frozenset

frozenset([iterable])

Returns a frozenset object whose elements are taken from iterable. Frozensets are sets that have no update methods but can be hashed and used as members of other sets or as dictionary keys. The elements of a frozenset must be immutable themselves. To represent sets of sets, the inner sets should also be frozenset objects. If iterable is not specified, returns a new empty set, frozenset([]). New in version 2.4.