Python 3 reference · Built-in Functions

Python frozenset() Function

Create an immutable set: unique items that can't be added or removed, so it is hashable and can be a dict key.

Syntax

frozenset(iterable=())

Parameter	Description
`iterable` optional	The items; duplicates are dropped.

A new frozenset.

Create and test membershipTry it yourself

vowels = frozenset("aeiou")
print("e" in vowels, "z" in vowels)
print(len(frozenset([1, 1, 2, 3])))

Output

True False
3

Use as a dictionary keyTry it yourself

routes = {frozenset({"A", "B"}): 12, frozenset({"B", "C"}): 7}
print(routes[frozenset({"B", "A"})])

Output

Set operations return new frozensetsTry it yourself

a, b = frozenset({1, 2, 3}), frozenset({2, 3, 4})
print(sorted(a & b), sorted(a | b), sorted(a - b))

Output

[2, 3] [1, 2, 3, 4] [1]

Version	Change
Python 3.0	Added

set() · Create a set: an unordered collection of unique items with fast membership tests and set operations.
frozendict() · Create an immutable, hashable dictionary (new in Python 3.15). It can be a dict key or set member and can't be changed.
tuple() · Create a tuple: an ordered, immutable sequence. Tuples are hashable when their items are, so they can be dict keys.
Python Sets · tutorial