Immutable sets

Python has an immutable set type called frozenset. It works pretty much exactly like set apart from not allowing methods or operations that change values such as the add() or clear() methods. There are several ways that this immutability can be useful. For example, since normal sets are mutable and therefore not hashable, they cannot be used as members of other sets. The frozenset, on the other hand, is immutable and therefore able to be used as a member of a set:

Also the immutable property of frozenset means we can use it for a key to a dictionary, for example: