Python - frozenset Example

With frozenset, we have an immutable set. We cannot add or remove elements. In every other way it is like a set. And because it cannot be changed, it can be used as a dictionary key.

A simple example. Here is an example that uses the frozenset keyword. We create an immutable set of strings (bird, plant and fish).

Tip A frozenset can be used where a set cannot. It can be a dictionary key or a set element itself.

Dictionary

Tip 2 We initialize a frozenset with the frozenset() built-in. We pass it a list of strings.

# Strings to put in frozenset. keys = ["bird", "plant", "fish"] # Create frozenset. f = frozenset(keys) print(f) # Cannot add to frozenset. try: f.add("cat") except AttributeError: print("Cannot add") # Can use frozenset as key to dictionary. d = {} d[f] = "awesome" print(d)

frozenset({'plant', 'bird', 'fish'}) Cannot add {frozenset({'plant', 'bird', 'fish'}): 'awesome'}

Nested frozensets. Let us try to create a frozenset of frozensets. With sets, this is not possible, but frozensets can be hashed like other values.

Here We create a frozenset that contains possible color pairs (which are stored as frozensets also).

Result We can test the frozenset to see whether a pair of colors exists. This uses hashing.

# Create 3 frozensets. colors1 = frozenset(["blue", "red"]) colors2 = frozenset(["orange", "black"]) colors3 = frozenset(["black", "white"]) # Create a frozenset of two frozensets. possible = frozenset([colors1, colors2]) # This frozenset is included. if colors1 in possible: print("Possible:", colors1) # This one is not. if colors3 not in possible: print("Not possible:", colors3)

Possible: frozenset({'blue', 'red'}) Not possible: frozenset({'white', 'black'})

Unhashable error. A set cannot be hashed. So when we try to create a set of sets we will get an "objects are unhashable" error (a TypeError).

# This example will not compile. example = {1, 2, 3} example2 = {example, {5, 6, 7}}

Traceback (most recent call last): File "C:\programs\file.py", line 6, in <module> example2 = {example, {5, 6, 7}} TypeError: 'set' objects are unhashable

A summary. Here we have an immutable and hashable set. We can test a frozenset for the existence of other pairs—this is a fast way to search for multiple elements.

Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.

Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.

This page was last updated on Sep 23, 2021 (edit).

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