Sets

Sets store unique values. Use them when you care about membership or deduplication more than order.

A set is like a bag of distinct items. Adding the same value twice does not create a duplicate. If you only need to know whether something is present, a set usually fits better than a list.

Creating sets

tags = {"docs", "guide", "acton"}
empty_tags: set[str] = set()

Use {...} for a non-empty set. Use set() for an empty one, because {} means an empty dictionary.

Checking and updating

tags = {"docs", "guide"}

print("docs" in tags)
print("api" not in tags)

tags.add("api")
tags.add("docs")
tags.discard("guide")
tags.update({"reference", "tutorial"})

print(tags)
print(len(tags))

add() inserts one value. discard() removes a value if it is present and does nothing if it is not. update() adds values from another set or any iterable of values.

Removing values

tags = {"docs", "guide", "acton"}

print(tags.pop())
print(tags)

pop() removes and returns one element. On an empty set it raises an exception, so check before calling it if the set may be empty.

Iteration and order

tags = {"docs", "guide", "acton"}

for tag in tags:
    print(tag)

print(sorted(tags))

Sets do not preserve order. Do not rely on the printed order when the exact sequence matters. If you need stable display order, sort the set first.

Set comprehensions

words = ["hello", "world", "hello", "acton"]
unique_words = {word for word in words}
long_words = {word.upper() for word in words if len(word) > 4}
remainder_classes = {n % 3 for n in range(10)}

Set comprehensions are a compact way to build a set while automatically removing duplicates.