Immutable membership collections
Representing a fixed, unordered collection of values
Python · Class
frozenset
frozenset creates an immutable, hashable set of unique elements. It is useful as a dictionary key or as an element of another set when the collection itself must remain fixed.
How it works
Call frozenset with an iterable to create an immutable set of its elements. Because the contents cannot change, a frozenset is hashable and can serve as a dictionary key or as an element inside another set; it supports set operations but not mutating operations.
Syntax
class frozenset(iterable=(), /)Parameters
| Name | Type | Description |
|---|---|---|
iterable | iterable | An optional iterable whose elements are used to create the new frozenset; it defaults to an empty iterable. |
Returns
A new frozenset object containing the elements taken from iterable.
Key points
- Python provides frozenset as the immutable counterpart to the mutable set type.
- Its contents cannot be changed after construction, and it can therefore participate in hash-based collections.
- Use a frozenset as a dictionary key when the key represents an unordered, fixed collection of values.
- A frozenset can be nested inside a set because it is hashable.
Common mistakes
- Do not use mutating methods or in-place update operators; choose a new set operation when you need a changed value.
Example: Use a frozenset as a dictionary key
allowed = frozenset({"read", "write"})
permissions = {allowed: "editor"}
print("write" in allowed)
print(permissions[allowed])Creates an immutable set of permissions and uses it as a dictionary key.
When to use it
Composite dictionary keys
Using a collection of values as a dictionary key
Sets of sets
Nesting one set-like collection inside another
Related
setfrozenset.unionfrozenset.intersectionfrozenset.differencefrozenset.symmetric_difference