5.6 Set Data Types

set

What Is It?

A mutable, unordered collection of unique, hashable values.

Why Is It Used?

Automatic deduplication and fast (O(1) average) membership testing — far faster than checking in on a list for large collections.

>>> s = {1, 2, 3, 2, 1}    # duplicates are automatically dropped
>>> s
{1, 2, 3}

frozenset

The immutable counterpart to set — same behavior, but can’t be modified after creation, which makes it hashable and therefore usable as a dict key or set member itself.

>>> fs = frozenset([1, 2, 3])
>>> fs.add(4)
Traceback (most recent call last):
AttributeError: 'frozenset' object has no attribute 'add'

Unique Values

The most common practical use of set: deduplicating a list in one line.

>>> ips = ["10.0.0.1", "10.0.0.2", "10.0.0.1"]
>>> unique_ips = set(ips)
>>> unique_ips
{'10.0.0.1', '10.0.0.2'}

Quick Interview Answer

“set is a mutable, unordered collection of unique, hashable values — it deduplicates automatically and gives O(1) average membership testing, versus O(n) for a list. frozenset is its immutable counterpart, which makes it hashable and therefore usable as a dict key or as a member of another set. set(some_list) is the standard one-line way to deduplicate.”

Common Mistakes

• Trying to add to a frozenset after creation — it has no .add(), since it’s immutable by design.
• Relying on set ordering — sets are unordered, so iteration order isn’t guaranteed the way it is for a list or (since 3.7) a dict.
• Putting an unhashable value (like a list) into a set, raising TypeError: unhashable type.