Immutable Set — frozenset

8.1 set-tuple

The frozenset collection is an immutable version of set. It gives you all the benefits of a set, but doesn't let you change its elements after it's created. frozenset is super handy when you need to store unique items and ensure the set isn't altered.

Creating a frozenset

To whip up a frozenset, you can call the frozenset() function, passing in an iterable (like a list, tuple, string, etc.).

Examples:

Creating frozenset from a list

# Creating frozenset from a list
fset1 = frozenset([1, 2, 3, 4])
print(fset1)  # Output: frozenset({1, 2, 3, 4})

Creating frozenset from a string

# Creating frozenset from a string
fset2 = frozenset("hello")
print(fset2)  # Output: frozenset({'h', 'e', 'l', 'o'})

Creating an empty frozenset

# Creating an empty frozenset
fset3 = frozenset()
print(fset3)  # Output: frozenset()
        

8.2 Main methods of frozenset

frozenset supports most methods available for mutable sets (set), but since it's immutable, any methods that modify the set aren't available.

Examples:

Union of sets (union)

fset1 = frozenset([1, 2, 3, 4])
fset2 = frozenset([3, 4, 5, 6])
            
# Union of sets (union)
print(fset1 | fset2)  # Output: frozenset({1, 2, 3, 4, 5, 6})
print(fset1.union(fset2))  # Output: frozenset({1, 2, 3, 4, 5, 6})
        

Intersection of sets (intersection)

fset1 = frozenset([1, 2, 3, 4])
fset2 = frozenset([3, 4, 5, 6])
            
# Intersection of sets (intersection)
print(fset1 & fset2)  # Output: frozenset({3, 4})
print(fset1.intersection(fset2))  # Output: frozenset({3, 4})
        

Difference of sets (difference)

fset1 = frozenset([1, 2, 3, 4])
fset2 = frozenset([3, 4, 5, 6])
            
# Difference of sets (difference) 
print(fset1 - fset2)  # Output: frozenset({1, 2})
print(fset1.difference(fset2))  # Output: frozenset({1, 2})
        

Symmetric difference (symmetric difference)

fset1 = frozenset([1, 2, 3, 4])
fset2 = frozenset([3, 4, 5, 6])
            
# Symmetric difference (symmetric difference)
print(fset1 ^ fset2)  # Output: frozenset({1, 2, 5, 6})
print(fset1.symmetric_difference(fset2))  # Output: frozenset({1, 2, 5, 6})
        

8.3 Using frozenset

frozenset is handy in these situations:

Using as a dictionary key:

Since frozenset is immutable, you can totally use it as a dictionary key.

fset1 = frozenset([1, 2, 3])
fset2 = frozenset([3, 4, 5])
        
d = {fset1: "first", fset2: "second"}
print(d)  # Output: {frozenset({1, 2, 3}): 'first', frozenset({3, 4, 5}): 'second'}
    

Storing immutable sets:

If you need to make a set of unique items that won't be changed later on, frozenset's a great choice.

Data safety:

Using frozenset ensures the data won't get accidentally changed while your script is running.