4.6. Frozensets

Contents

• Frozensets
  • Quick Example
  • Set of Sets
  • Using set as key in a dictionary
  • Methods

Sets are mutable, and may therefore not be used, for example, as keys in dictionaries.

Another problem is that sets themselves may only contain immutable (hashable) values, and thus may not contain other sets.

Because sets of sets often occur in practice, there is the frozenset type, which represents immutable (and, therefore, hashable) sets.

4.6.1. Quick Example

>>> a = frozenset([1, 2, 3])
>>> b = frozenset([2, 3, 4])
>>> a.union(b)
frozenset([1, 2, 3, 4])

4.6.2. Set of Sets

Sets may only contain immutable (hashable) values, and thus may not contain other sets, in which case frozensets can be useful. Consider the following example:

>>> a = set([1, 2, 3])
>>> b = set([2, 3, 4])
>>> a.add(b)
>>> a.add(frozenset(b))

4.6.3. Using set as key in a dictionary

If you want to use a set as a key dictionary, you will need frozenset:

>>> fa = {frozenset([1,2]): 1}
>>> fa[ frozenset([1,2]) ]
1

4.6.4. Methods

frozensets have less methods than sets.

There are some operators similar to sets (intersection(), union(), symmetric_difference(), difference(), issubset(), isdisjoint(), issuperset()) and a copy() method.