iterable. Every element stored in the frozenset must itself be hashable.

Create a frozenset from a List Top ↑

values = [1, 2, 3, 2]
frozen = frozenset(values)
print(frozen)

The duplicate 2 is stored only once. Printed order should not be treated as fixed.

frozenset Cannot Be Modified In Place Top ↑

frozen = frozenset({1, 2, 3})
frozen.add(4)

This raises AttributeError because frozenset has no mutating add() method.

Item assignment also fails:

frozen = frozenset([1, 2, 3])
frozen[1] = 8

This raises TypeError. A frozenset is also unordered, so positional assignment would not make sense.

Create a frozenset from a Tuple Top ↑

my_tuple = (1, 2, 3)
frozen = frozenset(my_tuple)
print(frozen)

The original tuple can still be rebound or extended independently; the frozenset remains unchanged.

my_tuple = (1, 2, 3)
frozen = frozenset(my_tuple)
my_tuple += (4, 5)
print(my_tuple)
print(frozen)

Set Operations Return New frozenset Objects Top ↑

Non-mutating set operations are supported and return new objects.

A = frozenset({1, 2, 3})
B = frozenset({3, 4, 5})
print(A | B)
print(A & B)
print(A - B)
print(A ^ B)

As with regular sets, displayed order is not guaranteed.

Use a frozenset as a Dictionary Key Top ↑

permissions = {
    frozenset({'read', 'write'}): 'editor',
    frozenset({'read'}): 'viewer',
}
print(permissions[frozenset({'write', 'read'})])

editor

This works because a frozenset is hashable. A regular set cannot be used as a dictionary key.

Use a frozenset Inside a Set Top ↑

groups = {
    frozenset({1, 2}),
    frozenset({3, 4}),
}
print(len(groups))

2

set vs frozenset Top ↑

Common Mistakes Top ↑

• The correct name is frozenset, not fronzenset.
• frozenset does not make an unhashable nested value valid; its elements still need to be hashable.
• The + operator is not a set-union operator. Use | or union().
• Operations such as A | B create a new frozenset; they do not mutate A.

When to Use frozenset Top ↑

• Represent a fixed collection of unique values.
• Use a set-like collection as a dictionary key.
• Store a set-like collection inside another set.
• Protect a collection from accidental mutation while retaining set operations.

Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

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