In what cases and frameworks is the frozenset data type used in Python?

frozenset is an immutable version of a set in Python. Its main features:

• Once created, elements of a frozenset cannot be changed (no adding or removing).
• It can be used as a key in dictionaries or as an element of other sets because it is hashable.

It is used when you need a set that should not change, but still want to use it as a key or an element of a set.

Example of usage:

fs = frozenset([1, 2, 3])

# Can be used as a key in a dictionary
d = {fs: "value"}

print(d[fs])  # outputs "value"