How can a set be used as a key in a Python dictionary?

In Python, a set cannot be used directly as a dictionary key because sets are mutable and unhashable.

If you need to use a set as a key, you can use frozenset — an immutable and hashable version of a set.

Example:

my_dict = {}
key = frozenset([1, 2, 3])
my_dict[key] = "value"
print(my_dict[frozenset([1, 2, 3])])  # Outputs: value

Thus, frozenset allows you to use a set as a dictionary key.