frozenset

Frozenset is an immutable, unordered collection of distinct elements in Python. It is the immutable counterpart to the built-in set type. Like a set, a frozenset cannot contain duplicates and supports membership testing, iteration, and standard set operations, but its contents cannot be changed after creation.

Creation and immutability: A frozenset is created with the frozenset() constructor, optionally with an iterable, e.g., frozenset([1, 2, 3]). An empty frozenset is produced by frozenset(). Because it is immutable, a frozenset is hashable if all its elements are hashable, which allows it to be used as a dictionary key or as an element of another set.

Operations: Frozensets support union, intersection, difference, and symmetric_difference using operators |, &, -, and ^, or the corresponding methods. The results of these operations are new frozenset objects. Membership tests (x in f) and iteration are supported, and issubset/issuperset comparisons (<=, >=) are available.

Limitations: A frozenset does not support in-place modification methods such as add, remove, discard, pop, or update. Its immutability means that you must create a new frozenset to reflect any change. All elements of a frozenset must be hashable to be included.

Usage: Frozensets are useful when a fixed collection must be used as a dictionary key or as an element of a set, or when a read-only view of a set is required. They are typically used to represent fixed sets of attributes, categories, or configuration flags.