Python frozenset() — A Simple Guide with Video – Be on the Right Side of Change

Python’s built-in frozenset() function creates and returns a new frozenset object. A frozenset is an immutable set—so you cannot change the frozenset after creation and set methods such as add() or remove() don’t work on the frozenset. Without an argument, frozenset() returns an empty frozenset. With the optional argument, frozenset(iter) initializes the new frozenset with the elements in the iterable.

Read more about sets in our full tutorial about Python Sets.

Usage

Learn by example! Here are some examples of how to use the frozenset() built-in function:

You can create an empty set by skipping the argument:

>>> frozenset()
frozenset()

If you pass an iterable—such as a list, tuple, set, or dictionary—you obtain a new frozenset object with elements obtained from the iterable:

>>> frozenset([1, 2, 3])
frozenset({1, 2, 3})

Note that it really creates a new frozenset object that is different from the one passed as an argument:

>>> set_a = {1, 2, 3}
>>> frozenset_a = frozenset(set_a)
>>> set_a == frozenset_a
True
>>> set_a is frozenset_a
False

The new set frozenset_a has the same elements as the original set set_a. But it’s still a different object as you can see from the check set_a is frozenset_a that returns False.

You cannot add or remove elements from a frozenset because it’s immutable:

>>> s = frozenset({'Alice', 'Bob', 'Ann'})
>>> s.add('Ben')
Traceback (most recent call last):
  File "<pyshell#14>", line 1, in <module>
    s.add('Ben')
AttributeError: 'frozenset' object has no attribute 'add'
>>> s.remove('Alice')
Traceback (most recent call last):
  File "<pyshell#15>", line 1, in <module>
    s.remove('Alice')
AttributeError: 'frozenset' object has no attribute 'remove'

This would work for Python sets that are mutable:

>>> s = {'Alice', 'Bob', 'Ann'}
>>> s.add('Ben')
>>> s.remove('Alice')
>>> s
{'Bob', 'Ann', 'Ben'}

Conceptually, the only difference between sets and frozensets is that the former are mutable and the latter are immutable. Consequently, the latter doesn’t provide the same methods to modify an existing set.

Video frozenset()

Syntax frozenset()

You can use the frozenset() method with or without the optional iterable argument.

Syntax: There are two ways of using the constructor:
frozenset() -> new empty frozenset
frozenset(iterable) -> new frozenset initialized with elements in iterable

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Frozenset Operations

Here are a few examples that show which operations you can perform on a frozenset:

# Frozenset Operations

# initialize frozensets A and B
A = frozenset(['Alice', 'Bob', 'Carl'])
B = frozenset(['Bob', 'Carl', 'David'])

# copy
C = A.copy()
print(C)
# frozenset({'Alice', 'Bob', 'Carl'})

# union
print(A.union(B))
# frozenset({'Alice', 'David', 'Bob', 'Carl'})

# intersection
print(A.intersection(B))
# frozenset({'Bob', 'Carl'})

# difference
print(A.difference(B))
# frozenset({'Alice'})

# symmetric difference
print(A.symmetric_difference(B))
# frozenset({'Alice', 'David'})

# isdisjoint()
print(A.isdisjoint(C))
# False

# issubset()
C = A.union(B)
print(C.issubset(B))
# False

# issuperset()
print(C.issuperset(B))
# True

As a rule of thumb: you can perform all set operations but you cannot modify an existing set.

Summary

Python’s built-in frozenset() function creates and returns a new frozenset object that is an immutable version of a Python set.

When used without an argument, it returns an empty frozenset. As you cannot change a frozenset object, this wouldn’t make a lot of sense.
When used with the optional iterable argument, it initializes the new frozenset with the elements in the iterable.

>>> frozenset()
frozenset()
>>> frozenset([1, 2, 3])
frozenset({1, 2, 3})

References: https://docs.python.org/3/library/stdtypes.html#frozenset

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