Answer.

Background

The frozenset type was introduced in Python as an immutable version of the standard set type. It was important to ensure that sets could be used as elements of other sets and as keys of dictionaries — hence the need for an immutable variant.

Problem

The practical task is to require uniqueness and the ability to quickly check membership, as well as to use a set as a key in a dictionary or an element of another set. A regular set is not suitable, as it is mutable and, accordingly, not hashable.

Solution

frozenset is an immutable set. Once created, its elements cannot be changed: added, removed, or modified. However, frozenset supports all the same operations as set, except for the mutating ones.

Code example:

fs = frozenset([1, 2, 3])
print(2 in fs)  # True
# fs.add(4)  # AttributeError: 'frozenset' object has no attribute 'add'
map_dict = {fs: 'frozen'}  # Allowed, as frozenset is hashable

Key features:

frozenset is an immutable, hashable set.
Supports all non-destructive methods of regular sets (union, intersection, difference).
Can be used as a key in a dictionary or an element of another set.

Tricky questions.

Can frozenset contain mutable objects (e.g., a list)?

No, only hashable/immutable objects can be elements of a set of any type. frozenset([[], 1]) will raise TypeError, just like a regular set.

Can a mutable set be obtained from a frozenset and back?

Both types can be converted into each other using standard constructors:

fs = frozenset([1,2,3])
s = set(fs)
print(type(s))  # <class 'set'>

Does frozenset support the methods add() or remove()?

No, these methods are absent; any attempt to call them will result in an AttributeError, as frozenset is immutable.

Common mistakes and anti-patterns

Trying to add or remove an element from a frozenset.
Using mutable types as elements of frozenset.
Expecting that frozenset can be modified after creation.

Real-life example

Negative case

Using a regular set as a key in a dict:

s = set([1, 2])
d = {s: 'value'}  # TypeError: unhashable type: 'set'

Pros:

Simple attempt, familiar type — less code.

Cons:

Runtime error.
Inability to achieve the intended behavior.

Positive case

Converting to frozenset:

fs = frozenset([1, 2])
d = {fs: 'value'}  # OK
print(d[frozenset([1, 2])])  # 'value'